Question

I’m trying to use the following code to check for a DBNull and set the variable to nothing if it is, or a short if it isn’t. The problem is it is failing to set the variable to Nothing and sets it to a 0 instead. Anybody know why?

variable = If(currentRow.Item("variable") Is DBNull.Value,
    Nothing, CShort(currentRow.Item("variable")))

Answer 1

If variable is declared As Short? then the code works with a slight tweak: you need to cast either operand of If to the target type first:

variable = If(condition, CType(Nothing, Short?), CShort(…))

(You could also have cast the third operand instead, or both.)

This cast is necessary because of how If deduces types: if the two result types mismatch, a common type is deduced which is the closest parent type, i.e. a type from which both inherit. However, with Nothing, new rules come into play because as far as VB is concerned, Nothing is already a valid Short – a default-initialised one (see old answer below for explanation). So VB doesn’t try any type coercion, it simply uses Short as the return value.

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Old answer below, assuming that OP had declared variable As Short:

You cannot set value types to Nothing. If you assign Nothing to a value type then it will be set to its type’s default value instead – which is 0 for Short.

You can test this easily:

Dim s as Short = Nothing
Console.WriteLine(s)

Setting a value type to Nothing is the same as invoking its default constructor (New Short()) or declaring a new variable of that type without initialising it. The corresponding operation in C# would be to assign default(T) (short s = default(short)).

If you want to represent null value types, you have to use nullable types:

Dim s as Short? = Nothing

Now s is of type Nullable<Short> (Short? is a shortcut of that) and can be assigned a proper Nothing.