The simplest form of configurable Dependency Injection

public class MyClass {

public static Func<string, string> 
     GetConfigValue = s => ConfigurationManager.AppSettings[s];

//...

Normal use:

string connectionString = MyClass.GetConfigValue("myConfigValue");

Custom (e.g. Unit Testing) use:

MyClass.GetConfigValue = s =>  s == "myConfigValue" ? "Hi", "string.Empty";