C# Language Design: explicit interface implementation of an event

user_196126 0
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Small question about C# language design :))

If I had an interface like this:

interface IFoo {
  int Value { get; set; }
}

It's possible to explicitly implement such interface using C# 3.0 auto-implemented properties:

sealed class Foo : IFoo {
  int IFoo.Value { get; set; }
}

But if I had an event in the interface:

interface IFoo {
  event EventHandler Event;
}

And trying to explicitly implement it using field-like event:

sealed class Foo : IFoo {
  event EventHandler IFoo.Event;
}

I will get the following compiler error:

error CS0071: An explicit interface implementation of an event must use event accessor syntax

I think that field-like events is the some kind of dualism for auto-implemented properties.

So my question is: what is the design reason for such restriction done?

explicit-implementation events interface c#
4 Answers

Interesting question. I did some poking around the language notes archive and I discovered that this decision was made on the 13th of October, 1999, but the notes do not give a justification for the decision.

Off the top of my head I don't see any theoretical or practical reason why we could not have field-like explicitly implemented events. Nor do I see any reason why we particularly need to. This may have to remain one of the mysteries of the unknown.

user_886560

When explicitly implementing an event that was declared in an interface, you must use manually provide the add and remove event accessors that are typically provided by the compiler. The accessor code can connect the interface event to another event in your class or to its own delegate type.

For example, this will trigger error CS0071:

public delegate void MyEvent(object sender);

interface ITest
{
    event MyEvent Clicked;
}

class Test : Itest
{
    event MyEvent ITest.Clicked;  // CS0071
    public static void Main() { }
}

The correct way would be:

public delegate void MyEvent(object sender);

interface ITest
{
    event MyEvent Clicked;
}

class Test : Itest
{
    private MyEvent clicked;

    event MyEvent Itest.Clicked
    {
        add
        {
            clicked += value;
        }

        remove
        {
            clicked -= value;
        }
    }

    public static void Main() { }
}

see Compiler Error CS0071

user_713800
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