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Posted 22 Sep 2003

Cheat Sheet - Casting in VB.NET and C#

, 22 Sep 2003 Public Domain
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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

GeneralToString Pin
George_Seremetidis28-Sep-04 13:40
memberGeorge_Seremetidis28-Sep-04 13:40 
GeneralDon't Forget System.Convert.ChangeType Pin
Shaun Hayward29-Jun-04 7:35
memberShaun Hayward29-Jun-04 7:35 
GeneralRe: Don't Forget System.Convert.ChangeType Pin
skjagini6-Sep-05 2:56
memberskjagini6-Sep-05 2:56 
JokeRe: Don't Forget System.Convert.ChangeType Pin
Artur M.7-Dec-06 21:45
memberArtur M.7-Dec-06 21:45 
GeneralProblems with casting objects Pin
simbakid3-Mar-04 3:04
membersimbakid3-Mar-04 3:04 
QuestionDifference between DirectCast and Ctype? Pin
FruitBatInShades26-Jan-04 5:36
memberFruitBatInShades26-Jan-04 5:36 
GeneralPlease add pointers to the next update! Pin
Anonymous4-Oct-03 13:20
sussAnonymous4-Oct-03 13:20 
GeneralOK, but could be better Pin
David Bennett30-Sep-03 4:23
sussDavid Bennett30-Sep-03 4:23 
GeneralInformative Pin
Nishant S23-Sep-03 20:14
editorNishant S23-Sep-03 20:14 
GeneralComments Pin
Nick Seng23-Sep-03 15:21
memberNick Seng23-Sep-03 15:21 
GeneralGood Pin
dog_spawn23-Sep-03 13:14
memberdog_spawn23-Sep-03 13:14 
GeneralRe: Good Pin
Furty23-Sep-03 13:36
memberFurty23-Sep-03 13:36 
GeneralRe: Good Pin
dog_spawn24-Sep-03 6:34
memberdog_spawn24-Sep-03 6:34 
GeneralRe: Good Pin
Rudi Breedenraedt25-Sep-03 5:22
memberRudi Breedenraedt25-Sep-03 5:22 
GeneralRe: Good Pin
dog_spawn25-Sep-03 6:52
memberdog_spawn25-Sep-03 6:52 
GeneralRe: Good Pin
peterchen29-Sep-03 6:29
memberpeterchen29-Sep-03 6:29 
GeneralRe: Good Pin
dog_spawn29-Sep-03 6:34
memberdog_spawn29-Sep-03 6:34 
GeneralRe: Good Pin
peterchen29-Sep-03 11:19
memberpeterchen29-Sep-03 11:19 
GeneralRe: Good Pin
dog_spawn29-Sep-03 11:23
memberdog_spawn29-Sep-03 11:23 
GeneralRe: Good Pin
peterchen29-Sep-03 12:26
memberpeterchen29-Sep-03 12:26 
GeneralRe: Good Pin
dog_spawn29-Sep-03 13:53
memberdog_spawn29-Sep-03 13:53 
GeneralRe: Good Pin
Rocky Moore23-Sep-03 14:14
memberRocky Moore23-Sep-03 14:14 
GeneralRe: Good Pin
Attila Hajdrik25-Sep-03 21:27
memberAttila Hajdrik25-Sep-03 21:27 
GeneralRe: Good Pin
rj4524-Apr-04 21:07
memberrj4524-Apr-04 21:07 
GeneralRe: Good Pin
Ngraham26-May-04 4:32
memberNgraham26-May-04 4:32 

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