Why is there a difference in checking null against a value in VB.NET and C#?

Question

In VB.NET this happens:

Dim x As System.Nullable(Of Decimal) = Nothing
Dim y As System.Nullable(Of Decimal) = Nothing
         


        

Answer 1

Look at the generated CIL (I've converted both to C#):

C#:

private static void Main(string[] args)
{
    decimal? x = null;
    decimal? y = null;
    y = 5M;
    decimal? CS$0$0000 = x;
    decimal? CS$0$0001 = y;
    if ((CS$0$0000.GetValueOrDefault() != CS$0$0001.GetValueOrDefault()) ||
        (CS$0$0000.HasValue != CS$0$0001.HasValue))
    {
        Console.WriteLine("true");
    }
    else
    {
        Console.WriteLine("false");
    }
}

Visual Basic:

[STAThread]
public static void Main()
{
    decimal? x = null;
    decimal? y = null;
    y = 5M;
    bool? VB$LW$t_struct$S3 = new bool?(decimal.Compare(x.GetValueOrDefault(), y.GetValueOrDefault()) != 0);
    bool? VB$LW$t_struct$S1 = (x.HasValue & y.HasValue) ? VB$LW$t_struct$S3 : null;
    if (VB$LW$t_struct$S1.GetValueOrDefault())
    {
        Console.WriteLine("true");
    }
    else
    {
        Console.WriteLine("false");
    }
}

You'll see that the comparison in Visual Basic returns Nullable<bool> (not bool, false or true!). And undefined converted to bool is false.

Nothing compared to whatever is always Nothing, not false in Visual Basic (it is the same as in SQL).