Is creating a C# generic method that accepts (nullable) value type and reference type possible?

Question

I want to create a simple method that accepts both value type and reference type parameters, i.e. int is value, and string is reference.

So this is what I start with:

public bool areBothNotNull<T>(T? p1, T? p2)
{
    return (p1.HasValue && p2.HasValue);
}

So I want to be able to use it like this:

var r1 = areBothNotNull<int>(3, 4); // will be true
var r2 = areBothNotNull<int>(3, null); // will be false
var r3 = areBothNotNull<string>("three", "four"); // will be true
var r4 = areBothNotNull<string>(null, "four"); // will be false

But the first issue I encounter is

The type 'T' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method 'System.Nullable'

To proceed I add a struct constraint to my method

public bool areBothNotNull<T>(T? p1, T? p2) where T : struct

But now the method won't accept the string based calls, and gives me this error:

The type 'string' must be a non-nullable value type in order to use it as parameter 'T' in the generic type or method.

Is this possible? Or why are we prevented from doing this?

Answer 1

Your problem is that you want generic type constraints that are conflicting with each other:

• Nullable<T> works with value types only
• Reference types are not value types

So you will need to have two overloads for your code to work:

public static bool areBothNotNull<T>(T? p1, T? p2) where T : struct
{            
    return (p1.HasValue && p2.HasValue);
}

public static bool areBothNotNull<T>(T p1, T p2)
{
    return (p1 != null && p2 != null);
}

Still, the following line will never compile:

var r3 = areBothNotNull<string>(3, 4);

There is a conflict here, where the generic type argument states that the parameters are of type string, but the code tries to pass ints instead.

Answer 2

If you want to achieve this, you need to use non nullable types in your parameters areBothNotNull<T>(T p1, T p2) and check whether T is a Nullable<> in your implementation

If so, use IsNull otherwise, use object.ReferenceEquals (I presume you want to ignore value types like int...)