C# equivalent of creating anonymous class that implements an interface

anonymous-class, c#, interface, java

Solution

Woof...ok, permit me to generalize a bit:

So in Java, you need a way to pass functions around. Java does not inherently support functions as first-class citizens, and this was one reason behind the implementation of anonymous classes - packaged groups of functions that can be declared inline and passed (as interfaces) to methods/other classes that will then call these functions.

In C#, functions are first-class citizens, and can be declared as either `Delegates`, `Func<>s`, or `Action<>s`. Let's try a comparison (of sorts):

Some sort of Java-y construct (my Java's fairly old, so bear with me):

public interface IDoSomething {
    public int Return42();
    public bool AmIPrettyOrNot(string name);
    public void Foo();
} 

public void Main(String[] args) {
    DoStuff(new IDoSomething() {
        public int Return42() { return 42; }
        public bool AmIPrettyOrNot(string name) { return name == "jerkimball"; }
        public bool Foo(int x) { ... }
    });
}

public void DoStuff(IDoSomething something) { ... }

The (very rough) equivalent of this in C# would be:

public void Main(string[] args)
{
    Func<int> returns42 = () => 42;
    Func<string,bool> amIPretty = name => name == "jerkimball";
    Action<int> foo = x => {};
}

Now, as others have mentioned, you usually see this pattern on the Java side when dealing with the handling of events - likewise on the C# side:

 public class Foo 
 {
     // define the shape of our event handler
     public delegate void HandlerForBarEvent(object sender, EventArgs args);
     // declare our event
     public event HandlerForBarEvent BarEvent;

     public void CallBar()
     {
         // omitted: check for null or set a default handler
         BarEvent(this, new EventArgs());
     }
 }    

 public void Main(string[] args)
 {
      var foo = new Foo();
      // declare the handler inline using lambda syntax
      foo.BarEvent += (sender, args) => 
      {
           // do something with sender/args
      }
      foo.CallBar();
 }

Note that we can also give it something with the same "shape":

 public void MyHandler(object sender, EventArgs args)
 {
     // do stuff
 }
 public void Main(string[] args)
 {
      var foo = new Foo();
      // that method above is the same "shape" as HandlerForBarEvent
      foo.BarEvent += MyHandler;
      foo.CallBar();
 }

But it's also used in Java to define what Threads do, if memory serves (i.e., `Runnable`) - and we can do this as well in C#:

var thread = new Thread((Action)(() => 
     {
         // I'm the threads "run" method!
     });
thread.Start();

Now, other stuff - enumeration:

public void processEvents(){
    for(Event event : eventList)
        eventList.execute();
}

C# has the same idea, just called differently:

public void processEvents()
{
    // edit: derp, 'event' is a keyword, so I'm
    // renaming this, since I won't get into why
    // you could also use @event...
    foreach(var evt in eventList)
    {
        evt.Execute();
    }
}

Problem

Original source

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