Casting Types in VB.Net and More

Source: Cheat Sheet - Casting in VB.NET and C#

Great little find, and since I’m a VB.Net whore, I’ll just post up the VB.Net version :)

Casting in VB.NET

  1. By default in VB, casting is automatically done for you when you assign objects to variables. The objects are then automatically casted to the variables' type.

    This behavior can be influenced by an option line on top of your code file:

    Option Strict OnOption Strict Off

    When on, casting is strict and not automatic.

  2. Explicit casting can be done with the cast operator CType() or DirectCast():

    textbox = CType(obj, TextBox)textbox = DirectCast(obj, TextBox)

    The difference between the two keywords is that CType succeeds as long as there is a valid conversion defined between the expression and the type, whereas DirectCast requires the run-time type of an object variable to be the same as the specified type. If the specified type and the run-time type of the expression are the same, however, the run-time performance of DirectCast is better than that of CType. DirectCast throws an InvalidCastException error if the argument types do not match.

  3. Testing if an object is of a particular type, can be done with the TypeOf...Is operator:

    If TypeOf obj Is TextBox Then...

  4. Obtaining a System.Type object for a given type can be done with the GetType operator:

    Dim t As System.Typet = GetType(String)MessageBox.Show(t.FullName)

  5. Obtaining a System.Type object for a given object can be done with the GetType method:

    Dim t as System.Typet = obj.GetType()MessageBox.Show(t.FullName)

Now to build on and add one more of my own…

Get the type based on an arbitrary string.


Problem with this though is you can not use it to cast using CType or DirectCast. Looking at the CType Function..

typename - Any expression that is legal within an As clause in a Dim statement, that is, the name of any data type, object, structure, class, or interface.

You can only use types that are compiled and can’t use types that show up at runtime. So trying to use CType(obj, System.Type.GetType(“System.String”)) will only get you an error.  Array bounds cannot appear in type specifiers.

How to get around this? I have no freaking idea, yet. I’m think I might could create a custom type of my own and do something so that it converts to the proper method but for now I’ll just catch a few ArgumentExceptions and try different cast types then.

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