Can I use params in Action or Func delegates?

How about this workaround?

private Action<string, object[]> writeToLogCallBack;
public void WriteToLogCallBack(string s, params object[] args)
{
  if(writeToLogCallBack!=null)
    writeToLogCallBack(s,args);
}

Or you could define your own delegate type:

delegate void LogAction(string s, params object[] args);

Variadic type parameters are not possible in C#.

That's why there're many declarations for Action<...>, Func<...>, and Tuple<...>, for example. It would be an interesting feature, though. C++0x has them.

You could try this. It allows for any number of arguments, and you'll get a compile time error if you pass the wrong number or type of arguments.

public delegate T ParamsAction<T>(params object[] oArgs);

public static T LogAction<T>(string s, ParamsAction<T> oCallback)
{
    Log(s);
    T result = oCallback();
    return T;
}

Foo foo = LogAction<Foo>("Hello world.", aoArgs => GetFoo(1,"",'',1.1));