remove global static Graph instance.

src/TensorFlowNET.Core/Graphs/DefaultGraphStack.cs

@@ -0,0 +1,40 @@
+using System;
+using System.Collections.Generic;
+using System.Linq;
+using System.Text;
+
+namespace Tensorflow
+{
+    public class DefaultGraphStack
+    {
+        List<StackModel> stack = new List<StackModel>();
+
+        public void set_controller(Graph @default)
+        {
+            if (!stack.Exists(x => x.Graph == @default))
+                stack.Add(new StackModel { Graph = @default, IsDefault = true });
+
+            foreach (var s in stack)
+                s.IsDefault = s.Graph == @default;
+        }
+
+        public Graph get_controller()
+        {
+            if (stack.Count == 0)
+                stack.Add(new StackModel { Graph = tf.Graph(), IsDefault = true });
+
+            return stack.First(x => x.IsDefault).Graph;
+        }
+
+        public void reset()
+        {
+            stack.Clear();
+        }
+    }
+
+    public class StackModel
+    {
+        public Graph Graph { get; set; }
+        public bool IsDefault { get; set; }
+    }
+}

src/TensorFlowNET.Core/Graphs/Graph.cs

@@ -87,7 +87,7 @@ namespace Tensorflow
         private Dictionary<string, object> _collections = new Dictionary<string, object>();
 
         public bool building_function;
-
+        
         public Graph()
         {
             _handle = c_api.TF_NewGraph();
@@ -113,7 +113,14 @@ namespace Tensorflow
             return _as_graph_element_locked(obj, allow_tensor, allow_operation);
         }
 
-        public Graph as_default() => ops.set_default_graph(this);
+        /// <summary>
+        /// Returns a context manager that makes this `Graph` the default graph.
+        /// </summary>
+        /// <returns></returns>
+        public Graph as_default()
+        {
+            return ops.set_default_graph(this);
+        }
 
         private Tensor _as_graph_element(object obj)
         {

src/TensorFlowNET.Core/Layers/Layer.cs

@@ -172,13 +172,12 @@ namespace Tensorflow.Layers
                 }
                 else
                 {
-                    with(tf.variable_scope(scope, default_name: _base_name),
-                        captured_scope =>
-                        {
-                            _scope = captured_scope;
-                        });
+                    with(tf.variable_scope(scope, default_name: _base_name), captured_scope =>
+                    {
+                        // convert variable_scope to VariableScope
+                        _scope = captured_scope;
+                    });
                 }
-
             }
         }
     }

src/TensorFlowNET.Core/Variables/variable_scope.py.cs

@@ -26,6 +26,7 @@ namespace Tensorflow
         private bool? _reuse;
         bool _in_graph_mode;
         protected Graph _graph;
+        bool _building_function;
 
         public variable_scope(string name, 
             string default_name = "",
@@ -70,6 +71,17 @@ namespace Tensorflow
 
         public void __enter__()
         {
+            // If the default graph is building a function, then we should not replace it
+            // with the cached graph.
+            if (ops.get_default_graph().building_function)
+                _building_function = true;
+            else
+                _building_function = false;
+            if (_in_graph_mode && !_building_function)
+            {
+                _graph.as_default();
+            }
+
             _scope = _enter_scope_uncached();
         }
 

src/TensorFlowNET.Core/ops.name_scope.cs

@@ -54,12 +54,13 @@ namespace Tensorflow
             public void Dispose()
             {
                 var g = get_default_graph();
-                // Console.WriteLine($"name_scope: {g._name_stack} -> {old_stack}");
                 g._name_stack = old_stack;
+                // Console.WriteLine($"name_scope: {g._name_stack} -> {old_stack}");
             }
 
             public void __exit__()
             {
+
             }
 
             /// <summary>

src/TensorFlowNET.Core/ops.py.cs

@@ -50,7 +50,8 @@ namespace Tensorflow
             return get_default_graph().get_collection_ref(key);
         }
 
-        private static Graph default_graph;
+        public static DefaultGraphStack default_graph_stack = new DefaultGraphStack();
+
         /// <summary>
         /// Returns the default graph for the current thread.
         /// 
@@ -68,15 +69,13 @@ namespace Tensorflow
         {
             //TODO: original source indicates there should be a _default_graph_stack!
             //return _default_graph_stack.get_default()
-            if (default_graph == null)
-                default_graph = tf.Graph();
-            return default_graph;
+            return default_graph_stack.get_controller();
         }
         public static Graph set_default_graph(Graph graph)
         {
             //TODO: original source does not have a 'set_default_graph' and indicates there should be a _default_graph_stack!
-            default_graph = graph;
-            return default_graph;
+            default_graph_stack.set_controller(graph);
+            return default_graph_stack.get_controller();
         }
 
         /// <summary>
@@ -96,10 +95,7 @@ namespace Tensorflow
             //    throw new InvalidOperationException("Do not use tf.reset_default_graph() to clear " +
             //                                    "nested graphs. If you need a cleared graph, " +
             //                                    "exit the nesting and create a new graph.");
-            //_default_graph_stack.reset();
-            if (default_graph!=null)
-                default_graph.Dispose();
-            default_graph = tf.Graph();
+            default_graph_stack.reset();
         }
 
         public static Graph _get_graph_from_inputs(params Tensor[] op_input_list)

test/TensorFlowNET.Examples/TextProcess/CnnTextClassification.cs

@@ -195,7 +195,7 @@ namespace TensorFlowNET.Examples
             return graph;
         }
 
-        private bool RunWithImportedGraph(Session sess, Graph graph)
+        private bool Train(Session sess, Graph graph)
         {
             var stopwatch = Stopwatch.StartNew();
 
@@ -274,8 +274,7 @@ namespace TensorFlowNET.Examples
         {
             var graph = IsImportingGraph ? ImportGraph() : BuildGraph();
 
-            return with(tf.Session(graph), sess 
-                => RunWithImportedGraph(sess, graph));
+            return with(tf.Session(graph), sess => Train(sess, graph));
         }
 
         public bool Predict()