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

Because x <> y returns Nothing instead of true. It is simply not defined since x is not defined. (similar to SQL null).

Note: VB.NET Nothing <> C# null.

You also have to compare the value of a Nullable(Of Decimal) only if it has a value.

So the VB.NET above compares similar to this(which looks less incorrect):

If x.HasValue AndAlso y.HasValue AndAlso x <> y Then
    Console.WriteLine("true")
Else
    Console.WriteLine("false")  
End If

The VB.NET language specification:

7.1.1 Nullable Value Types ... A nullable value type can contain the same values as the non-nullable version of the type as well as the null value. Thus, for a nullable value type, assigning Nothing to a variable of the type sets the value of the variable to the null value, not the zero value of the value type.

For example:

Dim x As Integer = Nothing
Dim y As Integer? = Nothing

Console.WriteLine(x) ' Prints zero '
Console.WriteLine(y) ' Prints nothing (because the value of y is the null value) '