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Modify appSettings in the custom config file which is referenced via 'file' attribute

In appSettings section of Web.config a file attribute is used referencing a custom config file. The goal is to have possibility to modify some app-settings in the custom config without causing the application to be restarted.
Web.config
<appSettings file="CustomAppSettings.config">
<add key="key1" value="val2" />
</appSettings>
CustomAppSettings.config
<?xml version="1.0" encoding="utf-8" ?>
<appSettings>
<add key="customKey1" value="custVal2"/>
</appSettings>
The following code does not work. It saves the value to Web.config but expected is to save it to the CustomAppSettings.config because so it will not restart the application (Source).
var configuration = WebConfigurationManager.OpenWebConfiguration("~/");
configuration.AppSettings.Settings[key].Value = value.ToString();
configuration.Save();
This does not work as well.
var configuration = WebConfigurationManager.OpenWebConfiguration("~/CustomAppSettings.config");
What am I doing wrong? Could someone point me to the right direction?

use configSource instead of file.
<appSettings configSource="CustomAppSettings.config" />
use ConfigurationSaveMode.Minimal on saving.
var configuration = WebConfigurationManager.OpenWebConfiguration("~/");
configuration.AppSettings.Settings[key].Value = value.ToString();
configuration.Save(ConfigurationSaveMode.Minimal);

Reading settings from app.config in .net 4.5

I've added a reference to System.Configuration. I've created App1.config in my project and populated it with the following code:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="ResistanceA" value="0.04"/>
<add key="ResistanceB" value="0.04"/>
<add key="ResistanceC" value="0.01"/>
<add key="TempBattLow" value="40"/>
<add key="TempBattHigh" value="45"/>
<add key="TempLoad" value="40"/>
</appSettings>
</configuration>
Then I try to read the values using the following code,
using System.Configuration;
string str = ConfigurationManager.AppSettings.Get("ResistanceA");
However I do not get the data. Any idea what I am doing wrong? Thanks.

Make sure the (app name here).config file actually appears in the same folder as your (app name here).exe file. Being that you called it App1.config, I'm guessing that you have more than one.
Visual Studio renames App.Config to the actual (app name here).config file during a build, not App1.config.

MVC 4 dedicated configuration file

I have to add some constants to MVC 4 project.
Adding them to in web.config will work:
<appSettings>
<add key="MyConst1" value="123"/>
<add key="MyConst2" value="321"/>
<add key="MyConst3" value="234"/>
</appSettings>
Is there a way to create separate config file for this constants?

I guess you want to separate your appSettings from web.config, you can store them in a separate file and then specify that in your web.config under appSettings's configSource like:
<appSettings configSource="MySettings.config" />
and then you can have your settings in MySettings.config as:
<?xml version="1.0" encoding="UTF-8"?>
<appSettings>
<add key="MyConst1" value="123"/>
<add key="MyConst2" value="321"/>
<add key="MyConst3" value="234"/>
</appSettings>
You may see: Using configSource to split configuration files

If you put your code snippet in a file called appSettings.config, you can simply reference that file in your web config:
<appSettings configSource="appSettings.config" />

Getting configuration values rom XML file located in the same project

I am making a little program to copy pictures form one location to another. The information for the pictures are stored in a database so I need connections string and also I create a txt file with the final output from the operation and I want to store these two values in a App.Config.xml file.
The structure of my project is very simple :
And the XML files itself is :
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<connectionStrings>
<add name="MyDB" connectionString="Data Source=.\\DVSQLEXPRESS08;Initial Catalog=**;Persist Security Info=True;User ID=**;Password=**;MultipleActiveResultSets=True" />
</connectionStrings>
<createResultFile>
<add key="ResultFile" value="C:\Users\dv\Desktop\Leron\PictureStatus.txt"/>
</createResultFile>
</configuration>
I want to use the connectionString and <createResultFile> value in my PictureTransferTool.cs. This is my first time working with XML file and C# (.NET in general) so I want what is the way to retrieve those config values?

You config file must be like below...
Config File :
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="ResultFile" value="C:\Users\dv\Desktop\Leron\PictureStatus.txt"/>
</appSettings>
<connectionStrings>
<add name="MyDB" connectionString="Data Source=.\\DVSQLEXPRESS08;Initial Catalog=**;Persist Security Info=True;User ID=**;Password=**;MultipleActiveResultSets=True" />
</connectionStrings>
</configuration>
C# :
You can read Connection String like below
var connectionString = System.Configuration.ConfigurationManager.ConnectionStrings["MyDB"].ConnectionString;
Console.WriteLine(connectionString);
You can read User Defined Settings like Below
var Resultfile = System.Configuration.ConfigurationManager.AppSettings["ResultFile"];
Console.WriteLine(Resultfile);

The Way I normally do user-defined parameters in my app.config is i put them in the appSettings tab.
<appSettings>
<add key="myStr" value="String Value" />
and then you can access it with
string myStr = System.Configuration.ConfigurationSettings.AppSettings["myStr"];

It works for me.
Linq;
using System.Xml.XPath;
...
var doc = XDocument.Load("test.xml");// You should put the way to your XML
var name = doc.XPathSelectElements("/configuration/connectionStrings/add").Value;
var name = doc.XPathSelectElements("/configuration/createResultFile/add").Value;

c# Application Configuration File: AppSettings Reads Empty?

This is my first time using an XML config file. In the solution explorer I right click, add, new item, application config. file.
I then edited the file to read...
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="Key1" value="1000" />
</appSettings>
</configuration>
...and tried to access it with...
Int32.Parse(ConfigurationManager.AppSettings.Get("Key1"));
...but it says that the parameter is null. I looked at just ConfigurationManager.AppSettings and it's AllKeys property has dimension 0.
What am I doing wrong?
Thanks!

Right-click on your project, choose Properties>Settings tab and edit your settings there because the config file is not the only place these values are stored at.
In your code use Settings.Default.MySetting to access the settings.
You'll need to add using MyProjectName.Properties; to your using statements in order to access the settings this way or you'll have to fully qualify it as MyProjectName.Properties.Settings.Default.MySetting...

I know this is super old but I just encountered this opening an old project. A reference to System.Configuration was missing.

To ensure get information from App.config using ConfigurationManager.AppSettings["<Key name>"]; be sure to set your values inside <appSettings> section.
Example:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.7.2" />
</startup>
<appSettings>
<add key ="IntervalSeconds" value ="60000"/>
<add key ="OutputPathLog" value ="\\myserver\hackingtrack\Projects\Log\"/>
<!--Jorgesys si Elenasys sunt puisori-->
<add key="InputFeeds1" value="https://cld.blahbla.com//portadaKick.json" />
<add key="InputFeeds2" value="https://cld.blahbla.com//portadaKick.json" />
<add key="InputFeeds3" value="https://cld.blahbla.com//portadaKick.json" />
</appSettings>
</configuration>
Retrieving values:
string intervalSeconds = ConfigurationManager.AppSettings["IntervalSeconds"];
string inputFeeds1 = ConfigurationManager.AppSettings["InputFeeds1"]; ;
string inputFeeds2 = ConfigurationManager.AppSettings["InputFeeds2"];
string inputFeeds3 = ConfigurationManager.AppSettings["InputFeeds3"];