Fix the error when saving model with GPU.

src/TensorFlowNET.Core/Checkpoint/SaveUtilV1.cs

@@ -53,8 +53,11 @@ public static class SaveUtilV1
             var g = to_graph.as_default();
             var (named_saveable_objects, graph_proto, _, registered_savers) = serialize_gathered_objects(graph_view,
                     object_map, call_with_mapped_captures, saveables_cache);
-            tf.device("/cpu:0");
-            var object_graph_tensor = constant_op.constant(graph_proto.ToByteArray());
+            var object_graph_tensor = tf_with(ops.device("/cpu:0"), _ =>
+            {
+                // TODO(Rinne): locate the error that causes transferring TF_STRING to this function throws an exception.
+                return constant_op.constant(graph_proto.ToByteArray());
+            });
             named_saveable_objects.Add(new NoRestoreSaveable(object_graph_tensor, Trackable.Constants.OBJECT_GRAPH_PROTO_KEY));
             g.Exit();
             return (named_saveable_objects, registered_savers);
@@ -65,8 +68,10 @@ public static class SaveUtilV1
             {
                 var (named_saveable_objects, graph_proto, _, registered_savers) = serialize_gathered_objects(graph_view,
                     object_map, call_with_mapped_captures, saveables_cache);
-                tf.device("/cpu:0");
-                var object_graph_tensor = constant_op.constant(graph_proto.ToString(), TF_DataType.TF_STRING);
+                var object_graph_tensor = tf_with(ops.device("/cpu:0"), _ =>
+                {
+                    return constant_op.constant(graph_proto.ToString());
+                });
                 named_saveable_objects.Add(new NoRestoreSaveable(object_graph_tensor, Trackable.Constants.OBJECT_GRAPH_PROTO_KEY));
                 return (named_saveable_objects, registered_savers);
             }

src/TensorFlowNET.Core/Checkpoint/checkpoint.cs

@@ -58,8 +58,10 @@ public class TrackableSaver
 
         if(object_graph_tensor is null)
         {
-            tf.device("/cpu:0");
-            object_graph_tensor = constant_op.constant(graph_proto.ToByteArray());
+            tf_with(ops.device("/cpu:0"), _ =>
+            {
+                object_graph_tensor = constant_op.constant(graph_proto.ToByteArray());
+            });
         }
         else
         {
@@ -230,13 +232,15 @@ public class TrackableSaver
         Tensor object_graph_string = reader.GetTensor(Trackable.Constants.OBJECT_GRAPH_PROTO_KEY, dtype: TF_DataType.TF_STRING);
 
         Dictionary<Tensor, string> file_prefix_feed_dict;
-        Tensor file_prefix_tensor;
+        Tensor file_prefix_tensor = null;
         if (graph_building)
         {
             if(_file_prefix_placeholder is null)
             {
-                tf.device("/cpu:0");
-                _file_prefix_placeholder = constant_op.constant("model");
+                _file_prefix_placeholder = tf_with(ops.device("/cpu:0"), _ =>
+                {
+                    return constant_op.constant("model");
+                });
             }
             file_prefix_tensor = _file_prefix_placeholder;
             file_prefix_feed_dict = new();
@@ -244,8 +248,10 @@ public class TrackableSaver
         }
         else
         {
-            tf.device("/cpu:0");
-            file_prefix_tensor = constant_op.constant(save_path);
+            file_prefix_tensor = tf_with(ops.device("/cpu:0"), _ =>
+            {
+                return constant_op.constant(save_path);
+            });
             file_prefix_feed_dict = null;
         }
         TrackableObjectGraph object_graph_proto = new();

src/TensorFlowNET.Core/Checkpoint/functional_saver.cs

@@ -211,9 +211,11 @@ namespace Tensorflow.Checkpoint
 
             string restore_device = string.IsNullOrEmpty(options.experimental_io_device) ? "cpu:0": options.experimental_io_device!;
 
-            // tf python has code `with ops.device(restore_device):` here.
-            tf.device(restore_device); // may be risky.
-            var restored_tensors = gen_ops.restore_v2(file_prefix, tensor_names.ToArray(), slice_specs.ToArray(), tensor_dtypes.ToArray());
+            Tensor[] restored_tensors = null;
+            tf_with(ops.device(restore_device), _ =>
+            {
+                restored_tensors = gen_ops.restore_v2(file_prefix, tensor_names.ToArray(), slice_specs.ToArray(), tensor_dtypes.ToArray());
+            });
 
             Dictionary<string, IDictionary<string, Tensor>> restored_tensor_dict = new();
             int idx = 0;
@@ -338,11 +340,14 @@ namespace Tensorflow.Checkpoint
                 options = new CheckpointOptions();
             }
 
-            tf.device("CPU"); // may be risky.
-            var sharded_suffix = array_ops.where(gen_ops.regex_full_match(file_prefix, tf.constant(@"^s3://.*")),
+            Tensor tmp_checkpoint_prefix = null;
+            tf_with(ops.device("CPU"), _ =>
+            {
+                var sharded_suffix = array_ops.where(gen_ops.regex_full_match(file_prefix, tf.constant(@"^s3://.*")),
                 constant_op.constant(".part"), constant_op.constant("_temp/part"));
-            var tmp_checkpoint_prefix = gen_ops.string_join(new Tensor[] { file_prefix, sharded_suffix });
-            IDictionary<string, Tensor> registered_paths = _registered_savers.Keys.ToDictionary(x => x, x => registered_saver_filename(file_prefix, x));
+                tmp_checkpoint_prefix = gen_ops.string_join(new Tensor[] { file_prefix, sharded_suffix });
+                IDictionary<string, Tensor> registered_paths = _registered_savers.Keys.ToDictionary(x => x, x => registered_saver_filename(file_prefix, x));
+            });
 
             Operation save_fn()
             {
@@ -364,16 +369,24 @@ namespace Tensorflow.Checkpoint
                     var saver = pair.Value;
                     last_device = device;
                     // skip the extra process of device name because of lack of API.
-                    tf.device(device);
-                    var shard_prefix = sharded_filename(tmp_checkpoint_prefix, shard, num_shards_tensor);
+                    Tensor shard_prefix = null;
+                    tf_with(ops.device(device), _ =>
+                    {
+                        shard_prefix = sharded_filename(tmp_checkpoint_prefix, shard, num_shards_tensor);
+                    });
                     saved_prefixes.Add(shard_prefix);
-                    sharded_saves.Add(saver.save(shard_prefix, options));
+                    tf_with(ops.device(device), _ =>
+                    {
+                        sharded_saves.Add(saver.save(shard_prefix, options));
+                    });
                 }
                 using (var controller = ops.control_dependencies(sharded_saves.ToArray()))
                 {
                     string merge_device = string.IsNullOrEmpty(options.experimental_io_device) ? last_device : options.experimental_io_device;
-                    tf.device(merge_device);
-                    return gen_ops.merge_v2_checkpoints(saved_prefixes.ToArray(), tf.constant(file_prefix), delete_old_dirs: true);
+                    return tf_with(ops.device(merge_device), _ =>
+                    {
+                        return gen_ops.merge_v2_checkpoints(saved_prefixes.ToArray(), tf.constant(file_prefix), delete_old_dirs: true);
+                    });
                 }
             }
 
@@ -407,54 +420,56 @@ namespace Tensorflow.Checkpoint
                 {
                     var device = single_saver.Key;
                     var saver = single_saver.Value;
-                    tf.device(device);
-                    var restored_tensor_dict = saver.restore(file_prefix, options);
-
-                    foreach(var pair in restored_tensor_dict)
+                    tf_with(ops.device(device), _ =>
                     {
-                        var checkpoint_key = pair.Key;
-                        var slice_and_tensor = pair.Value;
-                        foreach(var item in slice_and_tensor)
+                        var restored_tensor_dict = saver.restore(file_prefix, options);
+
+                        foreach (var pair in restored_tensor_dict)
                         {
-                            var slice_spec = item.Key;
-                            var tensor = item.Value;
-                            var restore_fn = _keys_to_restore_fn[(checkpoint_key, slice_spec)];
-                            var internal_dict = restore_fn_inputs.SetDefault(restore_fn, new Dictionary<string, Maybe<Tensor, IDictionary<string, Tensor>>>());
-                            if (!string.IsNullOrEmpty(slice_spec))
+                            var checkpoint_key = pair.Key;
+                            var slice_and_tensor = pair.Value;
+                            foreach (var item in slice_and_tensor)
                             {
-                                if (!internal_dict.ContainsKey(checkpoint_key))
+                                var slice_spec = item.Key;
+                                var tensor = item.Value;
+                                var restore_fn = _keys_to_restore_fn[(checkpoint_key, slice_spec)];
+                                var internal_dict = restore_fn_inputs.SetDefault(restore_fn, new Dictionary<string, Maybe<Tensor, IDictionary<string, Tensor>>>());
+                                if (!string.IsNullOrEmpty(slice_spec))
                                 {
-                                    Dictionary<string, Tensor> dict = new();
-                                    dict[slice_spec] = tensor;
-                                    internal_dict[checkpoint_key] = new Maybe<Tensor, IDictionary<string, Tensor>>(dict);
+                                    if (!internal_dict.ContainsKey(checkpoint_key))
+                                    {
+                                        Dictionary<string, Tensor> dict = new();
+                                        dict[slice_spec] = tensor;
+                                        internal_dict[checkpoint_key] = new Maybe<Tensor, IDictionary<string, Tensor>>(dict);
+                                    }
+                                    else
+                                    {
+                                        internal_dict[checkpoint_key].GetValue<IDictionary<string, Tensor>>()[slice_spec] = tensor;
+                                    }
                                 }
                                 else
                                 {
-                                    internal_dict[checkpoint_key].GetValue<IDictionary<string, Tensor>>()[slice_spec] = tensor;
+                                    internal_dict[checkpoint_key] = new Maybe<Tensor, IDictionary<string, Tensor>>(tensor);
                                 }
-                            }
-                            else
-                            {
-                                internal_dict[checkpoint_key] = new Maybe<Tensor, IDictionary<string, Tensor>>(tensor);
-                            }
-                            restore_fn_input_count[restore_fn]--;
+                                restore_fn_input_count[restore_fn]--;
 
-                            if (restore_fn_input_count[restore_fn] == 0)
-                            {
-                                Dictionary<string, Maybe<Tensor, IDictionary<string, Tensor>>> restored_tensors = new();
-                                foreach(var input in restore_fn_inputs[restore_fn])
+                                if (restore_fn_input_count[restore_fn] == 0)
                                 {
-                                    restored_tensors[TrackableUtils.extract_local_name(input.Key)] = input.Value;
-                                }
-                                var ret = restore_fn.DynamicInvoke(restored_tensors);
-                                if(ret is IDictionary<string, Operation>)
-                                {
-                                    var dict = (IDictionary<string, Operation>)ret;
-                                    restore_ops = restore_ops.Concat(dict).ToDictionary(x => x.Key, x => x.Value);
+                                    Dictionary<string, Maybe<Tensor, IDictionary<string, Tensor>>> restored_tensors = new();
+                                    foreach (var input in restore_fn_inputs[restore_fn])
+                                    {
+                                        restored_tensors[TrackableUtils.extract_local_name(input.Key)] = input.Value;
+                                    }
+                                    var ret = restore_fn.DynamicInvoke(restored_tensors);
+                                    if (ret is IDictionary<string, Operation>)
+                                    {
+                                        var dict = (IDictionary<string, Operation>)ret;
+                                        restore_ops = restore_ops.Concat(dict).ToDictionary(x => x.Key, x => x.Value);
+                                    }
                                 }
                             }
                         }
-                    }
+                    });
                 }
 
                 foreach(var item in _registered_savers)
@@ -500,21 +515,25 @@ namespace Tensorflow.Checkpoint
         private Tensor _traced_save(Tensor file_prefix)
         {
             var save_op = save(file_prefix);
-            tf.device("cpu:0");
-            using (ops.control_dependencies(new object[]{ save_op }))
+            return tf_with(ops.device("cpu:0"), _ =>
             {
-                return array_ops.identity(file_prefix);
-            }
+                return tf_with(ops.control_dependencies(new object[] { save_op }), __ =>
+                {
+                    return array_ops.identity(file_prefix);
+                });
+            });
         }
 
         private Tensor _traced_restore(Tensor file_prefix)
         {
             var restore_op = restore(file_prefix);
-            tf.device("cpu:0");
-            using (ops.control_dependencies(restore_op.Values.ToArray()))
+            return tf_with(ops.device("cpu:0"), _ =>
             {
-                return array_ops.identity(file_prefix);
-            }
+                return tf_with(ops.control_dependencies(restore_op.Values.ToArray()), __ =>
+                {
+                    return array_ops.identity(file_prefix);
+                });
+            });
         }
 
         public static MultiDeviceSaver from_saveables(IEnumerable<MySaveableObject> saveables, IDictionary<string, IDictionary<string, Trackable>>? registered_savers = null, bool call_with_mapped_captures = false)

src/TensorFlowNET.Core/Contexts/Context.Device.cs

@@ -21,6 +21,7 @@ using Tensorflow.Eager;
 using static Tensorflow.Binding;
 using Google.Protobuf;
 using Tensorflow.Device;
+using Tensorflow.Exceptions;
 using System.Collections.Generic;
 
 namespace Tensorflow.Contexts
@@ -30,10 +31,30 @@ namespace Tensorflow.Contexts
     /// </summary>
     public sealed partial class Context
     {
+        internal static Dictionary<(string, string), (string, DeviceSpec)> _device_parsing_cache = new();
+        internal List<LogicalDevice> _logical_devices = null;
+        internal List<string> _context_devices = null;
+        
         ContextDevicePlacementPolicy _device_policy;
         bool _log_device_placement;
+        int _num_gpus;
         Dictionary<PhysicalDevice, bool> _memory_growth_map = new Dictionary<PhysicalDevice, bool>();
 
+        public string DeviceName { get; set; } = "";
+        public DeviceSpec DeviceSpec { get; set; } = null;
+
+        internal List<string> Devices
+        {
+            get
+            {
+                if(_context_devices is null)
+                {
+                    throw new AssertionError("Context must be initialized first.");
+                }
+                return _context_devices;
+            }
+        }
+
         public void log_device_placement(bool enable)
         {
             if (_handle != null)
@@ -89,5 +110,57 @@ namespace Tensorflow.Contexts
 
             return results.ToArray();
         }
+
+        public EagerDeviceContext device(string name)
+        {
+            return new EagerDeviceContext(this, name);
+        }
+
+        internal void _set_device(string device_name, DeviceSpec device_spec)
+        {
+            DeviceSpec = device_spec;
+            DeviceName = device_name;
+        }
+
+        internal void _initialize_logical_devices()
+        {
+            List<LogicalDevice> logical_devices = new();
+            List<string> context_devices = new();
+            Status status = new();
+            var device_list = c_api.TFE_ContextListDevices(_handle, status);
+            status.Check(true);
+            try
+            {
+                this._num_gpus = 0;
+                string current_job = null;
+                int current_task = -1;
+                for(int i = 0; i < c_api.TF_DeviceListCount(device_list); i++)
+                {
+                    var dev_name = c_api.TF_DeviceListName(device_list, i, status);
+                    status.Check(true);
+                    context_devices.Add(DeviceUtils.canonical_name(dev_name));
+                    var spec = DeviceSpec.from_string(dev_name);
+                    if(spec.Job == "localhost")
+                    {
+                        spec = spec.replace(job: null, replica: -1, task: -1);
+                    }
+                    logical_devices.Add(new LogicalDevice(spec.ToString(), spec.DeviceType));
+                    var dev_type_memory = c_api.TF_DeviceListType(device_list, i, status);
+                    var dev_type = c_api.StringPiece(dev_type_memory);
+                    status.Check(true);
+                    if(dev_type == "GPU" && spec.Job == current_job && spec.Task == current_task)
+                    {
+                        _num_gpus++;
+                    }
+                }
+            }
+            finally
+            {
+                _logical_devices = logical_devices;
+                _context_devices = context_devices;
+            }
+        }
     }
+
+    public record class LogicalDevice(string name, string device_type);
 }

src/TensorFlowNET.Core/Contexts/Context.cs

@@ -34,7 +34,6 @@ namespace Tensorflow.Contexts
         public const int EAGER_MODE = 1;
 
         int defaultExecutionMode = EAGER_MODE;
-        public string DeviceName { get; set; } = "";
         public string ScopeName { get; set; } = "";
         bool initialized = false;
         ContextSwitchStack context_switches;
@@ -62,6 +61,8 @@ namespace Tensorflow.Contexts
             if (initialized)
                 return;
 
+            Debug.Assert(_context_devices is null);
+
             Config = MergeConfig();
             FunctionCallOptions.Config = Config;
             var config_str = Config.ToByteArray();
@@ -72,6 +73,7 @@ namespace Tensorflow.Contexts
             c_api.TFE_ContextOptionsSetDevicePlacementPolicy(opts, _device_policy);
             _handle = c_api.TFE_NewContext(opts, status);
             status.Check(true);
+            _initialize_logical_devices();
             initialized = true;
         }
 
@@ -174,6 +176,7 @@ namespace Tensorflow.Contexts
             {
                 c_api.TFE_ContextClearCaches(_handle);
             }
+            _device_parsing_cache.Clear();
         }
 
         public static implicit operator SafeContextHandle(Context ctx)

src/TensorFlowNET.Core/Contexts/EagerDeviceContext.cs

@@ -0,0 +1,71 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using Tensorflow.Device;
+
+namespace Tensorflow.Contexts
+{
+    public class EagerDeviceContext : ITensorFlowObject
+    {
+        private Context _ctx;
+        private string _device_name;
+        private Stack<(string, DeviceSpec, DeviceSpec)> _stack;
+
+        public EagerDeviceContext(Context ctx, string device_name)
+        {
+            _ctx = ctx;
+            _device_name = device_name;
+            _stack = new Stack<(string, DeviceSpec, DeviceSpec)>();
+        }
+        public void __enter__()
+        {
+            var ctx = _ctx;
+            var old_device_name = ctx.DeviceName;
+            var old_device_spec = ctx.DeviceSpec; 
+            var new_device_name = _device_name;
+            var cache_key = (old_device_name, new_device_name);
+            DeviceSpec new_device_spec;
+            if (Context._device_parsing_cache.ContainsKey(cache_key))
+            {
+                (new_device_name, new_device_spec) = Context._device_parsing_cache[cache_key];
+            }
+            else
+            {
+                if(new_device_name is not null)
+                {
+                    var device_spec = DeviceSpec.from_string(new_device_name);
+                    if (!string.IsNullOrEmpty(old_device_name))
+                    {
+                        new_device_spec = new DeviceSpec(old_device_spec);
+                    }
+                    else
+                    {
+                        ctx.ensure_initialized();
+                        new_device_spec = DeviceSpec.from_string(ctx._context_devices[0]);
+                    }
+                    new_device_spec = new_device_spec.make_merged_spec(device_spec);
+                }
+                else
+                {
+                    new_device_spec = DeviceSpec.from_string(ctx._context_devices[0]);
+                }
+                new_device_name = new_device_spec.ToString();
+                Context._device_parsing_cache[cache_key] = (new_device_name, new_device_spec);
+            }
+            ctx._set_device(new_device_name, new_device_spec);
+            _stack.Push((old_device_name, old_device_spec, new_device_spec));
+        }
+
+        public void __exit__()
+        {
+            var ctx = _ctx;
+            var (old_device_name, old_device_spec, new_device_spec) = _stack.Pop();
+            ctx._set_device(old_device_name, old_device_spec);
+        }
+
+        public void Dispose()
+        {
+
+        }
+    }
+}

src/TensorFlowNET.Core/Device/DeviceSpec.cs

@@ -0,0 +1,205 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+using System.Threading.Tasks;
+
+namespace Tensorflow.Device
+{
+    public class DeviceSpec
+    {
+        private static Dictionary<string, Components> _STRING_TO_COMPONENTS_CACHE = new();
+        private static Dictionary<Components, string> _COMPONENTS_TO_STRING_CACHE = new();
+        private string _job;
+        private int _replica;
+        private int _task;
+        private string _device_type;
+        private int _device_index;
+        private string _as_string;
+
+        public string Job => _job;
+        public int Replica => _replica;
+        public int Task => _task;
+        public string DeviceType => _device_type;
+        public int DeviceIndex => _device_index;
+
+        public DeviceSpec(string job = null, int replica = -1, int task = -1,
+            string device_type = null, int device_index = -1)
+        {
+            _job = job;
+            _replica = replica;
+            _task = task;
+            _device_type = device_type;
+            _device_index = device_index;
+            _as_string = _components_to_string(job, replica, task, device_type, _device_index);
+
+        }
+
+        public DeviceSpec(DeviceSpec other)
+        {
+            _job = other._job;
+            _replica = other._replica;
+            _task = other._task;
+            _device_type = other._device_type;
+            _device_index = other._device_index;
+            _as_string = other._as_string;
+        }
+
+        protected DeviceSpec(Components com)
+        {
+            _job = com.Job;
+            _replica = com.Replica;
+            _task = com.Task;
+            _device_type = com.DeviceType;
+            _device_index = com.DeviceIndex;
+            _as_string = _components_to_string(_job, _replica, _task, _device_type, _device_index);
+        }
+
+        public DeviceSpec replace(string job = null, int replica = -1, int task = -1, 
+            string device_type = null, int device_index = -1)
+        {
+            job = job ?? _job;
+            replica = replica == -1 ? _replica : replica;
+            task = task == -1 ? _task : task;
+            device_type = device_type ?? _device_type;
+            device_index = device_index == -1 ? _device_index : device_index;
+            return new DeviceSpec(job, replica, task, device_type, device_index);
+        }
+
+        public static DeviceSpec from_string(string spec)
+        {
+            var components = _string_to_components(spec);
+            return new DeviceSpec(components.Job, components.Replica, components.Task, components.DeviceType, components.DeviceIndex);
+        }
+
+        public DeviceSpec make_merged_spec(DeviceSpec dev)
+        {
+            return new DeviceSpec(_get_combined_properties(dev));
+        }
+
+        private Components _get_combined_properties(DeviceSpec dev)
+        {
+            return new Components(
+                dev.Job ?? _job,
+                dev.Replica == -1 ? _replica : dev.Replica,
+                dev.Task == -1 ? _task : dev.Task,
+                dev.DeviceType ?? _device_type,
+                dev.DeviceIndex == -1 ? _device_index : dev.DeviceIndex
+                );
+        }
+
+        private static string _components_to_string(string job, int replica, int task, string device_type, int device_index)
+        {
+            var key = new Components(job, replica, task, device_type, device_index);
+            if(_COMPONENTS_TO_STRING_CACHE.TryGetValue(key, out var cache_result))
+            {
+                return cache_result;
+            }
+
+            StringBuilder output = new();
+            if(job is not null)
+            {
+                output.Append($"/job:{job}");
+            }
+            if(replica != -1)
+            {
+                output.Append($"/replica:{replica}");
+            }
+            if(task != -1)
+            {
+                output.Append($"/task:{task}");
+            }
+            if (device_type is not null)
+            {
+                string device_index_string = "*";
+                if (device_index != -1)
+                {
+                    device_index_string = device_index.ToString();
+                }
+                output.Append($"/device:{device_type}:{device_index_string}");
+            }
+            var result = output.ToString();
+            _COMPONENTS_TO_STRING_CACHE[key] = result;
+            return result;
+        }
+
+        private static Components _string_to_components(string spec)
+        {
+            if(_STRING_TO_COMPONENTS_CACHE.TryGetValue(spec, out var cached_result))
+            {
+                return cached_result;
+            }
+            var raw_spec = spec;
+            var splits = spec.Split('/').Select(x => x.Split(':'));
+            var valid_device_types = _get_valid_device_types();
+            string job = null, device_type = null;
+            int replica = -1, task = -1, device_index = -1;
+            foreach (var y in splits)
+            {
+                var ly = y.Length;
+                if (ly > 0)
+                {
+                    if(ly == 2 && y[0] == "job")
+                    {
+                        job = y[1];
+                    }
+                    else if(ly == 2 && y[0] == "replica")
+                    {
+                        replica = int.Parse(y[1]);
+                    }
+                    else if(ly == 2 && y[0] == "task")
+                    {
+                        task = int.Parse(y[1]);
+                    }
+                    else if((ly == 1 || ly == 2) && valid_device_types.Contains(y[0].ToUpper()))
+                    {
+                        if (device_type is not null)
+                        {
+                            throw new ValueError($"Multiple device types are not allowed " +
+                                $"while parsing the device spec: {spec}.");
+                        }
+                        device_type = y[0].ToUpper();
+                        if(ly == 2 && y[1] != "*")
+                        {
+                            device_index = int.Parse(y[1]);
+                        }
+                    }
+                    else if(ly == 3 && y[0] == "device")
+                    {
+                        if(device_type is not null)
+                        {
+                            throw new ValueError($"Multiple device types are not allowed " +
+                                $"while parsing the device spec: {spec}.");
+                        }
+                        device_type = y[1];
+                        if (y[2] != "*")
+                        {
+                            device_index = int.Parse(y[2]);
+                        }
+                    }
+                    else if (y[0] != "")
+                    {
+                        throw new ValueError($"Unknown attribute '{y[0]}' is encountered " +
+                            $"while parsing the device spec: {spec}.");
+                    }
+                }
+            }
+
+            var output = new Components(job, replica, task, device_type, device_index);
+            _STRING_TO_COMPONENTS_CACHE[raw_spec] = output;
+            return output;
+        }
+
+        private static HashSet<string> _get_valid_device_types()
+        {
+            // TODO(Rinne): revise it to calling C API (need customized API).
+            return new HashSet<string>(new string[] { "CPU", "GPU" });
+        }
+
+        public override string ToString()
+        {
+            return _as_string;
+        }
+
+        protected record class Components(string Job, int Replica, int Task, string DeviceType, int DeviceIndex);
+    }
+}

src/TensorFlowNET.Core/Device/DeviceUtils.cs

@@ -0,0 +1,26 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Tensorflow.Device
+{
+    internal static class DeviceUtils
+    {
+        public static string canonical_name(string device)
+        {
+            if(device is null)
+            {
+                return "";
+            }
+            return DeviceSpec.from_string(device).ToString();
+        }
+        public static string canonical_name(DeviceSpec device)
+        {
+            if (device is null)
+            {
+                return "";
+            }
+            return device.ToString();
+        }
+    }
+}

src/TensorFlowNET.Core/Graphs/Graph.cs

@@ -19,6 +19,7 @@ using System.Collections;
 using System.Collections.Generic;
 using System.Collections.Specialized;
 using System.Linq;
+using Tensorflow.Graphs;
 using static Tensorflow.Binding;
 
 namespace Tensorflow
@@ -294,9 +295,15 @@ namespace Tensorflow
             return op;
         }
 
-        public void device(string device_name)
+        public ITensorFlowObject device(string device_name)
         {
-            
+            return new GraphDeviceContext(this, device_name);
+        }
+
+        private void add_device_to_stack(string device_name, int offset = 0)
+        {
+            // TODO(Rinne): deal with device spec.
+            int total_offset = offset + 1;
         }
 
         private void _create_op_helper(Operation op, bool compute_device = true)

src/TensorFlowNET.Core/Graphs/GraphDeviceContext.cs

@@ -0,0 +1,31 @@
+using System;
+using System.Collections.Generic;
+using System.Text;
+
+namespace Tensorflow.Graphs
+{
+    public class GraphDeviceContext : ITensorFlowObject
+    {
+        private Graph _graph;
+
+        public GraphDeviceContext(Graph graph, string device_name)
+        {
+            _graph = graph;
+        }
+
+        public void __enter__()
+        {
+
+        }
+
+        public void __exit__()
+        {
+
+        }
+
+        public void Dispose()
+        {
+
+        }
+    }
+}

src/TensorFlowNET.Core/Training/Saving/ResourceVariableSaveable.cs

@@ -42,16 +42,20 @@ namespace Tensorflow
             _var_device = var.Device;
             _var_shape = var.shape;
 
-            Tensor _read_variable_closure(BaseResourceVariable v)
+            Tensor? _read_variable_closure(BaseResourceVariable v)
             {
-                tf.device(v.Device);
-                if(tf.Context.executing_eagerly() && !((bool)v.is_initialized().numpy()))
+                return tf_with(ops.device(v.Device), _ =>
                 {
-                    return null;
-                }
-                var x = v.read_value_no_copy();
-                tf.device("/device:CPU:0");
-                return array_ops.identity(x);
+                    if (tf.Context.executing_eagerly() && !((bool)v.is_initialized().numpy()))
+                    {
+                        return null;
+                    }
+                    var x = v.read_value_no_copy();
+                    return tf_with(ops.device("/device:CPU:0"), __ =>
+                    {
+                        return array_ops.identity(x);
+                    });
+                });
             }
 
             this.handle_op = var.Handle;

src/TensorFlowNET.Core/Training/Saving/SavedModel/loader.cs

@@ -412,8 +412,10 @@ namespace Tensorflow
         {
             var variables_path = SavedModelUtils.get_variables_path(_export_dir);
             var saver = new TrackableSaver(new ObjectGraphView(get(0)));
-            tf.device("CPU");
-            saver.FilePrefixPlaceHolder = constant_op.constant(variables_path);
+            tf_with(ops.device("CPU"), _ =>
+            {
+                saver.FilePrefixPlaceHolder = constant_op.constant(variables_path);
+            });
             LoadStatus load_status;
             if (_save_options.allow_partial_checkpoint)
             {
@@ -598,14 +600,16 @@ namespace Tensorflow
 
             if (load_with_device)
             {
-                tf.device(saved_device);
-                return (new UninitializedVariable(
-                    shape: new Shape(proto.Shape.Dim.Select(x => (int)x.Size).ToArray()),
-                    dtype: (TF_DataType)proto.Dtype,
-                    name: name,
-                    trainable: trainable,
-                    aggregation: aggregation
-                ), setattr);
+                return tf_with(ops.device(saved_device), _ =>
+                {
+                    return (new UninitializedVariable(
+                        shape: new Shape(proto.Shape.Dim.Select(x => (int)x.Size).ToArray()),
+                        dtype: (TF_DataType)proto.Dtype,
+                        name: name,
+                        trainable: trainable,
+                        aggregation: aggregation
+                    ), setattr);
+                });
             }
             else
             {

src/TensorFlowNET.Core/Variables/BaseResourceVariable.cs

@@ -266,9 +266,11 @@ namespace Tensorflow
             BaseResourceVariable new_variable;
             if (save_options.experimental_variable_policy.save_variable_devices())
             {
-                tf.device(this.Device);
                 Debug.Assert(this is ResourceVariable);
-                new_variable = resource_variable_ops.copy_to_graph_uninitialized((ResourceVariable)this);
+                new_variable = tf_with(ops.device(this.Device), _ =>
+                {
+                    return resource_variable_ops.copy_to_graph_uninitialized((ResourceVariable)this);
+                });
             }
             else
             {

src/TensorFlowNET.Core/Variables/UninitializedVariable.cs

@@ -49,9 +49,12 @@ namespace Tensorflow.Variables
                     {
                         tf_with(ops.name_scope("Read"), _ =>
                         {
-                            tf.device(handle.Device);
-                            var value = gen_resource_variable_ops.read_variable_op(handle, dtype);
-                            // _maybe_set_handle_data(dtype, handle, value)
+                            var value = tf_with(ops.device(handle.Device), _ =>
+                            {
+                                var result = gen_resource_variable_ops.read_variable_op(handle, dtype);
+                                // TODO(Rinne): _maybe_set_handle_data(dtype, handle, value)
+                                return result;
+                            });
                             _graph_element = value;
                         });
                         ops.add_to_collection(ops.GraphKeys.GLOBAL_VARIABLES_, this);

src/TensorFlowNET.Core/ops.cs

@@ -577,6 +577,23 @@ namespace Tensorflow
             
         }
 
+        public static ITensorFlowObject device(string device_name)
+        {
+            if (tf.Context.executing_eagerly())
+            {
+                return tf.Context.device(device_name);
+            }
+            //else if (ops.executing_eagerly_outside_functions())
+            //{
+            //    throw new NotImplementedException();
+            //}
+            else
+            {
+                return get_default_graph().device(device_name);
+            }
+            // TODO(Rinne): deal with `ops.executing_eagerly_outside_functions()`.
+        }
+
         public class NullContextManager: IDisposable
         {
             public void Dispose()