Question

In VB.NET, why can’t I see a member of a structure when I make it a nullable type?

Example:

Public Structure myNullable  
    Dim myNullVar As Integer  
End Structure

Sub Main()  
    Dim myInstance As myNullable 'This works.  
    Dim myNullableInstance? As myNullable 'This works.   
    myInstance.myNullVar = 1  'This works.     
    myNullableInstance.myNullVar = 1   'This doesn't work.  
End Sub

Answer 1

As others have said, you need to use the Value property to fetch the value. However, System.Nullable<T> is immutable – the Value property is read-only. It will be returning a copy of the value, so even if you could change the field, it wouldn’t do what you want.

This is actually a good thing – value types should be immutable. Mutable structs are horrible – if the fact that Nullable<T> makes it hard to use your type pushes you down the immutability route, that’s great.

Make your field read-only, and add a constructor to let you pass in the value. Then instead of trying to modify the nullable value, assign an entirely new value to the variable.