Extension method resolution with nullable value type params

Question

public static class Extension
{
    public static void Test(this DateTime? dt)
    {
    }
}


void Main()
{
    var now = DateTime.Now;
    Extension.Test(now); //          


        

Answer 1

Because you wrote an extension for DateTime? and not for DateTime.

        DateTime? now = DateTime.Now;
        Extension.Test(now); // ok
        now.Test(); // no compile time error

or

        var now = new DateTime?(DateTime.Now);
        Extension.Test(now); // ok
        now.Test(); // no compile time error

will work.