ConfigurationManager return null instead of string values

If your config file use in different class library you must change your name YourClasslibraryDllname.dll.config and you must change config file copy to output directory property

Ex:
   YourSolution
       ClassLibrary_1
            ClassLibrary_1.dll.config
            ApplicationConfigurationReader.cs
            ConfigurationConst.cs
       ClassLibrary_2
       ConsoleApp

1. Rename your config file like this YourClasslibraryDllname.dll.config
2. Open Properties Window
3. Change Do Not Copy to Copy Always

4. Add reference -> Assembly -> System.Configuration

5. Add below clases in ClassLibrary_1 Project

ConfigurationConst Class using System.Configuration;

public static class ConfigurationConst
{
   public static KeyValueConfigurationCollection Configs;
}

ApplicationConfigurationReader class using System.Configuration;

internal class ApplicationConfigurationReader
    {
        public void Read()
        {
            // read assembly
            var ExecAppPath = this.GetType().Assembly.Location;

            // Get all app settings  in config file
            ConfigurationConst.Configs = ConfigurationManager.OpenExeConfiguration(ExecAppPath).AppSettings.Settings;

        }
    }

Read Config using ClassLibrary_1;

static void Main(string[] args)
{
    new ApplicationConfigurationReader().Read();
    var Configval = ConfigurationConst.Configs["provider"].Value;            
    Console.ReadKey();
}

i Hope you can get clean help

Did you ensure that the config file is placed correctly at the directory from which you're running the application? Is there actually a file called <app name>.exe.config in that directory?

I'm just guessing here - maybe you added the App.Config file in a different project then your exe assembly project...?

By the way, I copied your code and App.Config as is to a clean project, and this code worked for me. So I'd look in the direction of the config file itself and not in the code. The code is fine...

Hope this helps,

Ran