C#

We can supply an optional list of constraints for each type parameter. A constraint indicates a requirement
that a type shall fulfill in order to be accepted as a type argument. (For example, it might have to implement
a given interface or be derived from a given base class.) A constraint is declared using the word where,
followed by a type parameter and colon (: ), followed by a comma-separated list of constraints, which can
include a class type, interface types, other type parameters, the reference type constraint “class”, the value
type constraint “struct”, and the constructor constraint “new()”

public interface IAllowThese { }
public static class ExtensionTest
    {
    public static String AAAAAAA<T>(this T x) where T : IAllowThese
        {
        return "Extension Method Test";
        }
    }
public class TestClass1 : IAllowThese { }
public class TestClass2 : IAllowThese { }
public class TestClass3 { }

TestClass1 test1 = new TestClass1();
TestClass2 test2 = new TestClass2();
TestClass3 test3 = new TestClass3();

//this works and I want it to work
Console.WriteLine(test1.AAAAAAA());

//this line won't compile, doesn't work, and
//I want to add the same extension method
//to TestClass2 and don't know how
Console.WriteLine(test2.AAAAAAA());

//this line won't compile either, but I
//specifically don't want to add
//the extension method to TestClass3
Console.WriteLine(test3.AAAAAAA());

public interface UseAAAAAAA {}

public static class ExtensionTest
{
    public static String AAAAAAA<T>(this T x) where T : UseAAAAAAA 
    {
        return "Extension Method Test Class: " + typeof(T).Name;
    }
}

public class Test1 : UseAAAAAAA {}

public class Test2 : UseAAAAAAA {}

public class Test3 {}

Test1 test1 = new Test1();
Test2 test2 = new Test2();
Test3 test3 = new Test3();

Console.WriteLine(test1.AAAAAAA());
Console.WriteLine(test2.AAAAAAA());

// will throw an Error like: Test3' cannot be used as type parameter 'T' in the generic type or method // ExtensionTest.AAAAAAA<T>(T)'. There is no implicit reference conversion from 'Test3' to UseAAAAAAA'
// Console.WriteLine(test3.AAAAAAA());

BillWoodruff wrote:

inherit from a common Interface

private static void HelpFooBar<T>(T t) where T :baseclass

public static void FooBar(this RealObject o){
 HelpFooBar(o);
}
public static void FooBar(this AnotehrObject o){
 HelpFooBar(o);
}

public void PrintRectangle()
{
  Console.BackgroundColor = ConsoleColor.Green;
  Console.ForegroundColor = ConsoleColor.DarkGreen;
  Console.WriteLine("+----+");
  Console.WriteLine("|    |");
  Console.WriteLine("|    |");
  Console.WriteLine("|    |");
  Console.WriteLine("|    |");
  Console.WriteLine("+----+");
  Console.ResetColor();
}