public interface ICanHasDefaultMembers
{
    void FireDev();

    void FireAllDevs()
    {
        foreach (var dev in theCogworks)
        {
            FireDev();
        }
    }
}

public class MySweetClassWhichIDontWantToModify : ICanHasDefaultMembers
{
    public void FireDev()
    {
        // You know what to do :>
    }
}

public interface ICanHasDefaultMembers
{
    void FireDev();

    void FireAllDevs()
    {
        foreach (var dev in theCogworks)
        {
            FireDev();
        }
    }
}

public class MySweetClassWhichIDontWantToModify : ICanHasDefaultMembers
{
    public void FireDev()
    {
        // You know what to do :>
    }

    private void FireAllDevs() 
    {
        // My private member
    }
}

MySweetClassWhichIDontWantToModify myClass = new MySweetClassWhichIDontWantToModify();
myClass.FireAllDevs(); // Doesn't work because FireAllDevs() is private

ICanHasDefaultMembers myClassOnSteroids = (ICanHasDefaultMembers)myClass;
myClassOnSteroids.FireAllDevs(); // It works!

async Task<bool> ThisIsWellKnownAndUsedTaskAsync(){
    return true;
}

await ThisIsWellKnownAndUsedTaskAsync();

// ---------------------------------------------------

IAsyncEnumerable<T> willItBlowOrNotHmmmWeWillSee = ...

foreach await(var bang in willItBlowOrNotHmmmWeWillSee)
{
    // Thread is not blocked and item is retrieved
    // Move next when data is here!
}

public class SomeClassWhichNeedsToBeExtended
{

}

public static class SomeClassWhichNeedsToBeExtendedExtensions
{
    public static T MethodName<T>(this SomeClassWhichNeedsToBeExtended instance)
    {
        return default(T);
    }
}

extension MyClassExtension extends ClassToBeExtended
{
    public int AddAnotherHead()
    {
        this.Heads++; // it call the current instance of MyClassExtension
    }

    public int Heads()
    {
        get { ... }
    }
}

// --------------------------------------------------------------------

extension SomeComplexExtension extends MyClass : IMyInterface
{
    ...
}