java - Anonymous interface implementation

Question

I′ve read 'C# anonymously implement interface (or abstract class)' thread for implementing an interface anonymously. But I wondered if this is also possible using .NET 2.0 (NO LINQ) using delegates or any similar approach. I know from JAVA the following possible:

MyClass m = MyMethod(new MyInterface() {
    @override
    public void doSomething() {...}
}

(I hope I remember well, is a time ago that I used JAVA, but I suppose it was something similar). This might be helpful whenever a method needs an instance of an interface and is called only once so there is no need to create a new class for this single approach.

Answer 1

.NET 2.0 also supported anonymous delegates, it's just that the syntax was a bit more verbose compared to lambdas, and type inference didn't work. And there were no extension methods in C# 2.0 (although you were able to use C# 3.0 and compile against .NET 2.0), which are the basis of LINQ and being able to operate on interfaces.

Compare:

• .NET 2.0: delegate(int i) { return (i < 5); }
• .NET 3.5: i => i < 5

.NET 2.0 also lacks common generic delegate signatures (Func and Action), but you can also easily define them yourself (for all combinations of parameters you like):

public delegate void Action<T>(T item);
public delegate Tresult Func<T, Tresult>(T item);

So, whatever approach your linked answer used to mimic anonymous interfaces can be represented using .NET 2.0 delegates, at the expense of added verbosity. Making you ask yourself: "is this really that shorter to write?"

[Update]

If your interface is a single method interface, like:

interface IFoo
{
    string Bar(int value);
}

class SomeOtherClass
{
    void DoSomething(IFoo foo);
}

then you might get rid of it entirely and simply use a delegate instead:

class SomeOtherClass
{
    void DoSomething(Func<int, string> bar);
}

new SomeOtherClass().DoSomething(delegate(int i) { return i.ToString(); });

If you have an interface with many methods that you want to be able to implement inline in many different places, you can use something like this:

interface IFoo
{
    string GetSomething();
    void DoSomething(int value);
}

// conditional compile, only if .NET 2.0
#if NET_2_0
public delegate void Action<T>(T item);
public delegate Tresult Func<Tresult>();
#endif

class DelegatedFoo : IFoo
{
    private readonly Func<string> _get;
    private readonly Action<int> _do;

    public DelegatedFoo(Func<string> getStuff, Action<int> doStuff)
    {
        _get = getStuff;
        _do = doStuff;
    }

    #region IFoo members simply invoke private delegates

    public string GetSomething()
    { return _get(); }

    public void DoSomething(int value)
    { _do(value); }

    #endregion
}

Which would allow you to pass delegates to the DelegatedFoo class inline:

var delegated = new DelegatedFoo(
    delegate() { return ""; }, // string GetSomething()
    delegate(int i) { }        // void DoSomething(int)
);

Using .NET 4 the C# 4.0 syntax it would look a bit cleaner due to syntactic sweetness of lambdas and named parameters:

var delegated = new DelegatedFoo(
    getStuff: () => "",
    doStuff:  i => { }
);