Fix XmlSerialization for MongoSymbol.

source/MongoDB.Tests/UnitTests/TestMongoSymbol.cs

@@ -147,5 +147,18 @@ public void CanBeXmlSerialized()
 
             Assert.AreEqual(source, dest);
         }
+
+        [Test]
+        public void CanBeXmlSerializedWhenValueNull()
+        {
+            var source = new MongoSymbol(null);
+            var serializer = new XmlSerializer(typeof(MongoSymbol));
+
+            var writer = new StringWriter();
+            serializer.Serialize(writer, source);
+            var dest = (MongoSymbol)serializer.Deserialize(new StringReader(writer.ToString()));
+
+            Assert.AreEqual(source, dest);
+        }
     }
 }

source/MongoDB/MongoSymbol.cs

@@ -1,12 +1,15 @@
 using System;
+using System.Xml;
+using System.Xml.Schema;
+using System.Xml.Serialization;
 
 namespace MongoDB
 {
     /// <summary>
     ///   Type to hold an interned string that maps to the bson symbol type.
     /// </summary>
     [Serializable]
-    public struct MongoSymbol : IEquatable<MongoSymbol>, IEquatable<String>, IComparable<MongoSymbol>, IComparable<String>
+    public struct MongoSymbol : IEquatable<MongoSymbol>, IEquatable<String>, IComparable<MongoSymbol>, IComparable<String>, IXmlSerializable
     {
         /// <summary>
         /// Gets or sets the empty.
@@ -232,5 +235,38 @@ public MongoSymbol(string value)
         public override int GetHashCode(){
             return (Value != null ? Value.GetHashCode() : 0);
         }
+
+        /// <summary>
+        /// This method is reserved and should not be used. When implementing the IXmlSerializable interface, you should return null (Nothing in Visual Basic) from this method, and instead, if specifying a custom schema is required, apply the <see cref="T:System.Xml.Serialization.XmlSchemaProviderAttribute"/> to the class.
+        /// </summary>
+        /// <returns>
+        /// An <see cref="T:System.Xml.Schema.XmlSchema"/> that describes the XML representation of the object that is produced by the <see cref="M:System.Xml.Serialization.IXmlSerializable.WriteXml(System.Xml.XmlWriter)"/> method and consumed by the <see cref="M:System.Xml.Serialization.IXmlSerializable.ReadXml(System.Xml.XmlReader)"/> method.
+        /// </returns>
+        XmlSchema IXmlSerializable.GetSchema()
+        {
+            return null;
+        }
+
+        /// <summary>
+        /// Generates an object from its XML representation.
+        /// </summary>
+        /// <param name="reader">The <see cref="T:System.Xml.XmlReader"/> stream from which the object is deserialized.</param>
+        void IXmlSerializable.ReadXml(XmlReader reader)
+        {
+            if(reader.IsEmptyElement)
+                return;
+
+            Value = string.Intern(reader.ReadString());
+        }
+
+        /// <summary>
+        /// Converts an object into its XML representation.
+        /// </summary>
+        /// <param name="writer">The <see cref="T:System.Xml.XmlWriter"/> stream to which the object is serialized.</param>
+        void IXmlSerializable.WriteXml(XmlWriter writer)
+        {
+            if(Value != null)
+                writer.WriteString(Value);
+        }
     }
 }