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

If you look at Nullable Struct's documentation you can see that it requires to be a struct:

public struct Nullable where T : struct

I believe you will need to constraint T to be a struct:

interface IA where T : struct
{
    T? Read(Guid id);
    // Or Nullable Read(Guid id);
}


class A : IA
{
    public int? Read(Guid id) { Console.WriteLine("A"); return 0; }
}

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BTW. Could you give us an example of what types you want to use this class with?

Why not just use where T: class and return T (or even not have a constraint at all)?

interface IA where T : class
{
    T Read(Guid id);
}