Understanding C# generics and Nullable value type. Return null or nullable

Question

Suppose I have the following class:

public class GenericClass<T>
{
    public T Find()
    {
        //return T if found otherwise null or Nullable<T>
    }
}

Somewhere I'd like to specialize my class using T with a class, other times with a struct. I'm facing this issue: I can't return a Nullable<T> if T type isn't restricted to be a struct.

I would like to provide an implementation of my Find method that works if T is specialized with both a class or a struct. In case Find fails , I'd like to return null if T is a class otherwise Nullable<T>.

Is that possible without using reflection? If yes how?

Answer 1

You can return default(T).

For a class, this will be null. For any Nullable<T>, this will be a Nullable<T> without a value (effectively null).

That being said, if you use this with a struct and not a Nullable<T> as the type, default(T) will be the default value of the struct.

---

If you want to make this work uniformly for any class or struct, you would likely need to return two values - you could use the framework as inspiration here, and have a TryXXX method, ie:

public bool TryFind(out T)

You could then use default(T) when the value isn't found, and return false. This avoids the need for nullable types. You could also write this returning a Tuple<bool, T> or similar, if you wanted to avoid the out parameter, ie:

public Tuple<bool, T> Find()

A final option, potentially, would be to make your class non-generic, then use a pair of generic methods:

class YourClass // Non generic
{
    public T FindReference<T>() where T : class
    {
         // ...
        return null;
    }

    public Nullable<T> FindValue<T>() where T : struct
    {
         // ...
         return default(T);
    }
}

Note that you need distinct names, since you can't have an overloaded method purely based on the return type.