Casting in .NET

13. September 2007 04:20 by Mrojas in General  //  Tags: , , ,   //   Comments (0)

.NET has a more strict typing than VB6
So you must check in some circumstances if your object implements an interface or not.

So I had used the as and is operators in C# but I did not know how to do that.
I did I little research and I discovered some things about casting operators for VB.NET


Operator Example Observations
CType Dim testNumber As Long = 1000
' The following line of code sets testNewType to 1000.0.
Dim testNewType As Single = CType(testNumber, Single)
Throws InvalidCastException or OverflowException

It could be less eficient due to VB.Net helper routines.

This is a Narrowing and Widening operator.

It can be overloaded

Public Structure digit
Private dig As Byte
    Public Sub New(ByVal b As Byte)
        If (b OrElse b > 9) Then Throw New _
            System.ArgumentException("Argument outside range for Byte")
        Me.dig = b
    End Sub
    Public Shared Widening Operator CType(ByVal d As digit) As Byte
        Return d.dig
    End Operator
    Public Shared Narrowing Operator CType(ByVal b As Byte) As digit
        Return New digit(b)
    End Operator
End Structure


DirectCast Dim f As New System.Windows.Forms.Form
Dim c As System.Windows.Forms.Control
' The following conversion succeeds.
c = DirectCast(f, System.Windows.Forms.Control)
Throws InvalidCastException. Is more efficient than CType because it does not depend on the Visual Basic helper runtime functions. It can even detect some errors as invalid casts during compile time

However it requires a relationship of inheritance of implementation
For example:

Dim q As Object = 2.37
Dim i As Integer = CType(q, Integer)
' The following conversion fails at run time
Dim j As Integer = DirectCast(q, Integer)

The run-time type of q is Double. CType succeeds because Double can be converted to Integer. However, the first DirectCast fails at run time because the run-time type of Double has no inheritance relationship with Integer, even though a conversion exists

TryCast     Dim obj As MyType = TryCast(obj, MyType)
    If obj Is Nothing Then
      ' Object could not be cast
     ' Object was casted

   End If
Throws no exceptions.

All this information has been taken from the MSDN site. This is just a quick summary. For more information see:

Type Conversion Functions
Conversion Functions (Visual Basic)

Widening and Narrowing Conversions
Implicit and Explicit Conversions