Develop Reference

reading app settings as null from App.config

I am struggeling reading data from a configuration file, and all the methods online are not working for me...
I have this configuration file (App.config):
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="test" value="testVal"/>
</appSettings>
</configuration>
And this line in my C# code
string appSettings = ConfigurationManager.AppSettings["test"];
for some reason, appSettings remains null when expected to be testVal.

This should work. Put a break point right after that and check the value. I tested same code and getting the value.
If you use web application make sure you put this in web.config and not in the app.config

Something that may be happening is the App.config isn't being included in the build output. Check the bin folder of your project directory (Debug/Release), and ensure there's a file that matches the namespace of your project with a ".config" extension. You should see, for example:
YourApp.dll
YourApp.exe
YourApp.dll.config (edit this in notepad to verify it contains the
appsetting values)
...
Assuming this is the case and you don't see the config, or it doesn't have the correct values, ensure the App.config file in your project is included in the project itself (not just added manually to the project directory through file explorer).
If it is included and still not working, try deleting and re-creating a new "Application Configuration File" in your project in Visual Studio... that should clear things up (hopefully!). Be sure to save the original app.config contents beforehand when doing this.

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

Problems while reading value from app.config file

I am making one windows application in c# in where i added one file as app.config file.I have written some code in that file as
<appSettings>
<add key ="FlagForArchiving" value="true"/>
</appSettings>
In 'program.cs' file i am reading this value as
ConfigurationSettings.AppSettings["FlagForArchiving"].ToString();
On local machine i can retrieve value from config file but whenever i am building that application and running on any other machine then I cant read the value from config file.I am trying to run my application on windows 7.Please help me.Thanks in advance.

app.config is renamed to <MyProgramName>.exe.config when you build. When your program runs it will look for that <MyProgramName>.exe.config file, not app.config.
You need to deploy the renamed file (<MyProgramName>.exe.config) along with your program.
In your case, you need to copy over OBViewer.exe, OBViewer.exe.config, and any other files that OBViewer.exe depends on (e.g. other .dll assemblies in your debug/release directory).
By the way, this renamed file is often commonly referred to as "app.config", even if it doesn't have the same filename.

and the app.config file exists on the other machine?
Before reading check if it exists
The exception you get says whats incorrect: "FileNotFoundException"
EDIT
here is the correct way!
if (File.Exists(AppDomain.CurrentDomain.SetupInformation.ConfigurationFile))
{
MessageBox.Show(ConfigurationSettings.AppSettings["FlagForArchiving"].ToString());
}

Configuration from App.config isn't being pulled correctly

I'm trying to extract a URL I saved to the app.config file, but it's returning a blank string. Any ideas why?
string asdf = String.Format("{0}", ConfigurationManager.AppSettings["MemberUrl"]);
And the configuration file:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="ForumUrl" value="http://www.dreamincode.net/forums/xml.php?showforum=" />
<add key="MemberUrl" value="http://www.dreamincode.net/forums/xml.php?showuser=" />
</appSettings>
</configuration>

If the app.config is part of a class library it probably isn't being copied to the bin folder properly (if at all).
The config file must be named <exefilename>.config for it to be picked up by the running application.
The App.config file in the application project (the one that produces an exe file, Console, WinForms, etc.) will copy and rename on deployment. Or if this is being executed from a web project it needs to go in the web.config.
Does this help?

All config information that your class library needs must be in the main projects App.config or web.config. In other words, if your app.config file is attached to the library it will NOT be read.
Go to the main application and add the appropriate keys/values to it's config file.

Sergio I just tried this is a console application and it works perfectly.
I would suggest that it's a class library; and not a main assembly that you have added your app.config file to.
When you do a build; look in the binary output folder Debug or Release and in there you should see a file named yourEXEfilename.config; if that file is not there then you will not get any output from the line of code you have above.
AppSettings will return a NULL string.
Hope this is of use
Kind Regards
Noel

there's no reason why that wouldn't work - do you have any other pertinent info ?
FYI, you dont need String.Format for what you're doing, the following is fine
string asdf = ConfigurationManager.AppSettings["MemberUrl"];

Windows Service Config File C#

I've developed a windows service application using Visual Studio 2008 / C#.
I have an app.config file in the project. When installed, the app.exe.config file appears beside the executable but it appears not to be reading the values from it when I try to access them through ConfigurationManager.AppSettings.
Has it copied the config file elsewhere or is there some other problem I don't know about?
Thanks in advance,
Martin.
Edit:
The config file name is infact my_exe_file_name.exe.config, it looks like:
<?xml version="1.0" encoding="utf-8" ?>
<configuration>
<appSettings>
<add key="RuntimeFrequency" value="3" />
</appSettings>
</configuration>
and I am trying to read via:
ConfigurationManager.AppSettings["RuntimeFrequency"]
The debug value I continually see is '1' and not '3'. Am I doing something wrong here?

I located the error and it was related to file permissions. After installing the service, my local user account didn't have access to modify the app.exe.config file.
The tool I was using to edit was not informing me it was being denied access to save the file - that's notepad++ if anyone is interested - so I couldn't see that it wasn't saving over the old config file.
Solved now, thanks everyone.
Martin.

When you are in debug mode check and see what settings are in the my_exe_file_name.vshost.exe.config Also make sure you adjust this in the app.config file. Visual studio should update the final config file in your bin/debug folders.

Maybe you are updating the wrong config file. You should double check that using
System.Configuration.ConfigurationManager.OpenExeConfiguration(PATH_TO_CONFIG);

Generally for the Windows Services that I write, i drop the appName.exe.config file into C:\WINDOWS\system32\
Perhaps you have an older version in that directory, which is where your service is getting the value, even though you've updated the config file in your project.

App.config file should be renamed to your_exe_file_name.exe.config and placed near the exe file.

Is it possible that you have more than one instance of the RuntimeFrequency entry defined? The ConfigurationManager reads the file from top to bottom and processes each setting individually. Therefore, the last value of RuntimeFrequency that is defined in the file is the one it will use.
If you want to know for sure if your file is being used, I would simply remove or comment out any definition for RuntimeFrequency (copy/paste errors do happen) and wait to see an application error when ConfigurationManager attempts to reference an entry in AppSettings that does not exist.