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Read configuration from App.config instead of AppName.exe.config

I have developed a small windows service which performs few database operations. I have to give an option to the user to change server name on post-deploy. if user changes in app.config it doesn't affect the service, it is still reading connection strings from AppName.exe.config.
Here what I have tried.
<connectionStrings>
<add name="testString" connectionString="Data Source=ServerAddresss;Initial Catalog=DatabaseName;Integrated Security=True;" />
</connectionStrings>
C# code,
ConfigurationManager.ConnectionStrings["ProjectName.Properties.Settings.testString"].ConnectionString);
This returns a server connection string from AppName.exe.config file but I want to access it from App.config file.
can someone help me out in this?

App.config and AppName.exe.config aren't exactly separate - AppName.exe.config is the build output of app.config.
In other words, once the service is deployed, there shouldn't even be an app.config to change, and if there is, changing it won't do anything. AppName.exe.config is where your application gets "app.config" values.
If you need different configuration values for different environments, take a look at config transformations. They allow you to change or replace sections or individual values for different configurations such as debug, release, or other custom configurations.
This is useful because it means that all of the connection strings are part of the project and are in source control. You can do without this and just edit the config file in place on the server, but then there will be nothing in source control indicating where the connection string came from. And then if you redeploy the service the change will be overwritten unless someone remembers to make the same change every time. Having it in source control is much better.
For some reason that I do not understand, you can right-click a web.config and select "Add Config Transforms", but you can't do that with an app.config. Maybe there's a good reason.
You can install this extension which enables the same behavior for app.config. Then you can right-click on app.config and add a transformation for another configuration, like Release.
In that file (app.Release.config) add this:
<connectionStrings xdt:Transform="Replace">
<add name="testString" connectionString="...your Release connection string..." />
</connectionStrings>
When you build and deploy using the Release configuration it will replace the connectionStrings section with this one, leaving everything else just as it is. You can also right-click app.Release.config and select "Preview Config Transforms" to see the transformed file side-by-side with the original.

You should be able to just pull in an arbitrary file with a ConfigurationFileMap
System.Configuration.ConfigurationFileMap fileMap = new ConfigurationFileMap(strConfigPath); //Path to your config file
System.Configuration.Configuration configuration = System.Configuration.ConfigurationManager.OpenMappedMachineConfiguration(fileMap);
This question is a possible duplicate of: Using ConfigurationManager to load config from an arbitrary location
Also, you'll have to be sure that the service is restarted after updating the config file (either config file).

ASP.Net - use seperate configuration file for database and app settings

I have an asp.net project that is using Entity Framework that is used on several different client servers. This means a different web.config for each for connectionstrings and app settings.
Hasn't been an issue but I changed something that altered the web.config file recently and I manually had to adjust this for each client, I also have to exclude the web.config file in updates to ensure their own one is not overwritten.
What I would like to achieve is store these settings in maybe another config file that the project can pick up and use. Maybe that on Globals Application_Start gets these and imports them/overwrites the current web.config file perhaps.
Essentially I don't want to affect the current code that uses the connection string and ConfigurationManager.AppSettings used throughout the project but I do want to be able to let the web.config file update for each client and use a seperate file for some settings.
NOTE: I do not have access to publish directly to each server so I can't simply write a different deploy web.config for each one. I have to publish the files locally, store as zip and automated routine on servers downloads and extracts accordingly.
EDIT:
Please do say if this is considered a bad idea but an idea I had was to put something similar in Global.asax Application_Start method:
Does config file exist?
If no, create file and append all current settings in web.config
If yes, open and import those settings to the current web.config overwriting the original values
Hopefully then in a few weeks time, after I have asked all clients to perform a manual update they will have this code and I can begin to include the web.config in updates.

In VS, inside the Build menu, the last item is Configuration Manager.
In here you can specify various different release environments which can each have their own web.config transforms.
This is normally used for production/staging/test environments. However, I can see no reason why you could not use this and have a configuration file for each of your servers/environments.
You will then need to create the transformations for each environment, by rightclicking the web.config, then selecting Add Config Transform.
each of the environments you had setup can then override the settings in the main web.config. (That now acts as a template/default settings)
e.g.
In Web.EnvironmentA.Config
<add key="ConnectionString" value="ConStringA" xdt:Transform="SetAttributes" xdt:Locator="Match(key)"/>
In Web.EnvironmentB.Config
<add key="ConnectionString" value="ConStringB" xdt:Transform="SetAttributes" xdt:Locator="Match(key)"/>
Then when you do a publish, you simply setup which config file to use. If you use Azure or the VS publish method, it will save each of these settings, so then you can simply push to the relevant environment easily.
Just make sure you test this works as you intent first, before you start publishing everywhere ;)

What is App.config in C#.NET? How to use it?

I have done a project in C#.NET where my database file is an Excel workbook. Since the location of the connection string is hard coded in my coding, there is no problem for installing it in my system, but for other systems there is.
Is there a way to prompt the user to set a path once after the setup of the application is completed?
The answers I got was "Use App.Config"... can anyone tell what is this App.config and how to use it in my context here?

At its simplest, the app.config is an XML file with many predefined configuration sections available and support for custom configuration sections. A "configuration section" is a snippet of XML with a schema meant to store some type of information.
Overview (MSDN)
Connection String Configuration (MSDN)
Settings can be configured using built-in configuration sections such as connectionStrings or appSettings. You can add your own custom configuration sections; this is an advanced topic, but very powerful for building strongly-typed configuration files.
Web applications typically have a web.config, while Windows GUI/service applications have an app.config file.
Application-level config files inherit settings from global configuration files like machine.config. Web also applications inherit settings from applicationHost.config.
Reading from the App.Config
Connection strings have a predefined schema that you can use. Note that this small snippet is actually a valid app.config (or web.config) file:
<?xml version="1.0"?>
<configuration>
<connectionStrings>
<add name="MyKey"
connectionString="Data Source=localhost;Initial Catalog=ABC;"
providerName="System.Data.SqlClient"/>
</connectionStrings>
</configuration>
Once you have defined your app.config, you can read it in code using the ConfigurationManager class. Don't be intimidated by the verbose MSDN examples; it's actually quite simple.
string connectionString = ConfigurationManager.ConnectionStrings["MyKey"].ConnectionString;
Writing to the App.Config
Frequently changing the *.config files is usually not a good idea, but it sounds like you only want to perform one-time setup.
See: Change connection string & reload app.config at run time which describes how to update the connectionStrings section of the *.config file at runtime.
Note that ideally you would perform such configuration changes from a simple installer.
Location of the App.Config at Runtime
Q: Suppose I manually change some <value> in app.config, save it and then close it. Now when I go to my bin folder and launch the .exe file from here, why doesn't it reflect the applied changes?
A: When you compile an application, its app.config is copied to the bin directory1 with a name that matches your exe. For example, if your exe was named "test.exe", there should be a ("text.exe.config" in .net framework) or ("text.dll.config" in .net core) in your bin directory. You can change the configuration without a recompile, but you will need to edit the config file that was created at compile time, not the original app.config.
1: Note that web.config files are not moved, but instead stay in the same location at compile and deployment time. One exception to this is when a web.config is transformed.
.NET Core
New configuration options were introduced with .NET Core and continue with the unified .NET (version 5+). The way that *.config files works hasn't fundamentally changed, but developers are free to choose new, more flexible configuration paradigms.
As with .NET Framework configuration .NET Core can get quite complex, but implementation can be as simple as a few lines of configuration with a few lines of c# to read it.
Configuration in ASP.NET Core
Configuration in .NET Core

Simply, App.config is an XML based file format that holds the Application Level Configurations.
Example:
<?xml version="1.0"?>
<configuration>
<appSettings>
<add key="key" value="test" />
</appSettings>
</configuration>
You can access the configurations by using ConfigurationManager as shown in the piece of code snippet below:
var value = System.Configuration.ConfigurationManager.AppSettings["key"];
// value is now "test"
Note: ConfigurationSettings is obsolete method to retrieve configuration information.
var value = System.Configuration.ConfigurationSettings.AppSettings["key"];

App.Config is an XML file that is used as a configuration file for your application. In other words, you store inside it any setting that you may want to change without having to change code (and recompiling). It is often used to store connection strings.
See this MSDN article on how to do that.

Just to add something I was missing from all the answers - even if it seems to be silly and obvious as soon as you know:
The file has to be named "App.config" or "app.config" and can be located in your project at the same level as e.g. Program.cs.
I do not know if other locations are possible, other names (like application.conf, as suggested in the ODP.net documentation) did not work for me.
PS. I started with Visual Studio Code and created a new project with "dotnet new". No configuration file is created in this case, I am sure there are other cases.
PPS. You may need to add a nuget package to be able to read the config file, in case of .NET CORE it would be "dotnet add package System.Configuration.ConfigurationManager --version 4.5.0"

You can access keys in the App.Config using:
ConfigurationSettings.AppSettings["KeyName"]
Take alook at this Thread

Just adding one more point
Using app.config some how you can control application access, you want apply particular change to entire application use app config file and you can access the settings like below
ConfigurationSettings.AppSettings["Key"]

Application settings enable you to store application information dynamically. Settings allow you to store information on the client computer that shouldn't be included in the application code (for example a connection string), user preferences, and other information you need at runtime.
To add an application configuration file to a C# project:
In Solution Explorer, right-click the project node, and then select Add > New Item.
The Add New Item dialog box appears.
Expand Installed > Visual C# Items.
In the middle pane, select the Application Configuration File template.
Select the Add button.
A file named App.config is added to your project.
take a look at this article

How to access a connection string from within a library

I have a web project (mvc) and data access layer in a separated class library project. I need to access to a connection string in app.config which sits in that library project.
ConfigurationManager.ConnectionStrings[0].ConnectionString pulls something strange. I don't have this kind of settings neither in the library's config nor in the web project's config files.
the App.config looks like that:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<connectionStrings>
<add name="DALConnectionString" connectionString="User ID=sa;Password=pass;Initial Catalog=db;Data Source=srv\SQL2005;" />
</connectionStrings>
</configuration>

Your library should use dependency injection in this case for inversion of control.
Your class in the data access layer (DAL) library should take the connection string as a constructor argument or a property value.
This will make sure that your DAL can be used in other projects also and is not tied to your your mvc web application.
Let the code which will consume the DAL read the connection string from the config file and inject it into your class's constructor.

By default, a class library can't access a config file.
The client of the class library, in this case your web project, can provide config settings.
Therefore, put all the relevant settings, the connection strings, in the web's config file. The ConfigurationManager code in the class library will use the web projects config settings.

you should add the fragment shown above in the web.config then at runtime the configuration manager will use it even if running inside your class library.

You cannot access a app.config for a DLL.
app.config only works for the entry point assembly or web.config for a web project.
Try copying the connection to the entry point config or load the config by parsing the configuration XML - not recommended.

Relocating app.config file to a custom path

Is it possible to relocate the whole App.Config file to a custom path?
It seems a bit odd that the config file resides in the same folder as the exe, with Windows' new approcah of saving all program settings in c:\ProgramData and all.
An additional requirement we have is to programatically specify where to find the app.config file. The reason for this being that we spawn different service instances from the same exes, and would like to store each service's app.config in that service's settings folder under c:\ProgramData\\.

If still relevant, we have used the following I found on another suggested answer to another question here on Stack Overflow...
AppDomain.CurrentDomain.SetData ("APP_CONFIG_FILE", "path to config file")
Worked great for us when we had issues loading app.config from DLL only...

Each AppDomain has/can have its own configuration file. The default AppDomain created by CLR host uses programname.exe.config; if you want to provide your own configuration file, create separate AppDomain. Example:
// get the name of the assembly
string exeAssembly = Assembly.GetEntryAssembly().FullName;
// setup - there you put the path to the config file
AppDomainSetup setup = new AppDomainSetup();
setup.ApplicationBase = System.Environment.CurrentDirectory;
setup.ConfigurationFile = "<path to your config file>";
// create the app domain
AppDomain appDomain = AppDomain.CreateDomain("My AppDomain", null, setup);
// create proxy used to call the startup method
YourStartupClass proxy = (YourStartupClass)appDomain.CreateInstanceAndUnwrap(
exeAssembly, typeof(YourStartupClass).FullName);
// call the startup method - something like alternative main()
proxy.StartupMethod();
// in the end, unload the domain
AppDomain.Unload(appDomain);
Hope that helps.

I know this is an old question, but for those who just want to have their app.config in a different location then their binary output build location, the following works as microsoft intended it (and thus no need re-read and re-write the file to disk)
Define an app.config as you always do.
Define another config file where you want to have the actual configuration file
Change the app.config so that it refers to the configuration file
At runtime the settings from the configuration file will override the settings in the app.config (if present). And you are done.
Example app.config
<?xml version="1.0" encoding="utf-8"?>
<configuration>
<startup>
<supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.0"/>
</startup>
<appSettings file="..\Config\settings.config">
<add key="port" value="1001"/>
</appSettings>
</configuration>
Note the file="..\Config\settings.config". You are completely free to define the path to the location where you want your users to change settings.
Example of the actual configuration file
<?xml version="1.0" encoding="utf-8"?>
<appSettings>
<add key="port" value="1234"/>
</appSettings>
At runtime the setting port will have the value 1234.
More info see msdn

This worked for me.. (taken from http://msdn.microsoft.com/en-us/library/system.configuration.appsettingssection.aspx)
// open config
System.Configuration.Configuration config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);
// update appconfig file path
config.AppSettings.File = "C:\\dev\\App.config";
// Save the configuration file.
config.Save(ConfigurationSaveMode.Modified);
// Force a reload in memory of the changed section.
ConfigurationManager.RefreshSection("appSettings");
Then when you call
NameValueCollection settings = System.Configuration.ConfigurationManager.AppSettings;
or any operation to fetch the app config, the new path is used.
Hope this might help someone else who has same problem!

I am sorry if I misunderstand your request but can you not use
ConfigurationManager.OpenExeConfiguration Method (String)
Based on the change, is it possible that you can use
AppDomainSetup.ConfigurationFile Property

This is an ancient question, but I ran into this same problem and came up with a hacky workaround from a few minutes in reflector:
static public class ConfigHack {
static public void OverrideAppConfig(string path) {
((AppDomainSetup)
typeof(AppDomain)
.GetField("_FusionStore", BindingFlags.NonPublic | BindingFlags.Instance)
.GetValue(AppDomain.CurrentDomain))
.ConfigurationFile = path;
}
static public void ResetConfigManager() {
typeof(ConfigurationManager)
.GetField("s_initState", BindingFlags.Static | BindingFlags.NonPublic)
.SetValue(null, 0);
}
}
I've only used it on .NET2, but it looks the same in 4 in reflector. Of course I wouldn't recommend shipping this :P I only use it for quick internal things.

You could use astander's approach of calling OpenExeConfiguration. If you literally want to relocate your config file, you'll have to create your own app domain. In the process of setting up your app domain you get the chance to specify where the config file is located.
BTW, .NET config files aren't great for configuration, at least not the sort that users can modify: they're not like INI files or the registry. If you want flexibility over where your configuration comes from you're better off storing it separately.

If u use the registry, your problem can be resolved.

MSDN probably would help...
The element
simplifies servicing for component
assemblies. If one or more
applications use an assembly that has
a configuration file residing in a
well-known location, the configuration
files of the applications that use the
assembly can use the
element to
include the assembly configuration
file, rather than including
configuration information directly.
When the component assembly is
serviced, updating the common
configuration file provides updated
configuration information to all
applications that use the assembly