fix default graph and operation issue when import model.

src/TensorFlowNET.Core/Buffers/Buffer.cs

@@ -30,8 +30,8 @@ namespace Tensorflow
             get 
             {
                 var data = new byte[buffer.length];
-                if (buffer.length > 0)
-                    Marshal.Copy(buffer.data, data, 0, (int)buffer.length);
+                if (data.Length > 0)
+                    Marshal.Copy(buffer.data, data, 0, data.Length);
                 return data;
             }
         }

src/TensorFlowNET.Core/Framework/c_api_util.cs

@@ -128,7 +128,7 @@ namespace Tensorflow
             IntPtr c_op;
             while ((c_op = c_api.TF_GraphNextOperation(graph, ref pos)) != IntPtr.Zero)
             {
-                yield return c_op;
+                yield return new Operation(c_op, graph);
             }
         }
     }

src/TensorFlowNET.Core/Graphs/Graph.Operation.cs

@@ -38,6 +38,31 @@ namespace Tensorflow
             return c_api.TF_NewOperation(_handle, opType, opName);
         }
 
+        public unsafe Operation[] ReturnOperations(IntPtr results)
+        {
+            TF_Operation return_oper_handle = new TF_Operation();
+            int num_return_opers = 0;
+            c_api.TF_ImportGraphDefResultsReturnOperations(results, ref num_return_opers, ref return_oper_handle);
+            Operation[] return_opers = new Operation[num_return_opers];
+            for (int i = 0; i < num_return_opers; i++)
+            {
+                var handle = return_oper_handle.node + Marshal.SizeOf<TF_Operation>() * i;
+                return_opers[i] = new Operation(*(IntPtr*)handle);
+            }
+
+            return return_opers;
+        }
+
+        public Operation OperationByName(string operName)
+        {
+            return c_api.TF_GraphOperationByName(_handle, operName);
+        }
+
+        public ITensorOrOperation[] get_operations()
+        {
+            return _nodes_by_name.Values.Select(x => x).ToArray();
+        }
+
         /// <summary>
         /// Returns the `Operation` with the given `name`.
         /// 

src/TensorFlowNET.Core/Graphs/Graph.cs

@@ -15,6 +15,7 @@
 ******************************************************************************/
 
 using System;
+using System.Collections;
 using System.Collections.Generic;
 using System.Linq;
 using System.Runtime.InteropServices;
@@ -72,7 +73,7 @@ namespace Tensorflow
         all variables that are created during the construction of a graph. The caller
         may define additional collections by specifying a new name.     
      */
-    public partial class Graph : IPython, IDisposable
+    public partial class Graph : IPython, IDisposable, IEnumerable<Operation>
     {
         private IntPtr _handle;
         private Dictionary<int, ITensorOrOperation> _nodes_by_id;
@@ -121,6 +122,10 @@ namespace Tensorflow
             _nodes_by_name = new Dictionary<string, ITensorOrOperation>();
             _names_in_use = new Dictionary<string, int>();
             _graph_key = $"grap-key-{ops.uid()}/";
+        }
+
+        public void __enter__()
+        {
         }
 
         public ITensorOrOperation as_graph_element(object obj, bool allow_tensor = true, bool allow_operation = true)
@@ -409,31 +414,6 @@ namespace Tensorflow
             return return_outputs;
         }
 
-        public unsafe Operation[] ReturnOperations(IntPtr results)
-        {
-            TF_Operation return_oper_handle = new TF_Operation();
-            int num_return_opers = 0;
-            c_api.TF_ImportGraphDefResultsReturnOperations(results, ref num_return_opers, ref return_oper_handle);
-            Operation[] return_opers = new Operation[num_return_opers];
-            for (int i = 0; i < num_return_opers; i++)
-            {
-                var handle = return_oper_handle.node + Marshal.SizeOf<TF_Operation>() * i;
-                return_opers[i] = new Operation(*(IntPtr*)handle);
-            }
-            
-            return return_opers;
-        }
-
-        public Operation OperationByName(string operName)
-        {
-            return c_api.TF_GraphOperationByName(_handle, operName);
-        }
-
-        public ITensorOrOperation[] get_operations()
-        {
-            return _nodes_by_name.Values.Select(x => x).ToArray();
-        }
-
         public string[] get_all_collection_keys()
         {
             return _collections.Keys.Where(x => !x.StartsWith("__")).ToArray();
@@ -481,17 +461,46 @@ namespace Tensorflow
         public Tensor get_tensor_by_name(string name)
         {
             return (Tensor)this.as_graph_element(name, allow_tensor: true, allow_operation: false);
-        }
-
-        public void __enter__()
-        {
+        }
+
+        public TensorShape GetTensorShape(TF_Output output)
+        {
+            var status = new Status();
+            var ndim = c_api.TF_GraphGetTensorNumDims(_handle, output, status);
+            status.Check();
+
+            if (ndim == -1)
+                return new TensorShape();
+
+            var dims = new long[ndim];
+            c_api.TF_GraphGetTensorShape(_handle, output, dims, dims.Length, status);
+            status.Check();
+
+            return new TensorShape(dims.Select(x => (int)x).ToArray());
+        }
+
+        public override string ToString()
+        {
+            int len = 0;
+            return c_api.TF_GraphDebugString(_handle, out len);
         }
 
         public void __exit__()
         {
 
-        }
+        }
+
+        private IEnumerable<Operation> GetEnumerable()
+            => c_api_util.tf_operations(this);
 
+        IEnumerator<Operation> IEnumerable<Operation>.GetEnumerator()
+            => GetEnumerable().GetEnumerator();
+
+        IEnumerator IEnumerable.GetEnumerator()
+        {
+            throw new NotImplementedException();
+        }
+
         public static implicit operator IntPtr(Graph graph)
         {
             return graph._handle;

src/TensorFlowNET.Core/Graphs/c_api.graph.cs

@@ -43,6 +43,9 @@ namespace Tensorflow
         [DllImport(TensorFlowLibName)]
         public static extern void TF_DeleteImportGraphDefResults(IntPtr results);
 
+        [DllImport(TensorFlowLibName)]
+        public static extern string TF_GraphDebugString(IntPtr graph, out int len);
+
         [DllImport(TensorFlowLibName)]
         public static extern void TF_GraphGetOpDef(IntPtr graph, string op_name, IntPtr output_op_def, IntPtr status);
 
@@ -100,6 +103,7 @@ namespace Tensorflow
         /// <param name="status">TF_Status*</param>
         [DllImport(TensorFlowLibName)]
         public static extern void TF_GraphImportGraphDef(IntPtr graph, IntPtr graph_def, IntPtr options, IntPtr status);
+        
         /// <summary>
         /// Iterate through the operations of a graph.
         /// </summary>

src/TensorFlowNET.Core/Operations/Operation.Implicit.cs

@@ -23,7 +23,10 @@ namespace Tensorflow
     /// </summary>
     public partial class Operation
     {
-        public static implicit operator Operation(IntPtr handle) => new Operation(handle);
+        // make sure the new op is in the same graph instance
+        public static implicit operator Operation(IntPtr handle) 
+            => new Operation(handle);
+
         public static implicit operator IntPtr(Operation op) => op._handle;
         public static implicit operator Tensor(Operation op) => op.output;
 

src/TensorFlowNET.Core/Operations/Operation.Output.cs

@@ -35,6 +35,8 @@ namespace Tensorflow
 
         public int OutputNumConsumers(int index) => c_api.TF_OperationOutputNumConsumers(new TF_Output(_handle, index));
 
+        public TF_Output this[int index] => _tf_output(index);
+
         public unsafe TF_Input[] OutputConsumers(int index, int max_consumers)
         {
             int size = Marshal.SizeOf<TF_Input>();

src/TensorFlowNET.Core/Sessions/Session.cs

@@ -15,6 +15,7 @@
 ******************************************************************************/
 
 using System;
+using System.Runtime.InteropServices;
 
 namespace Tensorflow
 {
@@ -26,8 +27,8 @@ namespace Tensorflow
 
         }
 
-        public Session(IntPtr handle)
-            : base("", null, null)
+        public Session(IntPtr handle, Graph g = null)
+            : base("", g, null)
         {
             _session = handle;
         }
@@ -50,8 +51,10 @@ namespace Tensorflow
             var graph = c_api.TF_NewGraph();
             var status = new Status();
             var opt = c_api.TF_NewSessionOptions();
+
             var tags = new string[] { "serve" };
             var buffer = new TF_Buffer();
+
             var sess = c_api.TF_LoadSessionFromSavedModel(opt,
                 IntPtr.Zero,
                 path,
@@ -61,14 +64,13 @@ namespace Tensorflow
                 ref buffer,
                 status);
 
-            //var bytes = new Buffer(buffer.data).Data;
-            //var meta_graph = MetaGraphDef.Parser.ParseFrom(bytes);
-
+            // load graph bytes
+            // var data = new byte[buffer.length];
+            // Marshal.Copy(buffer.data, data, 0, (int)buffer.length);
+            // var meta_graph = MetaGraphDef.Parser.ParseFrom(data);*/
             status.Check();
 
-            new Graph(graph).as_default();
-
-            return sess;
+            return new Session(sess, g: new Graph(graph).as_default());
         }
 
         public static implicit operator IntPtr(Session session) => session._session;

test/TensorFlowNET.Examples/BasicModels/LogisticRegression.cs

@@ -118,7 +118,7 @@ namespace TensorFlowNET.Examples
                 float acc = accuracy.eval(new FeedItem(x, mnist.Test.Data), new FeedItem(y, mnist.Test.Labels));
                 print($"Accuracy: {acc.ToString("F4")}");
 
-                return acc > 0.88;
+                return acc > 0.9;
             });
         }