static void Main()

{ int index = 10; var update = Update<TestClass>.Set(x => x.List[index].Name, "Blah"); Console.WriteLine(update.ToString()); }

private class TestClass
{
public ObjectId Id

{ get; set; }

public List<SubTestClass> List { get; set; }

}

private class SubTestClass
{
public string Name

{ get; set; }

}