public class C
{
    public int Id;
    public int X;
}

collection.Insert(new C { Id = 1, X = 1 });
collection.Insert(new C { Id = 2, X = 2 });

var count = collection.AsQueryable()
    .Select(a => a.X)
    .Distinct()
    .Count();

System.NotSupportedException: Distinct must be used with Select to identify the field whose distinct values are to be found.
   at MongoDB.Driver.Linq.SelectQuery.ExecuteDistinct(IMongoQuery query) in C:\work\rstam\mongo-csharp-driver\Driver\Linq\Translators\SelectQuery.cs:line 302
   at MongoDB.Driver.Linq.SelectQuery.Execute() in C:\work\rstam\mongo-csharp-driver\Driver\Linq\Translators\SelectQuery.cs:line 137
   at MongoDB.Driver.Linq.MongoQueryProvider.Execute(Expression expression) in C:\work\rstam\mongo-csharp-driver\Driver\Linq\MongoQueryProvider.cs:line 155
   at MongoDB.Driver.Linq.MongoQueryProvider.Execute[TResult](Expression expression) in C:\work\rstam\mongo-csharp-driver\Driver\Linq\MongoQueryProvider.cs:line 131
   at System.Linq.Queryable.Count[TSource](IQueryable`1 source)
   at ConsoleApplication1.Program.Main(String[] _args) in C:\Users\Robert Stam\documents\visual studio 2010\Projects\ConsoleApplication1\ConsoleApplication1\Pro
gram.cs:line 39