[c#] Write values in app.config file

As others mentioned, you can do this with ConfigurationManager.AppSettings.Settings. But: Using Settings[key] = value will not work if the key doesn't exist.
Using Settings.Add(key, value), if the key already exists, it will join the new value to its value(s) separated by a comma, something like <add key="myKey" value="value1, value2, value3" />

To avoid these unexpected results, you have to handle two scenario's

• If entry with the given key exists? then update its value
• if entry with the given key doesn't exist? then create new entry(key,value)

Code

public static void Set(string key, string value)
{
    var config = ConfigurationManager.OpenExeConfiguration(ConfigurationUserLevel.None);

    var entry = config.AppSettings.Settings[key];
    if (entry == null)
        config.AppSettings.Settings.Add(key, value);
    else
        config.AppSettings.Settings[key].Value = value;

    config.Save(ConfigurationSaveMode.Modified);
}

For more info about the check entry == null, check this post.
Hope this will help someone.