Long story short, if I call MongoCollection.Save<T>() on a class with
an IList<T> property it throws an exception. If I change it it List<T>
it does not.

Long Explanation where I take actual code, and scrub it:

So I have class that kinda looks like this:

[DataContract]
public class ChildClass : ParentClass
{
[BsonIgnoreIfNull]
[DataMember(Order = 1)]
[BsonKnownTypes(typeof(List<SomeClass>))]
public IList<SomeClass> SomeProperty

{ get; set; }
}


And then I do something like this:

var obj = new ChildClass{
SomeProperty = new List<SomeClass>();
};
_db["someCollection"].Save(obj, SafeMode.True);

Then I get the following
GenericArraySerializer cannot be used with type:
System.Collections.Generic.IList`1[[SomeClass, SomeAssembly
Version=1.0.0.0, Culture=neutral, PublicKeyToken=null]]
at MongoDB.Bson.DefaultSerializer.GenericArraySerializer.VerifyNominalType(Type
nominalType) in d:
justin's
projects\\mongo-csharp\\Bson\\DefaultSerializer\\Serializers
GenericArraySerializer.cs:line
113\r\n at MongoDB.Bson.DefaultSerializer.GenericArraySerializer.Serialize(BsonWriter
bsonWriter, Type nominalType, Object value, Boolean serializeIdFirst)
in d:
justin's
projects\\mongo-csharp\\Bson\\DefaultSerializer\\Serializers
GenericArraySerializer.cs:line
83\r\n at MongoDB.Bson.DefaultSerializer.BsonClassMapSerializer.SerializeMember(BsonWriter
bsonWriter, Object obj, BsonMemberMap memberMap) in d:
justin's
projects\\mongo-csharp\\Bson\\DefaultSerializer
BsonClassMapSerializer.cs:line
221\r\n at MongoDB.Bson.DefaultSerializer.BsonClassMapSerializer.Serialize(BsonWriter
bsonWriter, Type nominalType, Object value, Boolean serializeIdFirst)
in d:
justin's
projects\\mongo-csharp\\Bson\\DefaultSerializer
BsonClassMapSerializer.cs:line
181\r\n at MongoDB.Bson.Serialization.BsonSerializer.Serialize(BsonWriter
bsonWriter, Type nominalType, Object value, Boolean serializeIdFirst)
in d:
justin's
projects\\mongo-csharp\\Bson\\Serialization
BsonSerializer.cs:line
245\r\n at MongoDB.Bson.Serialization.BsonSerializer.Serialize(BsonWriter
bsonWriter, Type nominalType, Object value) in d:
justin's
projects\\mongo-csharp\\Bson\\Serialization
BsonSerializer.cs:line
229\r\n at MongoDB.Bson.DefaultSerializer.EnumerableSerializer`1.Serialize(BsonWriter
bsonWriter, Type nominalType, Object value, Boolean serializeIdFirst)
in d:
justin's
projects\\mongo-csharp\\Bson\\DefaultSerializer\\Serializers
CollectionSerializersGeneric.cs:line
89\r\n at MongoDB.Bson.DefaultSerializer.BsonClassMapSerializer.SerializeMember(BsonWriter
bsonWriter, Object obj, BsonMemberMap memberMap) in d:
justin's
projects\\mongo-csharp\\Bson\\DefaultSerializer
BsonClassMapSerializer.cs:line
221\r\n at MongoDB.Bson.DefaultSerializer.BsonClassMapSerializer.Serialize(BsonWriter
bsonWriter, Type nominalType, Object value, Boolean serializeIdFirst)
in d:
justin's
projects\\mongo-csharp\\Bson\\DefaultSerializer
BsonClassMapSerializer.cs:line
181\r\n at MongoDB.Bson.Serialization.BsonSerializer.Serialize(BsonWriter
bsonWriter, Type nominalType, Object value, Boolean serializeIdFirst)
in d:
justin's
projects\\mongo-csharp\\Bson\\Serialization
BsonSerializer.cs:line
245\r\n at MongoDB.Bson.Serialization.BsonSerializer.Serialize(BsonWriter
bsonWriter, Type nominalType, Object value) in d:
justin's
projects\\mongo-csharp\\Bson\\Serialization
BsonSerializer.cs:line
229\r\n at MongoDB.Bson.DefaultSerializer.EnumerableSerializer`1.Serialize(BsonWriter
bsonWriter, Type nominalType, Object value, Boolean serializeIdFirst)
in d:
justin's
projects\\mongo-csharp\\Bson\\DefaultSerializer\\Serializers
CollectionSerializersGeneric.cs:line
89\r\n at MongoDB.Bson.DefaultSerializer.BsonClassMapSerializer.SerializeMember(BsonWriter
bsonWriter, Object obj, BsonMemberMap memberMap) in d:
justin's
projects\\mongo-csharp\\Bson\\DefaultSerializer
BsonClassMapSerializer.cs:line
221\r\n at MongoDB.Bson.DefaultSerializer.BsonClassMapSerializer.Serialize(BsonWriter
bsonWriter, Type nominalType, Object value, Boolean serializeIdFirst)
in d:
justin's
projects\\mongo-csharp\\Bson\\DefaultSerializer
BsonClassMapSerializer.cs:line
181\r\n at MongoDB.Bson.Serialization.BsonSerializer.Serialize(BsonWriter
bsonWriter, Type nominalType, Object value, Boolean serializeIdFirst)
in d:
justin's
projects\\mongo-csharp\\Bson\\Serialization
BsonSerializer.cs:line
245\r\n at MongoDB.Bson.Serialization.BsonSerializer.Serialize[T](BsonWriter
bsonWriter, T value, Boolean serializeIdFirst) in d:
justin's
projects\\mongo-csharp\\Bson\\Serialization
BsonSerializer.cs:line
221\r\n at MongoDB.Driver.Internal.MongoUpdateMessage`2.WriteBody()
in d:
justin's
projects\\mongo-csharp\\Driver\\Internal
MongoUpdateMessage.cs:line
58\r\n at MongoDB.Driver.Internal.MongoRequestMessage.WriteToBuffer()
in d:
justin's
projects\\mongo-csharp\\Driver\\Internal
MongoRequestMessage.cs:line
81\r\n at MongoDB.Driver.Internal.MongoConnection.SendMessage(MongoRequestMessage
message, SafeMode safeMode) in d:
justin's
projects\\mongo-csharp\\Driver\\Internal
MongoConnection.cs:line
324\r\n at MongoDB.Driver.MongoCollection.Update[TQuery,TUpdate](TQuery
query, TUpdate update, UpdateFlags flags, SafeMode safeMode) in
d:
justin's projects\\mongo-csharp\\Driver\\Core
MongoCollection.cs:line
662\r\n at MongoDB.Driver.MongoCollection.Save[TDocument](TDocument
document, SafeMode safeMode) in d:
justin's
projects\\mongo-csharp\\Driver\\Core
MongoCollection.cs:line 584\r\n
at
... ... ... My Code .......


So the fix is as follows:

[DataContract]
public class ChildClass : ParentClass
{
[BsonIgnoreIfNull]
[DataMember(Order = 1)]
- [BsonKnownTypes(typeof(List<SomeClass>))]
- public IList<SomeClass> SomeProperty { get; set; }

+ public List<SomeClass> SomeProperty

{ get; set; }

}