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

var isn't a type. The actual type is figured out at compile-time. If you set DateTime.Now for a var, it will recognize as a DateTime type, not a Nullable, and that's why it doesn't compile.

var variables are also known as Implicitly Typed Local Variables (C# Programming Guide)

By the way, you can also create a generic extension method for nullable types:

public static T? GenericMethod(this T? source) where T : struct
{
    //Do something
}

and you can call whatever is nullable, declaring its type:

DateTime? dateTimeNullable = DateTime.Now;
dateTimeNullable.GenericMethod();

int? intNullable = 0;
intNullable.GenericMethod();