extension method on type and nullable<type>

Question

For sake of simplicity, let's assume I want to write an extension method for the type int? and int:

public static class IntExtentions
{
    public static int AddOne(this int? number)
    {
        var dummy = 0;
        if (number != null)
            dummy = (int)number;

        return dummy.AddOne();
    }

    public static int AddOne(this int number)
    {
        return number + 1;
    }
}

Can this be done using only 1 method?

Answer 1

Unfortunately not. You can make the int? (or whichever nullable type you are using) method call the non nullable method very easily though, so you don't need to duplicate any logic with 2 methods - e.g.

public static class IntExtensions
{
    public static int AddOne(this int? number)
    {
        return (number ?? 0).AddOne();
    }

    public static int AddOne(this int number)
    {
        return number + 1;
    }
}

Answer 2

No you cannot. This can be verified experimentally by compiling the following code

public static class Example {
  public static int Test(this int? source) {
    return 42;
  }
  public void Main() {
    int v1 = 42;
    v1.Test();  // Does not compile
  }
}

You will need to write an extension method for each type (nullable and not nullable) if you want it used on both types.