.NET Framework Class Library
ExpressionPrefixAttribute Class

Specifies the prefix attribute to use for the expression builder. This class cannot be inherited.

Inheritance Hierarchy
SystemObject
   SystemAttribute
    System.Web.CompilationExpressionPrefixAttribute

Namespace:   System.Web.Compilation
Assembly:  System.Web (in System.Web.dll)
Syntax
<[%$TOPIC/sbf78twf_en-us_VS_110_2_0_0_0_0%]([%$TOPIC/sbf78twf_en-us_VS_110_2_0_0_0_1%].Class, AllowMultiple := False)> _
Public NotInheritable Class ExpressionPrefixAttribute _
	Inherits [%$TOPIC/sbf78twf_en-us_VS_110_2_0_0_0_2%]
[[%$TOPIC/sbf78twf_en-us_VS_110_2_0_1_0_0%]([%$TOPIC/sbf78twf_en-us_VS_110_2_0_1_0_1%].Class, AllowMultiple = false)]
public sealed class ExpressionPrefixAttribute : [%$TOPIC/sbf78twf_en-us_VS_110_2_0_1_0_2%]
[[%$TOPIC/sbf78twf_en-us_VS_110_2_0_2_0_0%]([%$TOPIC/sbf78twf_en-us_VS_110_2_0_2_0_1%]::Class, AllowMultiple = false)]
public ref class ExpressionPrefixAttribute sealed : public [%$TOPIC/sbf78twf_en-us_VS_110_2_0_2_0_2%]
[<[%$TOPIC/sbf78twf_en-us_VS_110_2_0_3_0_0%]>]
[<[%$TOPIC/sbf78twf_en-us_VS_110_2_0_3_0_1%]([%$TOPIC/sbf78twf_en-us_VS_110_2_0_3_0_2%].Class, AllowMultiple = false)>]
type ExpressionPrefixAttribute =  
    class 
        inherit [%$TOPIC/sbf78twf_en-us_VS_110_2_0_3_0_3%] 
    end

The ExpressionPrefixAttribute type exposes the following members.

Constructors
  NameDescription
Public method ExpressionPrefixAttributeInitializes a new instance of the ExpressionPrefixAttribute class.
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Properties
  NameDescription
Public property ExpressionPrefixGets the prefix value for the current ExpressionBuilder object.
Public property TypeIdWhen implemented in a derived class, gets a unique identifier for this Attribute. (Inherited from Attribute.)
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Methods
  NameDescription
Public method EqualsInfrastructure. Returns a value that indicates whether this instance is equal to a specified object. (Inherited from Attribute.)
Public method GetHashCodeReturns the hash code for this instance. (Inherited from Attribute.)
Public method GetTypeGets the Type of the current instance. (Inherited from Object.)
Public method IsDefaultAttributeWhen overridden in a derived class, indicates whether the value of this instance is the default value for the derived class. (Inherited from Attribute.)
Public method MatchWhen overridden in a derived class, returns a value that indicates whether this instance equals a specified object. (Inherited from Attribute.)
Public method ToStringReturns a string that represents the current object. (Inherited from Object.)
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Explicit Interface Implementations
  NameDescription
Explicit interface implemetation Private method _AttributeGetIDsOfNamesMaps a set of names to a corresponding set of dispatch identifiers. (Inherited from Attribute.)
Explicit interface implemetation Private method _AttributeGetTypeInfoRetrieves the type information for an object, which can be used to get the type information for an interface. (Inherited from Attribute.)
Explicit interface implemetation Private method _AttributeGetTypeInfoCountRetrieves the number of type information interfaces that an object provides (either 0 or 1). (Inherited from Attribute.)
Explicit interface implemetation Private method _AttributeInvokeProvides access to properties and methods exposed by an object. (Inherited from Attribute.)
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Remarks

The ExpressionPrefixAttribute class is used at design time with expressions that are not defined in the configuration file. Use the ExpressionPrefix property to get the name of the prefix that is associated with the ExpressionPrefixAttribute object. An expression builder looks for any statements with the following form:

<%$ [expressionPrefix]:[expressionValue] %>

Then the expression builder, based on the prefix of the expression, generates code for the property assignment. The expressionPrefix parameter refers to a configured expression builder, which is defined in the configuration file or through an ExpressionPrefixAttribute object.

Examples

The following code examples demonstrate how to use the ExpressionPrefixAttribute class. The attribute is applied to a custom expression builder that implements the ExpressionBuilder abstract class. This implementation of ExpressionBuilder returns an evaluated statement that is passed to the expression. To run this example, you must first register the custom expression builder in the Web.config file. The first code example demonstrates how to register the custom expression builder in the Web.config file.

<configuration>
    <system.web>
       <compilation>
          <expressionBuilders>
              <add expressionPrefix="MyCustomExpression"
               type="MyCustomExpressionBuilder"/>
          </expressionBuilders>
       </compilation>
    </system.web>
</configuration>

The second code example demonstrates how to reference the expression in an .aspx file.

<asp:Label ID="Label1" runat="server" 
Text="<%$ MyCustomExpression:Hello, world! %>" />

The third code example demonstrates how to develop a customized expression builder by deriving from ExpressionBuilder. To run this code example, you must place the class in the App_Code folder.

Imports System
Imports System.CodeDom
Imports System.Web.UI
Imports System.ComponentModel
Imports System.Web.Compilation
Imports System.Web.UI.Design

' Apply ExpressionEditorAttributes to allow the  
' expression to appear in the designer.
<ExpressionPrefix("MyCustomExpression")> _
<ExpressionEditor("MyCustomExpressionEditor")> _
Public Class MyExpressionBuilder
    Inherits ExpressionBuilder
    ' Create a method that will return the result  
    ' set for the expression argument. 
    Public Shared Function GetEvalData(ByVal expression As String, _
       ByVal target As Type, ByVal entry As String) As Object 
        Return expression
    End Function 

    Public Overrides Function EvaluateExpression(ByVal target As Object, _
       ByVal entry As BoundPropertyEntry, ByVal parsedData As Object, _
       ByVal context As ExpressionBuilderContext) As Object 
        Return GetEvalData(entry.Expression, target.GetType(), entry.Name)
    End Function 

    Public Overrides Function GetCodeExpression(ByVal entry _
       As BoundPropertyEntry, ByVal parsedData As Object, ByVal context _
       As ExpressionBuilderContext) As CodeExpression
        Dim type1 As Type = entry.DeclaringType
        Dim descriptor1 As PropertyDescriptor = _
           TypeDescriptor.GetProperties(type1)(entry.PropertyInfo.Name)
        Dim expressionArray1(2) As CodeExpression
        expressionArray1(0) = New CodePrimitiveExpression(entry.Expression.Trim())
        expressionArray1(1) = New CodeTypeOfExpression(type1)
        expressionArray1(2) = New CodePrimitiveExpression(entry.Name)
        Return New CodeCastExpression(descriptor1.PropertyType, _
           New CodeMethodInvokeExpression(New CodeTypeReferenceExpression _
           (MyBase.GetType()), "GetEvalData", expressionArray1))
    End Function 

    Public Overrides ReadOnly Property SupportsEvaluate() As Boolean 
        Get 
            Return True 
        End Get 
    End Property 
End Class
using System;
using System.CodeDom;
using System.Web.UI;
using System.ComponentModel;
using System.Web.Compilation;
using System.Web.UI.Design;

// Apply ExpressionEditorAttributes to allow the  
// expression to appear in the designer.
[ExpressionPrefix("MyCustomExpression")]
[ExpressionEditor("MyCustomExpressionEditor")]
public class MyExpressionBuilder : ExpressionBuilder
{
    // Create a method that will return the result  
    // set for the expression argument. 
    public static object GetEvalData(string expression, Type target, string entry)
    {
        return expression;
    }

    public override object EvaluateExpression(object target, BoundPropertyEntry entry, 
	object parsedData, ExpressionBuilderContext context)
    {
        return GetEvalData(entry.Expression, target.GetType(), entry.Name);
    }

    public override CodeExpression GetCodeExpression(BoundPropertyEntry entry, 
	object parsedData, ExpressionBuilderContext context)
    {
        Type type1 = entry.DeclaringType;
        PropertyDescriptor descriptor1 = TypeDescriptor.GetProperties(type1)[entry.PropertyInfo.Name];
        CodeExpression[] expressionArray1 = new CodeExpression[3];
        expressionArray1[0] = new CodePrimitiveExpression(entry.Expression.Trim());
        expressionArray1[1] = new CodeTypeOfExpression(type1);
        expressionArray1[2] = new CodePrimitiveExpression(entry.Name);
        return new CodeCastExpression(descriptor1.PropertyType, new CodeMethodInvokeExpression(new 
	   CodeTypeReferenceExpression(base.GetType()), "GetEvalData", expressionArray1));
    }

    public override bool SupportsEvaluate
    {
        get { return true; }
    }
}
Version Information

.NET Framework

Supported in: 4.5.2, 4.5.1, 4.5, 4, 3.5, 3.0, 2.0
Platforms

Windows 8.1, Windows Server 2012 R2, Windows 8, Windows Server 2012, Windows 7, Windows Vista SP2, Windows Server 2008 (Server Core Role not supported), Windows Server 2008 R2 (Server Core Role supported with SP1 or later; Itanium not supported)

The .NET Framework does not support all versions of every platform. For a list of the supported versions, see .NET Framework System Requirements.

Thread Safety
Any public static (Shared in Visual Basic) members of this type are thread safe. Any instance members are not guaranteed to be thread safe.