Incorporated Brian Knight's fix for CSHARP-310 with minor changes.

Bson/Serialization/BsonClassMap.cs

@@ -878,7 +878,7 @@ namespace MongoDB.Bson.Serialization {
 
             var defaultValue = conventions.DefaultValueConvention.GetDefaultValue(memberInfo);
             if (defaultValue != null) {
-                memberMap.SetDefaultValue(defaultValue, memberMap.SerializeDefaultValue);
+                memberMap.SetDefaultValue(defaultValue);
             }
 
             // see if the class has a method called ShouldSerializeXyz where Xyz is the name of this member

Bson/Serialization/BsonMemberMap.cs

@@ -242,7 +242,9 @@ namespace MongoDB.Bson.Serialization {
         public BsonMemberMap SetDefaultValue(
             object defaultValue
         ) {
-            return SetDefaultValue(defaultValue, true); // serializeDefaultValue
+            this.defaultValue = defaultValue;
+            this.hasDefaultValue = true;
+            return this;
         }
 
         /// <summary>
@@ -255,9 +257,8 @@ namespace MongoDB.Bson.Serialization {
             object defaultValue,
             bool serializeDefaultValue
         ) {
-            this.hasDefaultValue = true;
-            this.serializeDefaultValue = serializeDefaultValue;
-            this.defaultValue = defaultValue;
+            SetDefaultValue(defaultValue);
+            SetSerializeDefaultValue(serializeDefaultValue);
             return this;
         }
 

BsonUnitTests/Jira/CSharp310Tests.cs

@@ -24,23 +24,20 @@ using MongoDB.Bson;
 using MongoDB.Bson.Serialization;
 using MongoDB.Bson.Serialization.Conventions;
 
-
 namespace MongoDB.BsonUnitTests.Jira {
     [TestFixture]
     public class CSharp310Tests {
         private class C {
             public int Id;
-            public Guid G;
+            public Guid G = Guid.Empty;
         }
 
         private class EmptyGuidDefaultValueConvention : IDefaultValueConvention {
             public object GetDefaultValue(MemberInfo memberInfo) {
-                var type = (memberInfo.MemberType == MemberTypes.Field)
-                    ? ((FieldInfo)memberInfo).FieldType : ((PropertyInfo)memberInfo).PropertyType;
+                var type = (memberInfo.MemberType == MemberTypes.Field) ? ((FieldInfo) memberInfo).FieldType : ((PropertyInfo) memberInfo).PropertyType;
                 if (type == typeof(Guid)) {
                     return Guid.Empty;
-                }
-                else {
+                } else {
                     return null;
                 }
             }