The following works in LINQ2 but not in LINQ3:

 var find = collection
     .Find(x => x.Id == 1)
     .Project(x => new { R = MyFunction(x.X) }); // MyFunction cannot be translated to MQL

The following doesn't work with either LINQ provider:

var queryable = collection.AsQueryable()
    .Where(x => x.Id == 1)
    .Select(x => new { R = MyFunction(x.X) }); // MyFunction cannot be translated to MQL

When writing the new LINQ3 provider we made the deliberate decision to not support implicit client side projections for the following reasons:

• they only work on the final Select
• adding any additional stages after a final Select with implicit client side projections breaks the query
• they can hide inefficiency because more data than expected might be fetched to the client

We felt that client side projections should be explicit so that users writing and reading code know exactly what is happening server side and what is happening client side.

The above two examples can be refactored to use explicit client side projections as follows:

var find = collection
    .Find(x => x.Id == 1)
    .ToEnumerable() // execute the rest of this chain client side
    .Select(x => new { R = MyFunction(x.X) });

and

var enumerable = collection.AsQueryable()
    .Where(x => x.Id == 1)
    .AsEnumerable() // execute the rest of this chain client side
    .Select(x => new { R = MyFunction(x.X) });

While our decision to not support implicit client side projections is reasonable, nonetheless it is a breaking change from LINQ2 (at least with respect to `Find`). In addition, in spite of the potential inefficiencies that implicit client side projections might hide some users want the convenience of it anyway.