C# 'AS' equivalent for nullable types

[MattP]

C#'s 'as' keyword will let you do this:

int? input = value as int?

Here's what I would assume the vb.net equivalent would be:

Dim input As Integer? = TryCast(value, Integer?)

There's an intellisense error in the TryCast stating the operand must be a reference type but Integer? is a value type.

Intellisense on Nullable(Of Integer) says 'Represents an object whose underlying type is a value type that can also be assigned null like a reference type.'

It seems C#'s 'as' handles this like a reference type where TryCast doesn't have this built in.

In VB10 I was able to take advantage of the new CTypeDynamic function to do the casting. Conversion.CTypeDynamic Method (Microsoft.VisualBasic)

Dim input As Integer? = CTypeDynamic(Of Integer?)(value)

Or:

Dim input As Integer? = CTypeDynamic(Value, GetType(Integer?))

There's a cost here as CTypeDynamic examines the type at runtime.

My question is what is the elegant way to handle this without CTypeDynamic?

 

[JohnH]

DirectCast or CType.

Nullable(Of T) is a structure, ie a value type.

 

[MattP]

Yeah, was hoping there was a way to treat nullables like a reference type in this scenario like C# has.

There's a feedback thread on it here: TryCast should work with Value types and Nullable(Of T) | Microsoft Connect where there's a clunky workaround.

 

[JohnH]

I don't see how that is treating the value type as a reference type. You can to this:

input= CType(If(value, Nothing), Integer?)

but achieve nothing over a plain DirectCast/CType.

 

[JohnH]

Perhaps you can use a generic method like this:

Public Function TryCasts(Of T)(ByVal value As Object) As T
    Try            
        Return DirectCast(value, T)
    Catch
    End Try
    Return Nothing
End Function

Dim i = TryCasts(Of Integer?)(value)