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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#.

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:

    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)
    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.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:

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

    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

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About the Author

Rudi Breedenraedt
Architect Wolters Kluwer Belgium
Belgium Belgium
Rudi is a Software Architect at Wolters Kluwer Belgium.

Comments and Discussions

 
GeneralGood Pinmemberdog_spawn23-Sep-03 13:14 
GeneralRe: Good PinmemberFurty23-Sep-03 13:36 
GeneralRe: Good Pinmemberdog_spawn24-Sep-03 6:34 
GeneralRe: Good PinmemberRudi Breedenraedt25-Sep-03 5:22 
GeneralRe: Good Pinmemberdog_spawn25-Sep-03 6:52 
Rudi Breedenraedt wrote:
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.
 
C# is a strongly typed language. However the collections are not, hence my comments about bad design.
 
I think you have got mixed up here D'Oh! | :doh: You do not need casting with a strongly typed language. That is the point of types. C# requires casting so frequently because of the bad design of the collections.
 
Rudi Breedenraedt wrote:
I believe the contrary
 
I agree it is bad design to have GetAsSubclass method Smile | :) However that is a better solution than casting. This problem can be solved by good design. For example GetElementsByTagName in the XML part of the framework should return a list of XmlElements, not a list of XmlNodes as it currently does.
GeneralRe: Good Pinmemberpeterchen29-Sep-03 6:29 
GeneralRe: Good Pinmemberdog_spawn29-Sep-03 6:34 
GeneralRe: Good Pinmemberpeterchen29-Sep-03 11:19 
GeneralRe: Good Pinmemberdog_spawn29-Sep-03 11:23 
GeneralRe: Good Pinmemberpeterchen29-Sep-03 12:26 
GeneralRe: Good Pinmemberdog_spawn29-Sep-03 13:53 
GeneralRe: Good PinmemberRocky Moore23-Sep-03 14:14 
GeneralRe: Good PinmemberAttila Hajdrik25-Sep-03 21:27 
GeneralRe: Good Pinmemberrj4524-Apr-04 21:07 
GeneralRe: Good PinmemberNgraham26-May-04 4:32 
GeneralRe: Good PinmemberDBuckner26-May-04 18:57 

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