How do you use Func<> and Action<> when designing applications?

Question

All the examples I can find about Func<> and Action<> are simple as in the one below where you see how they technically work but I would like

Answer 1

One thing I use it for is Caching of expensive method calls that never change given the same input:

public static Func Memoize(this Func f)
{
    Dictionary values;

    var methodDictionaries = new Dictionary>();

    var name = f.Method.Name;
    if (!methodDictionaries.TryGetValue(name, out values))
    {
        values = new Dictionary();

        methodDictionaries.Add(name, values);
    }

    return a =>
    {
        TResult value;

        if (!values.TryGetValue(a, out value))
        {
            value = f(a);
            values.Add(a, value);
        }

        return value;
    };
}

The default recursive fibonacci example:

class Foo
{
  public Func Fibonacci = (n) =>
  {
    return n > 1 ? Fibonacci(n-1) + Fibonacci(n-2) : n;
  };

  public Foo()
  {
    Fibonacci = Fibonacci.Memoize();

    for (int i=0; i<50; i++)
      Console.WriteLine(Fibonacci(i));
  }
}