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Cheat Sheet - Casting in VB.NET and C#

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22 Sep 2003Public Domain2 min read 965.7K   94   42
Describes several casting and type related operations in VB.NET and C#.

Introduction

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:

    VB.NET
    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():

    VB.NET
    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:

    VB.NET
    If TypeOf obj Is TextBox Then...
  4. Obtaining a System.Type object for a given type can be done with the GetType operator:

    VB.NET
    Dim t As System.Type
    t = GetType(String)
    MessageBox.Show(t.FullName)
  5. Obtaining a System.Type object for a given object can be done with the GetType method:

    VB.NET
    Dim t as System.Type
    t = obj.GetType()
    MessageBox.Show(t.FullName)

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:

    C#
    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:

    C#
    Form f = (Form)a.b;

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

    C#
    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:

    C#
    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:

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

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

    C#
    System.Type t;
    t = obj.GetType();
    MessageBox.Show(t.FullName);

License

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


Written By
Architect AREBIS
Belgium Belgium
Senior Software Architect and independent consultant.

Comments and Discussions

 
JokeRe: Don't Forget System.Convert.ChangeType Pin
Artur M.7-Dec-06 21:45
Artur M.7-Dec-06 21:45 
GeneralProblems with casting objects Pin
simbakid3-Mar-04 3:04
simbakid3-Mar-04 3:04 
QuestionDifference between DirectCast and Ctype? Pin
FruitBatInShades26-Jan-04 5:36
FruitBatInShades26-Jan-04 5:36 
GeneralPlease add pointers to the next update! Pin
Anonymous4-Oct-03 13:20
Anonymous4-Oct-03 13:20 
GeneralOK, but could be better Pin
David Bennett30-Sep-03 4:23
David Bennett30-Sep-03 4:23 
GeneralInformative Pin
Nish Nishant23-Sep-03 20:14
sitebuilderNish Nishant23-Sep-03 20:14 
GeneralComments Pin
Nick Seng23-Sep-03 15:21
Nick Seng23-Sep-03 15:21 
GeneralGood Pin
dog_spawn23-Sep-03 13:14
dog_spawn23-Sep-03 13:14 
I liked the style of this article as you consider both C# and VB.NET. I learnt something new - I did not know about the as operator Smile | :)

Do any C++ purists agree with me when I say: casting is never needed in good OO design.

Even in an XML parser you don't need it. Consider a simple example: if you can always predict what nodes you might need to 'cast' into you can provide a function:

CommentElement Node::GetAsComment()

...where Node is the superclass of all Xml element nodes.
GeneralRe: Good Pin
Furty23-Sep-03 13:36
Furty23-Sep-03 13:36 
GeneralRe: Good Pin
dog_spawn24-Sep-03 6:34
dog_spawn24-Sep-03 6:34 
GeneralRe: Good Pin
Rudi Breedenraedt25-Sep-03 5:22
Rudi Breedenraedt25-Sep-03 5:22 
GeneralRe: Good Pin
dog_spawn25-Sep-03 6:52
dog_spawn25-Sep-03 6:52 
GeneralRe: Good Pin
peterchen29-Sep-03 6:29
peterchen29-Sep-03 6:29 
GeneralRe: Good Pin
dog_spawn29-Sep-03 6:34
dog_spawn29-Sep-03 6:34 
GeneralRe: Good Pin
peterchen29-Sep-03 11:19
peterchen29-Sep-03 11:19 
GeneralRe: Good Pin
dog_spawn29-Sep-03 11:23
dog_spawn29-Sep-03 11:23 
GeneralRe: Good Pin
peterchen29-Sep-03 12:26
peterchen29-Sep-03 12:26 
GeneralRe: Good Pin
dog_spawn29-Sep-03 13:53
dog_spawn29-Sep-03 13:53 
GeneralRe: Good Pin
Rocky Moore23-Sep-03 14:14
Rocky Moore23-Sep-03 14:14 
GeneralRe: Good Pin
Attila Hajdrik25-Sep-03 21:27
Attila Hajdrik25-Sep-03 21:27 
GeneralRe: Good Pin
rj4524-Apr-04 21:07
rj4524-Apr-04 21:07 
GeneralRe: Good Pin
Ngraham26-May-04 4:32
Ngraham26-May-04 4:32 
GeneralRe: Good Pin
DBuckner26-May-04 18:57
DBuckner26-May-04 18:57 

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