Implemented CSHARP-424. Circular references now throw a BsonSerializationException which can be caught instead of a StackOverflowException. Default maximum serialization depth is 100, but can be set higher if required.

Author: Robert Stam

Bson/BsonDefaults.cs

@@ -28,6 +28,7 @@ public static class BsonDefaults
         // private static fields
         private static GuidRepresentation __guidRepresentation = GuidRepresentation.CSharpLegacy;
         private static int __maxDocumentSize = 4 * 1024 * 1024; // 4MiB
+        private static int __maxSerializationDepth = 100;
 
         // public static properties
         /// <summary>
@@ -49,5 +50,14 @@ public static int MaxDocumentSize
             get { return __maxDocumentSize; }
             set { __maxDocumentSize = value; }
         }
+
+        /// <summary>
+        /// Gets or sets the default max serialization depth (used to detect circular references during serialization). The default is 100.
+        /// </summary>
+        public static int MaxSerializationDepth
+        {
+            get { return __maxSerializationDepth; }
+            set { __maxSerializationDepth = value; }
+        }
     }
 }

Bson/IO/BsonBinaryWriter.cs

@@ -254,6 +254,7 @@ public override void WriteEndArray()
                 ThrowInvalidContextType("WriteEndArray", _context.ContextType, ContextType.Array);
             }
 
+            base.WriteEndArray();
             _buffer.WriteByte(0);
             BackpatchSize(); // size of document
 
@@ -276,6 +277,7 @@ public override void WriteEndDocument()
                 ThrowInvalidContextType("WriteEndDocument", _context.ContextType, ContextType.Document, ContextType.ScopeDocument);
             }
 
+            base.WriteEndDocument();
             _buffer.WriteByte(0);
             BackpatchSize(); // size of document
 
@@ -478,6 +480,7 @@ public override void WriteStartArray()
                 ThrowInvalidState("WriteStartArray", BsonWriterState.Value);
             }
 
+            base.WriteStartArray();
             _buffer.WriteByte((byte)BsonType.Array);
             WriteNameHelper();
             _context = new BsonBinaryWriterContext(_context, ContextType.Array, _buffer.Position);
@@ -497,6 +500,7 @@ public override void WriteStartDocument()
                 ThrowInvalidState("WriteStartDocument", BsonWriterState.Initial, BsonWriterState.Value, BsonWriterState.ScopeDocument, BsonWriterState.Done);
             }
 
+            base.WriteStartDocument();
             if (State == BsonWriterState.Value)
             {
                 _buffer.WriteByte((byte)BsonType.Document);

Bson/IO/BsonDocumentWriter.cs

@@ -161,6 +161,7 @@ public override void WriteEndArray()
                 ThrowInvalidContextType("WriteEndArray", _context.ContextType, ContextType.Array);
             }
 
+            base.WriteEndArray();
             var array = _context.Array;
             _context = _context.ParentContext;
             WriteValue(array);
@@ -182,6 +183,7 @@ public override void WriteEndDocument()
                 ThrowInvalidContextType("WriteEndDocument", _context.ContextType, ContextType.Document, ContextType.ScopeDocument);
             }
 
+            base.WriteEndDocument();
             if (_context.ContextType == ContextType.ScopeDocument)
             {
                 var scope = _context.Document;
@@ -376,6 +378,7 @@ public override void WriteStartArray()
                 ThrowInvalidState("WriteStartArray", BsonWriterState.Value);
             }
 
+            base.WriteStartArray();
             _context = new BsonDocumentWriterContext(_context, ContextType.Array, new BsonArray());
             State = BsonWriterState.Value;
         }
@@ -391,6 +394,7 @@ public override void WriteStartDocument()
                 ThrowInvalidState("WriteStartDocument", BsonWriterState.Initial, BsonWriterState.Value, BsonWriterState.ScopeDocument, BsonWriterState.Done);
             }
 
+            base.WriteStartDocument();
             switch (State)
             {
                 case BsonWriterState.Initial:

Bson/IO/BsonWriter.cs

@@ -35,6 +35,7 @@ public abstract class BsonWriter : IDisposable
         private string _name;
         private bool _checkElementNames;
         private bool _checkUpdateDocument;
+        private int _serializationDepth;
 
         // constructors
         /// <summary>
@@ -66,6 +67,14 @@ public bool CheckUpdateDocument
             set { _checkUpdateDocument = value; }
         }
 
+        /// <summary>
+        /// Gets the current serialization depth.
+        /// </summary>
+        public int SerializationDepth
+        {
+            get { return _serializationDepth; }
+        }
+
         /// <summary>
         /// Gets the settings of the writer.
         /// </summary>
@@ -324,12 +333,18 @@ public void WriteDouble(string name, double value)
         /// <summary>
         /// Writes the end of a BSON array to the writer.
         /// </summary>
-        public abstract void WriteEndArray();
+        public virtual void WriteEndArray()
+        {
+            _serializationDepth--;
+        }
 
         /// <summary>
         /// Writes the end of a BSON document to the writer.
         /// </summary>
-        public abstract void WriteEndDocument();
+        public virtual void WriteEndDocument()
+        {
+            _serializationDepth--;
+        }
 
         /// <summary>
         /// Writes a BSON Int32 to the writer.
@@ -506,7 +521,14 @@ public void WriteRegularExpression(string name, string pattern, string options)
         /// <summary>
         /// Writes the start of a BSON array to the writer.
         /// </summary>
-        public abstract void WriteStartArray();
+        public virtual void WriteStartArray()
+        {
+            _serializationDepth++;
+            if (_serializationDepth > _settings.MaxSerializationDepth)
+            {
+                throw new BsonSerializationException("Maximum serialization depth exceeded (does the object being serialized have a circular reference?).");
+            }
+        }
 
         /// <summary>
         /// Writes the start of a BSON array element to the writer.
@@ -521,7 +543,14 @@ public void WriteStartArray(string name)
         /// <summary>
         /// Writes the start of a BSON document to the writer.
         /// </summary>
-        public abstract void WriteStartDocument();
+        public virtual void WriteStartDocument()
+        {
+            _serializationDepth++;
+            if (_serializationDepth > _settings.MaxSerializationDepth)
+            {
+                throw new BsonSerializationException("Maximum serialization depth exceeded (does the object being serialized have a circular reference?).");
+            }
+        }
 
         /// <summary>
         /// Writes the start of a BSON document element to the writer.

Bson/IO/BsonWriterSettings.cs

@@ -29,6 +29,7 @@ public abstract class BsonWriterSettings
         // private fields
         private GuidRepresentation _guidRepresentation = BsonDefaults.GuidRepresentation;
         private bool _isFrozen;
+        private int _maxSerializationDepth = BsonDefaults.MaxSerializationDepth;
 
         // constructors
         /// <summary>
@@ -69,6 +70,19 @@ public bool IsFrozen
             get { return _isFrozen; }
         }
 
+        /// <summary>
+        /// Gets or sets the max serialization depth allowed (used to detect circular references).
+        /// </summary>
+        public int MaxSerializationDepth
+        {
+            get { return _maxSerializationDepth; }
+            set
+            {
+                if (_isFrozen) { ThrowFrozenException(); }
+                _maxSerializationDepth = value;
+            }
+        }
+
         // public methods
         /// <summary>
         /// Creates a clone of the settings.

Bson/IO/JsonWriter.cs

@@ -271,6 +271,7 @@ public override void WriteEndArray()
                 ThrowInvalidState("WriteEndArray", BsonWriterState.Value);
             }
 
+            base.WriteEndArray();
             _textWriter.Write("]");
 
             _context = _context.ParentContext;
@@ -288,6 +289,7 @@ public override void WriteEndDocument()
                 ThrowInvalidState("WriteEndDocument", BsonWriterState.Name);
             }
 
+            base.WriteEndDocument();
             if (_jsonWriterSettings.Indent && _context.HasElements)
             {
                 _textWriter.Write(_jsonWriterSettings.NewLineChars);
@@ -559,6 +561,7 @@ public override void WriteStartArray()
                 ThrowInvalidState("WriteStartArray", BsonWriterState.Value, BsonWriterState.Initial);
             }
 
+            base.WriteStartArray();
             WriteNameHelper(Name);
             _textWriter.Write("[");
 
@@ -577,6 +580,7 @@ public override void WriteStartDocument()
                 ThrowInvalidState("WriteStartDocument", BsonWriterState.Value, BsonWriterState.Initial, BsonWriterState.ScopeDocument);
             }
 
+            base.WriteStartDocument();
             if (State == BsonWriterState.Value || State == BsonWriterState.ScopeDocument)
             {
                 WriteNameHelper(Name);

BsonUnitTests/BsonUnitTests.csproj

@@ -95,6 +95,7 @@
     <Compile Include="DefaultSerializer\SerializeFlagsTests.cs" />
     <Compile Include="DefaultSerializer\Serializers\AnimalHierarchyWithAttributesTests.cs" />
     <Compile Include="DefaultSerializer\Serializers\AnimalHierarchyWithoutAttributesTests.cs" />
+    <Compile Include="DefaultSerializer\Serializers\CircularReferencesTests.cs" />
     <Compile Include="DefaultSerializer\Serializers\JaggedArraySerializerTests.cs" />
     <Compile Include="DefaultSerializer\Serializers\ObjectSerializerTests.cs" />
     <Compile Include="DefaultSerializer\Serializers\ThreeDimensionalArraySerializerTests.cs" />

BsonUnitTests/DefaultSerializer/Serializers/CircularReferencesTests.cs

@@ -0,0 +1,94 @@
+/* Copyright 2010-2012 10gen Inc.
+*
+* Licensed under the Apache License, Version 2.0 (the "License");
+* you may not use this file except in compliance with the License.
+* You may obtain a copy of the License at
+*
+* http://www.apache.org/licenses/LICENSE-2.0
+*
+* Unless required by applicable law or agreed to in writing, software
+* distributed under the License is distributed on an "AS IS" BASIS,
+* WITHOUT WARRANTIES OR CONDITIONS OF ANY KIND, either express or implied.
+* See the License for the specific language governing permissions and
+* limitations under the License.
+*/
+
+using System;
+using System.Collections.Generic;
+using System.IO;
+using System.Linq;
+using System.Text;
+using NUnit.Framework;
+
+using MongoDB.Bson;
+using MongoDB.Bson.IO;
+using MongoDB.Bson.Serialization;
+
+namespace MongoDB.BsonUnitTests.Serialization
+{
+    [TestFixture]
+    public class CircularReferencesTests
+    {
+        public class C
+        {
+            public int X { get; set; }
+            public C NestedDocument { get; set; }
+            public BsonArray BsonArray { get; set; }
+        }
+
+        [Test]
+        public void TestCircularBsonArray()
+        {
+            // note: setting a breakpoint in this method will crash the debugger if the locals window is open
+            // because it tries to display the value of array (presumably it's getting an internal stack overflow)
+            var array = new BsonArray();
+            array.Add(array);
+            var c1 = new C { X = 1, BsonArray = array };
+            Assert.Throws<BsonSerializationException>(() => { var json = c1.ToBson(); });
+            Assert.Throws<BsonSerializationException>(() => { var json = c1.ToBsonDocument(); });
+            Assert.Throws<BsonSerializationException>(() => { var json = c1.ToJson(); });
+        }
+
+        [Test]
+        public void TestCircularDocument()
+        {
+            var c1 = new C { X = 1 };
+            c1.NestedDocument = c1;
+            Assert.Throws<BsonSerializationException>(() => { var json = c1.ToBson(); });
+            Assert.Throws<BsonSerializationException>(() => { var json = c1.ToBsonDocument(); });
+            Assert.Throws<BsonSerializationException>(() => { var json = c1.ToJson(); });
+        }
+
+        [Test]
+        public void TestNoCircularReference()
+        {
+            var c2 = new C { X = 2 };
+            var c1 = new C { X = 1, NestedDocument = c2 };
+
+            var json = c1.ToJson();
+            var expected = "{ 'X' : 1, 'NestedDocument' : { 'X' : 2, 'NestedDocument' : null, 'BsonArray' : null }, 'BsonArray' : null }".Replace("'", "\"");
+            Assert.AreEqual(expected, json);
+
+            var memoryWriter = new MemoryStream();
+            using (var writer = BsonWriter.Create(memoryWriter))
+            {
+                BsonSerializer.Serialize(writer, c1);
+                Assert.AreEqual(0, writer.SerializationDepth);
+            }
+
+            var document = new BsonDocument();
+            using (var writer = BsonWriter.Create(document))
+            {
+                BsonSerializer.Serialize(writer, c1);
+                Assert.AreEqual(0, writer.SerializationDepth);
+            }
+
+            var stringWriter = new StringWriter();
+            using (var writer = BsonWriter.Create(stringWriter))
+            {
+                BsonSerializer.Serialize(writer, c1);
+                Assert.AreEqual(0, writer.SerializationDepth);
+            }
+        }
+    }
+}