How can I read/write app.config settings at runtime without using user settings?

Well, I haven't yet wanted to change application settings at runtime (that's what I use user settings for), but what I have been able to do is write application settings at install time. I imagine that a similar approach might work at runtime. You could try it out since there don't seem to be any other propsed solutions ATM.

    exePath = Path.Combine( exePath, "MyApp.exe" );
    Configuration config = ConfigurationManager.OpenExeConfiguration( exePath );
    var setting = config.AppSettings.Settings[SettingKey];
    if (setting != null)
    {
        setting.Value = newValue;
    }
    else
    {
        config.AppSettings.Settings.Add( SettingKey, newValue);
    }

    config.Save();

Hope that helps!

This is the method which allows you to change entries in the <AppSettings>:

    internal static bool SetSetting(string Key, string Value)
    {
        bool result = false;
        try
        {
            System.Configuration.Configuration config =
              ConfigurationManager.OpenExeConfiguration(
                                   ConfigurationUserLevel.None);

            config.AppSettings.Settings.Remove(Key); 
            var kvElem= new KeyValueConfigurationElement(Key, Value);
            config.AppSettings.Settings.Add(kvElem);

            // Save the configuration file.
            config.Save(ConfigurationSaveMode.Modified);

            // Force a reload of a changed section.
            ConfigurationManager.RefreshSection("appSettings");                

            result = true;
        }
        finally
        { }
        return result;
    } // function

Note that I have found it is necessary to refresh the section appSettings after the update.

The function removes a key before it adds it to avoid double entries. This works also if the key does not previously exist. If there is any error it returns false, on success true. The method to read settings is trivial and just listed for completeness:

    internal static string GetSetting(string Key)
    {
        string result = null;
        try
        {
            result = ConfigurationManager.AppSettings[Key];
        }
        finally
        { }
        return result;
    } // function

Note that I've surrounded it by a try ... finally block to suppress errors. If any errors occur, then GetSetting simply returns null while SetSetting returns false. That makes handling easier, however if you require the exceptions you can still add

        catch (Exception) { throw; }

to throw the exception up to the caller. Or, for debugging you could add:

        #if DEBUG
        catch (Exception ex) { 
                System.Diagnostics.Debug.WriteLine(ex.ToString()); 
        }
        #endif

Which will show the exception in the Output window of Visual Studio if you have selected the "Debug" configuration, but will continue with the code.

Note (cross-reference to a similar topic):

• The applicationSettings section is different, since it distinguishes between "User" and "Application" scope and it supports different datatypes, not just strings. If you want to know how you can handle applicationSettings, you can find it here (on stackoverflow):
  How to access applicationSettings

• If you are uncertain whether you should use AppSettings or applicationSettings, then read this before you decide it.

• If you encounter the warning 'ConfigurationSettings.AppSettings' is obsolete, then this hint can help you.

• If you're using the .NET Core framework, check out this link: AppSettings in .NET Core

WPF applications are able to access the app.config file just like WinForms apps through the

ConfigurationManager.OpenExeConfiguration()

method. The trick is to have the values you want to access in the AppSettings tag of your App.config file (also available in WPF applications).

The trick to all of this is to make sure to call the following methods when you're done modifying your properties:

MyConfig.Save(ConfigurationSaveMode.Modified)
ConfigurationManager.RefreshSection("appSettings")

I wrote a complete "how to" on this a little while back that explains it all here.