从 Web.Config 中读取变量

Assuming the key is contained inside the <appSettings> node:

ConfigurationSettings.AppSettings["theKey"];

As for "writing" - put simply, dont.

The web.config is not designed for that, if you're going to be changing a value constantly, put it in a static helper class.

I would suggest you to don't modify web.config from your, because every time when change, it will restart your application.

However you can read web.config using System.Configuration.ConfigurationManager.AppSettings

Ryan Farley has a great post about this in his blog, including all the reasons why not to write back into web.config files: Writing to Your .NET Application's Config File

Given the following web.config:

<appSettings>
<add key="ClientId" value="127605460617602"/>
<add key="RedirectUrl" value="http://localhost:49548/Redirect.aspx"/>
</appSettings>

Example usage:

using System.Configuration;


string clientId = ConfigurationManager.AppSettings["ClientId"];
string redirectUrl = ConfigurationManager.AppSettings["RedirectUrl"];

If you want the basics, you can access the keys via:

string myKey = System.Configuration.ConfigurationManager.AppSettings["myKey"].ToString();
string imageFolder = System.Configuration.ConfigurationManager.AppSettings["imageFolder"].ToString();

To access my web config keys I always make a static class in my application. It means I can access them wherever I require and I'm not using the strings all over my application (if it changes in the web config I'd have to go through all the occurrences changing them). Here's a sample:

using System.Configuration;


public static class AppSettingsGet
{
public static string myKey
{
get { return ConfigurationManager.AppSettings["myKey"].ToString(); }
}


public static string imageFolder
{
get { return ConfigurationManager.AppSettings["imageFolder"].ToString(); }
}


// I also get my connection string from here
public static string ConnectionString
{
get { return ConfigurationManager.ConnectionStrings["ConnectionString"].ConnectionString; }
}
}

I am siteConfiguration class for calling all my appSetting like this way. I share it if it will help anyone.

add the following code at the "web.config"

<configuration>
<configSections>
<!-- some stuff omitted here -->
</configSections>
<appSettings>
<add key="appKeyString" value="abc" />
<add key="appKeyInt" value="123" />
</appSettings>
</configuration>

Now you can define a class for getting all your appSetting value. like this

using System;
using System.Configuration;
namespace Configuration
{
public static class SiteConfigurationReader
{
public static String appKeyString  //for string type value
{
get
{
return ConfigurationManager.AppSettings.Get("appKeyString");
}
}


public static Int32 appKeyInt  //to get integer value
{
get
{
return ConfigurationManager.AppSettings.Get("appKeyInt").ToInteger(true);
}
}


// you can also get the app setting by passing the key
public static Int32 GetAppSettingsInteger(string keyName)
{
try
{
return Convert.ToInt32(ConfigurationManager.AppSettings.Get(keyName));
}
catch
{
return 0;
}
}
}
}

Now add the reference of previous class and to access a key call like bellow

string appKeyStringVal= SiteConfigurationReader.appKeyString;
int appKeyIntVal= SiteConfigurationReader.appKeyInt;
int appKeyStringByPassingKey = SiteConfigurationReader.GetAppSettingsInteger("appKeyInt");