Implemented CSHARP-247. It is now possible to deserialize a document to an interface provided the document contains a discriminator indicating the actual type.

Bson/Serialization/BsonSerializer.cs

@@ -213,8 +213,20 @@ namespace MongoDB.Bson.Serialization {
                 return BsonDocument.ReadFrom(bsonReader);
             }
 
-            var serializer = LookupSerializer(nominalType);
-            return serializer.Deserialize(bsonReader, nominalType, options);
+            // if nominalType is an interface find out the actualType and use it instead
+            if (nominalType.IsInterface) {
+                var discriminatorConvention = BsonDefaultSerializer.LookupDiscriminatorConvention(nominalType);
+                var actualType = discriminatorConvention.GetActualType(bsonReader, nominalType);
+                if (actualType == nominalType) {
+                    var message = string.Format("Unable to determine actual type of object to deserialize. NominalType is the interface {0}.", nominalType);
+                    throw new FileFormatException(message);
+                }
+                var serializer = LookupSerializer(actualType);
+                return serializer.Deserialize(bsonReader, actualType, options);
+            } else {
+                var serializer = LookupSerializer(nominalType);
+                return serializer.Deserialize(bsonReader, nominalType, options);
+            }
         }
 
         /// <summary>

DriverOnlineTests/DriverOnlineTests.csproj

@@ -102,6 +102,7 @@
     <Compile Include="Jira\CSharp218Tests.cs" />
     <Compile Include="Jira\CSharp230Tests.cs" />
     <Compile Include="Jira\CSharp231Tests.cs" />
+    <Compile Include="Jira\CSharp247Tests.cs" />
     <Compile Include="Jira\CSharp77Tests.cs" />
     <Compile Include="Jira\CSharp92Tests.cs" />
     <Compile Include="Jira\CSharp93Tests.cs" />

DriverOnlineTests/Jira/CSharp247Tests.cs

@@ -0,0 +1,67 @@
+/* Copyright 2010-2011 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.Runtime.Serialization;
+using System.Text;
+using NUnit.Framework;
+
+using MongoDB.Bson;
+using MongoDB.Bson.Serialization.Attributes;
+using MongoDB.Driver;
+using MongoDB.Driver.Builders;
+
+namespace MongoDB.DriverOnlineTests.Jira.CSharp247 {
+    [TestFixture]
+    public class CSharp247Tests {
+        public interface I {
+            int X { get; set; }
+        }
+
+        public class C : I {
+            public ObjectId Id { get; set; }
+            public int X { get; set; }
+        } 
+
+        private MongoServer server;
+        private MongoDatabase database;
+        private MongoCollection<BsonDocument> collection;
+
+        [TestFixtureSetUp]
+        public void TestFixtureSetup() {
+            server = MongoServer.Create("mongodb://localhost/?safe=true");
+            database = server["onlinetests"];
+            collection = database.GetCollection("testcollection");
+        }
+
+        [Test]
+        public void TestDeserializeInterface() {
+            collection.RemoveAll();
+
+            var c = new C { X = 1 };
+            collection.Insert<I>(c);
+            var id = c.Id;
+
+            var i = collection.FindOneAs<I>();
+            Assert.IsInstanceOf<C>(i);
+            var r = (C) i;
+            Assert.AreEqual(id, r.Id);
+            Assert.AreEqual(1, r.X);
+        }
+    }
+}