VB.NET - CType Use - Dot Net Perls

CType. In VB.NET we have many ways to convert one type to another. The CType operator converts a local variable to another type, and only succeeds if the types are compatible.

Similar in ways to DirectCast and TryCast, CType is versatile and can handle both reference and value types. Unlike TryCast, CType can cause an exception, and this can cause issues.

TryCast

CStr

CInt

Reference types. For testing CType and other conversion approaches, we introduce an Example class. This class (like all classes) has Object as its ultimate base class.

Object

Part 1 Suppose have an Object reference and want to cast it to a more derived type. CType is successful here.

Part 2 With reference types (like the Example class) DirectCast can be used in the same way as CType.

Part 3 An implicit cast (where no casting syntax is used, and we just assign to a different type) has the same effect.

Part 4 TryCast allows us to cast without causing exceptions. A Nothing value is returned if the cast does not succeed.

Nothing

Class Example Public Dim _name As String = "abc" End Class Module Module1 Sub Main() Dim o As Object = New Example() ' Part 1: cast Object to Example with CType. Dim s As Example = CType(o, Example) Console.WriteLine(s._name) ' Part 2: use DirectCast to cast. Dim s2 As Example = DirectCast(o, Example) Console.WriteLine(s2._name) ' Part 3: use implicit cast expression. Dim s3 As Example = o Console.WriteLine(s3._name) ' Part 4: use TryCast for exception-safe casting. Dim s4 As Example = TryCast(o, Example) If Not IsNothing(s4) Console.WriteLine(s4._name) End If End Sub End Module

abc abc abc abc

Value types. With CType, we can also convert value types, which are specified as Structures. Types like Integer and Short are value types.

Structure

Part 1 CType allows us to cast value types like Integer and Short. This can cause an exception.

Part 2 An implicit cast can be used instead of the CType operator (and this can also cause exceptions).

Part 3 When a numeric value cannot be represented in a smaller type, an OverflowException is thrown.

Module Module1 Sub Main() ' Part 1: use CType to cast Integer to Short. Dim value As Integer = 100 Dim v As Short = CType(value, Short) Console.WriteLine(v) ' Part 2: use implicit cast instead of CType. Dim v2 As Short = value Console.WriteLine(v2) ' Part 3: use CType when the numeric cast fails, which causes an exception. Dim value2 As Integer = Integer.MaxValue Dim v3 As Short = CType(value2, Short) End Sub End Module

100 100 Unhandled exception. System.OverflowException: Arithmetic operation resulted in an overflow. at vbtest.Module1.Main() in C:\Users\...\Program.vb:line 14

Summary. It is possible to use CType to cast from one compatible type to another. But often, using an implicit cast has the same effect (and may be easier to understand).