c# - The method 'Where' cannot follow the method 'Select' or is not supported

Question

Why am I getting:

The method 'Where' cannot follow the method 'Select' or is not supported. Try writing the query in terms of supported methods or call the 'AsEnumerable' or 'ToList' method before calling unsupported methods.

...when using the WHERE clause, like when calling:

XrmServiceContext.CreateQuery<Contact>().Project().To<Person>().Where(p => p.FirstName == "John").First();

?

This works:

XrmServiceContext.CreateQuery<Contact>().Project().To<Person>().First();

Also this works:

XrmServiceContext.CreateQuery<Contact>().Where(p => p.FirstName == "John").First();

I'm using AutoMapper QueryableExtension.

Additional info:

• I don't want to call ToList() before the Where clause. I know it will works that way.
• CreateQuery<TEntity>() returns IQueryable<TEntity>.

Answer 1

It's because whatever query provider you are using isn't able to handle this. It's not invalid in the general case; in fact most query providers do support filtering after projecting. Certain query providers simply aren't as robust as others, or they are representing a query model that is less flexible/powerful than the LINQ interface (or both). As a result, LINQ operations that are correct from the C# compiler's point of view might still not be translatable by the query provider, so the best it can do is throw an exception at runtime.