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AppSettingsExpressionBuilder Class

AppSettingsExpressionBuilder Class

Retrieves values, as specified in a declarative expression, from the <appSettings> section of the Web.config file.


Namespace:  System.Web.Compilation
Assembly:  System.Web (in System.Web.dll)

public class AppSettingsExpressionBuilder : ExpressionBuilder

The AppSettingsExpressionBuilder type exposes the following members.

Public methodAppSettingsExpressionBuilderInitializes a new instance of the AppSettingsExpressionBuilder class.

Public propertySupportsEvaluateReturns a value indicating whether an expression can be evaluated in a page that is not compiled. (Overrides ExpressionBuilder.SupportsEvaluate.)

Public methodEquals(Object)Determines whether the specified object is equal to the current object. (Inherited from Object.)
Public methodEvaluateExpressionReturns a value from the <appSettings> section of the Web.config file. (Overrides ExpressionBuilder.EvaluateExpression(Object, BoundPropertyEntry, Object, ExpressionBuilderContext).)
Protected methodFinalizeAllows an object to try to free resources and perform other cleanup operations before it is reclaimed by garbage collection. (Inherited from Object.)
Public methodStatic memberGetAppSetting(String)Returns a value from the <appSettings> section of the Web.config file.
Public methodStatic memberGetAppSetting(String, Type, String)Returns a value from the <appSettings> section of the Web.config file with the value converted to a target type.
Public methodGetCodeExpressionReturns a code expression that is used to perform the property assignment in the generated page class. (Overrides ExpressionBuilder.GetCodeExpression(BoundPropertyEntry, Object, ExpressionBuilderContext).)
Public methodGetHashCodeServes as the default hash function. (Inherited from Object.)
Public methodGetTypeGets the Type of the current instance. (Inherited from Object.)
Protected methodMemberwiseCloneCreates a shallow copy of the current Object. (Inherited from Object.)
Public methodParseExpressionWhen overridden in a derived class, returns an object that represents the parsed expression. (Inherited from ExpressionBuilder.)
Public methodToStringReturns a string that represents the current object. (Inherited from Object.)

The AppSettingsExpressionBuilder class provides access to values in the <appSettings> section of the Web.config file. The <appSettings> section of the Web.config file contains application-specific values in key/value pairs.

An application value is retrieved by assigning an expression of the form

<%$ AppSettings: appSettingsKey %>

to a property in a control declaration. The part of the expression before the colon (:) designates the type of expression to be retrieved, and the part after the colon signifies the key. The preceding expression would retrieve the following value from the Web.config file.


<add key="appSettingsKey" value="appSettingsValue"/>


When the page parser encounters an expression with the AppSettings prefix, it creates an instance of the AppSettingsExpressionBuilder class to handle the expression.

If the expression is encountered in a page that will be compiled, the AppSettingsExpressionBuilder object generates code to retrieve the specified value from the Web.config file. This code is executed during the execution of the generated page class. If the expression is encountered in a page that will not be compiled, the AppSettingsExpressionBuilder object returns the value from the Web.config file when the page is parsed and executed.

The following code example demonstrates how to retrieve an application settings value in a page that is not compiled.

<%@ Page Language="C#" CompilationMode="Never" %>

<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN" "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">

<html xmlns="http://www.w3.org/1999/xhtml" >
<head runat="server">
    <title>Noncompiled Page</title>
    <form id="form1" runat="server">
        Welcome to the 
        <asp:Literal ID="Literal1" runat="server" 
        Text="<%$ AppSettings: thisSeason %>" /> Sale!

This code retrieves the following expression from the Web.config file.

<configuration xmlns="http://schemas.microsoft.com/.NetConfiguration/v2.0">


<add key="thisSeason" value="Fall"/>



.NET Framework

Supported in: 4.6, 4.5, 4, 3.5, 3.0, 2.0

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