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I want to get different tags of App.config File in C#

I have to get string inside Setting-> Value in C#. How to reach there through code and fetch that value from App.config file. Please help me. Thank you.
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<configSections>
<sectionGroup name="applicationSettings" type="System.Configuration.ApplicationSettingsGroup, System, Version=4.0.0.0,Culture=neutral, PublicKeyToken=b77a5c561934e089" >
<section name="MyApplication.Properties.Settings" type="System.Configuration.ClientSettingsSection, System, Version=4.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" requirePermission="false" />
</sectionGroup>
</configSections>
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
</startup>
<applicationSettings>
<MyApplication.Properties.Settings>
**<setting name="ConnectionString" serializeAs="String">
<value>server=127.0.0.1;uid=root;pwd=root;database=MyApplication_db;</
</setting>**
</VASI_Application.Properties.Settings>
</applicationSettings>
</configuration>

You can use the following code:
var section = (ClientSettingsSection)ConfigurationManager.GetSection("applicationSettings/MyApplication.Properties.Settings");
var settingElement = section.Settings.Get("ConnectionString");
var settingValue = settingElement.Value.ValueXml.InnerText;
// settingValue equals "server=127.0.0.1;uid=root;pwd=root;database=MyApplication_db;"
First, you retrieve the ClientSettingsSection section. The name of the section is a combination of the name of the "applicationSettings" section and its inner "MyApplication.Properties.Settings" section. Then, you retrieve the actual settings by calling Settings.Get("ConnectionString") on the returned ClientSettingsSection instance. Finally, you can use the Value.ValueXml.InnerText property of the returned setting to get to the actual value.
By the way, you have two small errors in your XML file:
The <MyApplication.Properties.Settings> element in incorrectly closed with </VASI_Application.Properties.Settings>
The <value> element is not correctly closed with </value>

Don't you have the property ConnectionString?
As documented here, usually you can access it in this way:
var setting = Properties.Settings.Default.ConnectionString;
By the way, your application tag starts with MyApplication.Properties.Settings but ends with VASI_Application.Properties.Settings

If you do actually want to go ahead with a custom config file section like in your example then you will need to follow this Microsoft guide for creating custom configuration sections.
https://msdn.microsoft.com/en-us/library/2tw134k3.aspx
If you get stuck following this guide then raise another question.

"Configuration system failed to initialize" error

I'm getting this exception when using the code:
var appSettings = ConfigurationManager.AppSettings; //Exception here
string result = appSettings["ILFSsqlServer"] ?? "Not Found"; //trying to get to this point
I'm not really sure why and from previous answers to this question I think I'm doing exactly what they suggest but to no avail.
My app.config is:
<?xml version="1.0"?>
<configuration>
<configSections>
<sectionGroup name="userSettings"
type="System.Configuration.UserSettingsGroup, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" >
</sectionGroup>
</configSections>
<AppSettings>
<add key="ILFSsqlServer" value="ODBC;DSN=sql server copycloas;Trusted_Connection=Yes;APP=Microsoft Office 2010;DATABASE=ILFSView;"/>
</AppSettings>
</configuration>

On your app.config you should write appSettings, lowercase A.

How to deal with changed ConfigurationSection-Definition?

I have a Configuration-File that I read with the ConfigurationManager. There are some Sections that I defined by myself.
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<configSections>
<section name="applicationWideSettingsSection" type="MyTestApp.ApplicationSettings.ApplicationWideSettingsSection, MyTestAssembly, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null" />
</configSections>
<applicationWideSettingsSection>
<pathToSomeFile value="C:\Users\..." />
</applicationWideSettingsSection>
</configuration>
There are new requirements: now I have to store another value in the applicationWideSettingsSection like this:
<applicationWideSettingsSection>
<pathToSomeFile value="C:\Users\..." />
<pathToSomeOtherFile value="C:\Programs\..." />
</applicationWideSettingsSection>
So I change my definition of the applicationWideSettingsSection. If I now run the application with an old config-File it throws an exception on this line:
var configSection = _config.GetSection("applicationWideSettingsSection");
because there is only the pathToSomeFile-Setting and the other one is missing.
Is there a way to manually add another (default value) to this section?
How would you deal with this??
Thanks in advance, Joerg

Because I didn't want to change my design away from my custom Setting-Types I found a different way:
I set the IsRequired to false so no exception is thrown when the setting is missing. If it's missing I set it to some value from code.
In a different approach I wanted to override some of the Configuration-Section methods but did not get it to work...

now I have to store another value in the applicationWideSettingsSection
Instead of your custom type ApplicationWideSettingsSection you can use a key-value pairs, so
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<configSections>
<section name="applicationWideSettingsSection"
type="System.Configuration.NameValueSectionHandler" />
</configSections>
<applicationWideSettingsSection>
<pathToSomeFile value="C:\Users\..." />
</applicationWideSettingsSection>
</configuration>
This will store 1 or many parameters and allow you to enumerate on the inner parameters. You can also try DictionarySectionHandler. I found sample code in this post.

Multiple AppSettings files, is it possible?

I want to create 3 AppSettings config files:
Database.config
Messages.config
Global.config
And after add in my App.config:
<appSettings file="Database.config" />
<appSettings file="Messages.config" />
<appSettings file="Global.config" />
But when I try to access a key that there is in one of three files with the ConfigurationManager, I got the following error:
Configuration system failed to initialize. Sections must only appear once per config file.
I cannot have more than one AppSettings config file?

You can't have more than one appsettings because that's the name of a section. You can add a new section though that uses the same kind of section definition as appsettings. E.g.,
<configuration>
<configSections>
<section name="DatabaseConfig" type="System.Configuration.NameValueFileSectionHandler, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089"/>
</configSections>
....
<DatabaseConfig>
<add key="Whatever" value="stuff"/>
</DatabaseConfig>
</configuration>

Code for separate file:
Web.config:
<configSections>
<section name="DatabaseConfig" type="System.Configuration.NameValueFileSectionHandler, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
<section name="MessageConfig" type="System.Configuration.NameValueFileSectionHandler, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
<section name="GlobalConfig" type="System.Configuration.NameValueFileSectionHandler, System, Version=2.0.0.0, Culture=neutral, PublicKeyToken=b77a5c561934e089" />
</configSections>
<DatabaseConfig configSource="database.config">
</DatabaseConfig>
<MessageConfig configSource="message.config">
</MessageConfig>
<GlobalConfig configSource="global.config">
</GlobalConfig>
database.config:
<DatabaseConfig>
<add key="Name" value="ServerName" />
</DatabaseConfig>
etc...
Can be accessed via code like this:
var databaseConfiguration = (NameValueCollection)ConfigurationManager.GetSection("DatabaseConfig");
string name = databaseConfiguration["Name"];

App.config - custom section not working

I recently started building a console version of a web application. I copied my custom sections from my web.config. to my app.config. When I go to get config information i get this error:
An error occurred creating the configuration section handler for x/y: Could not load type 'x' from assembly 'System.Configuration
The line that it is not liking is:
return ConfigurationManager.GetSection("X/Y") as Z;
Anyone run into something like this?
I was able to add
<add key="IsReadable" value="0"/>
in the appSettings and read it.
Addition:
I do actually have this defined about the custom section:
<configSections>
<sectionGroup name="x">
<section name="y" type="zzzzz"/>
</sectionGroup>
</configSections>

it sounds like your config-section handler is not defined
<configSection>
<section
name="YOUR_CLASS_NAME_HERE"
type="YOUR.NAMESPACE.CLASSNAME, YOUR.NAMESPACE, Version=1.1.0.0, Culture=neutral, PublicKeyToken=PUBLIC_TOKEN_ID_FROM_ASSEMBLY"
allowLocation="true"
allowDefinition="Everywhere"
/>
</configSection>

I had this identical issue recently. I created a custom sectiongroup for a web application(ran just fine), but when I ported this layer to a console app, the sectiongroup was failing.
You were correct in your question regarding how much of the "type" is required in your section definition. I've modified your configuration section with an example below:
<configSection>
<section
name="yourClassName"
type="your.namespace.className, your.assembly"
allowLocation="true"
allowDefinition="Everywhere" />
</configSection>
You'll notice, the type now has class name followed by assembly name. This is required for interaction outside of a web environment.
NOTE: Assembly name does not necessarily equal your namespace(for a given section).

If you want a custom config handler you have to define the class and reference it as shown by Steven Lowe. You can inherit from predefined handlers, or you can just use the value/key pair that is offered in appSetting section as you noted.

At the top of your file you require to have configSection tag with inside the section.
You can have sectionGroup too. Example:
<configuration>
<configSections>
<sectionGroup name="x">
<section name="y" type="a, b"/>
</sectionGroup>
<configSections>
</configuration>

This class works as a general custom configuration section handler for any type...
public class XmlConfigurator : IConfigurationSectionHandler
{
public object Create(object parent, object configContext, XmlNode section)
{
if (section == null) return null;
Type sectionType = Type.GetType((string)(section.CreateNavigator()).Evaluate("string(#configType)"));
XmlSerializer xs = new XmlSerializer(sectionType);
return xs.Deserialize(new XmlNodeReader(section));
}
}
In your app.config, add
<section name="NameofConfigSection" type="NameSpace.XmlConfigurator, NameSpace.Assembly"/>
And in the configuration section element, add an attribute to specify the type you want the root element deserialized into..
<?xml version="1.0" encoding="utf-8" ?>
<NameofConfigSection configType="NameSpace.NameofTypeToDeserializeInto, Namespace.Assembly" >
...
</NameofConfigSection>