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Posted 22 Sep 2003

Cheat Sheet - Casting in VB.NET and C#

, 22 Sep 2003
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Describes several casting and type related operations in VB.NET and C#.


This article describes several casting and type related operations in VB.NET and C#.

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

    Option Strict On
    Option 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.Type
    t = GetType(String)
  5. Obtaining a System.Type object for a given object can be done with the GetType method:

    Dim t as System.Type
    t = obj.GetType()

Casting in C#

  1. C# is a strictly typed language. Whenever types don't match, casting is necessary.

    Regular casting in C# follows the C(++) and Java syntax:

    string s = (string)obj;

    The casting operator applies to the complete chain on the right of it, so in the following example, not a, but a.b is casted to a Form:

    Form f = (Form)a.b;

    To cast parts of the chain, use brackets. In the following example, obj is casted to a Form:

    string s = ((Form)obj).Text;
  2. C# knows an additional casting operator: as.

    The as operator is like a cast except that it yields null on conversion failure instead of raising an exception. In the following situation, btn gets the value null:

    Object obj = new TextBox();
    Button btn = obj as Button;
  3. Testing if an object is of a particular type, can be done with the is operator:

    if (obj is TextBox) {...}
  4. Obtaining a System.Type object for a given type can be done with the typeof operator:

    System.Type t;
    t = typeof(String);
  5. Obtaining a System.Type object for a given object can be done with the GetType method:

    System.Type t;
    t = obj.GetType();


This article, along with any associated source code and files, is licensed under A Public Domain dedication


About the Author

Rudi Breedenraedt
Architect RealDolmen
Belgium Belgium
Rudi is a Software Architect at RealDolmen.

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Comments and Discussions

GeneralPlease add pointers to the next update! Pin
Anonymous4-Oct-03 14:20
sussAnonymous4-Oct-03 14:20 
GeneralOK, but could be better Pin
David Bennett30-Sep-03 5:23
sussDavid Bennett30-Sep-03 5:23 
GeneralInformative Pin
Nishant S23-Sep-03 21:14
editorNishant S23-Sep-03 21:14 
GeneralComments Pin
Nick Seng23-Sep-03 16:21
memberNick Seng23-Sep-03 16:21 
GeneralGood Pin
dog_spawn23-Sep-03 14:14
memberdog_spawn23-Sep-03 14:14 
GeneralRe: Good Pin
Furty23-Sep-03 14:36
memberFurty23-Sep-03 14:36 
GeneralRe: Good Pin
dog_spawn24-Sep-03 7:34
memberdog_spawn24-Sep-03 7:34 
GeneralRe: Good Pin
Rudi Breedenraedt25-Sep-03 6:22
memberRudi Breedenraedt25-Sep-03 6:22 
First, thanks for the compliment!

Concerning your good OO design (btw, I don't know if C++ purists are by definition to be considered as OO purists, but please, let's not start that discussion here...D'Oh! | :doh: ):

Casting is in my opinion not a matter of design. Casting originates from the fact that the implementation is done in a strong-typed language. Would you implement in a purely object-oriented language as SmallTalk, then you would never have to cast, since it does no typechecking, and even does not know what casting is...

Now, should you design in such a way that casting will not be necessary in an implementation language ? My answer would again be no. Good OO designs are caracterised by their good distribution of responsibilities. Is it a good distribution of responsibility to have the superclass know about all it's (current but also future !?!) subclasses, since it should provide GetAsSubclass methods for each one of them ? I believe the contrary. By the way, as I pointed out, you can only provide GetAs methods for known (=current) subclasses, so you disallow extension by subclassing by 3rd parties. Wasn't OO meant to be extended that way ?

Consequently, if C# requires casting, it's not due to bad design (although design could have been better), but due to it's strong-typed character.

GeneralRe: Good Pin
dog_spawn25-Sep-03 7:52
memberdog_spawn25-Sep-03 7:52 
GeneralRe: Good Pin
peterchen29-Sep-03 7:29
memberpeterchen29-Sep-03 7:29 
GeneralRe: Good Pin
dog_spawn29-Sep-03 7:34
memberdog_spawn29-Sep-03 7:34 
GeneralRe: Good Pin
peterchen29-Sep-03 12:19
memberpeterchen29-Sep-03 12:19 
GeneralRe: Good Pin
dog_spawn29-Sep-03 12:23
memberdog_spawn29-Sep-03 12:23 
GeneralRe: Good Pin
peterchen29-Sep-03 13:26
memberpeterchen29-Sep-03 13:26 
GeneralRe: Good Pin
dog_spawn29-Sep-03 14:53
memberdog_spawn29-Sep-03 14:53 
GeneralRe: Good Pin
Rocky Moore23-Sep-03 15:14
memberRocky Moore23-Sep-03 15:14 
GeneralRe: Good Pin
Attila Hajdrik25-Sep-03 22:27
memberAttila Hajdrik25-Sep-03 22:27 
GeneralRe: Good Pin
rj4524-Apr-04 22:07
memberrj4524-Apr-04 22:07 
GeneralRe: Good Pin
Ngraham26-May-04 5:32
memberNgraham26-May-04 5:32 
GeneralRe: Good Pin
DBuckner26-May-04 19:57
memberDBuckner26-May-04 19:57 

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