How to communicate from class library

Question

I'm having some trouble conceptualizing what I'm supposed to do to facilitate communication between my class library and programs that use it -- in this case, a Windows Forms app:

// Class in library
class Foo()
{
    public Foo(){}

    public void DoWork()
    {
        log("Working...");
    }

    private void log( string s )
    {
        Console.Writeline(s);
    }

}

// Forms App
class Form1()
{
    public Form1()
    {
        Foo MyFoo = new Foo();
        MyFoo.DoWork();
    }
}

Since there is nothing listening to the console in a winforms app, calls to log() show nothing. Is there a way to dynamically overwrite the Foo.Log method, or possibly assign a method with that signature to the Foo object that is more suitable for a forms app at runtime?

Thank you!

Answer 1

Declare a logger interface and inject a concrete logger into the library class

public interface ILogger
{
    void Log(string message);
}

public class Foo
{
    private ILogger _logger;

    public Foo (ILogger logger)
    {
        _logger = logger;
    }

    public void DoWork ()
    {
        _logger.Log("Working...");
    }
}

Use it like this

var foo = new Foo(new ConsoleLogger());
var foo = new Foo(new FileLogger());
var foo = new Foo(new MsgBoxLogger());

Where all these loggers implement the interface.

public class ConsoleLogger : ILogger
{
    public void Log(string message)
    {
        Console.WriteLine(message);
    }
}

---

UPDATE

As Mike Panter has pointed out already, you can use a delegate

public class Foo
{
    private Action<string> _writeLog;

    public Foo (Action<string> writeLog)
    {
        _writeLog = writeLog;
    }

    public void DoWork ()
    {
        _writeLog("Working...");
    }
}

you can call it like this

var foo = new Foo(s => Console.WriteLine(s));

Advantages of the delegate injection

• Very simple to declare
• Very simple to use
• You can pass additional parameters through variable capturing.

        See Jon Skeets article The Beauty of Closures for variable capturing.

Advantages of the interface injection

• You can easily pass parameters to the logger through its constructor, like a file name for the FileLogger. Since the constructor is not part of the interface, different loggers can have different constructor parameters.
• You can easily test it with unit tests, what you cannot do with anonymous lambda expressions, since there is no way to access them from the unit test.
• You can use a mock logger in unit tests of your library classes.
• You can create a new specialized logger by deriving it from an existing one.
• You can declare private helper methods within a logger.