By Sara Gamage

2010-10-23 07:56:25 8 Comments

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?


@Fredrik Mörk 2010-10-23 08:09:37

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.

@Andrew Bezzub 2010-10-23 08:44:20

I would also add where T : struct and where T : class constraints to methods.

@Sara Gamage 2010-10-23 10:44:00

Sorry it was a copy/paste mistake for line r3, I'll update the question, "three", "four" should be the parameters.

@Fredrik Mörk 2010-10-23 13:45:08

@Sara: regarding understanding Nullable<T>, I wrote a blog post about that a while ago:…

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