Receiving error about nullable type parameter even when parameter has notnull constraint

Question

I have a generic interface IDataAdapter; implementors of the interface should be able to read POCOs with Guid IDs from a data source.

Answer 1

I think this issue is very similar to what is happening in this post.

Note that a T? where T : class and a T? where T : struct are represented very differently in the CLR. The former is just the CLR type T. There are not separate types in the CLR to differentiate between T and T?. T? in C# just adds extra compile time checking by the C# compiler. On the other hand, The latter is represented by the CLR type Nullable.

So let's consider your method:

T? Read (Guid id);

How should this be represented in the CLR? What is the return type? The compiler don't know whether T is a reference type or a value type, so the compiler cannot decide whether the method signature should be:

T Read (Guid id);

or:

Nullable Read (Guid id);