Access modifiers on interface members in C#

Question

I am getting a compile error from the following property.
The error is:

"The modifier 'public' is not valid for this item"

public System.Collections.Specialized.StringDictionary IWorkItemControl.Properties
{
    get { return properties; }
    set { properties = value; }
}

but if I remove the IWorkItemControl it compiles fine.

Why am I getting this error and what is the difference of having / not having the interface name in the signature?

Answer 1

Explicit interface implementation does not let you specify any access modifiers. When you implement an interface member explicitly (by specifying the interface name before the member name), you can access that member only using that interface. Basically, if you do:

System.Collections.Specialized.StringDictionary IWorkItemControl.Properties
{
    get { return properties; }
    set { properties = value; }
}

You can't do:

MyClass x = new MyClass();
var test = x.Properties; // fails to compile
// You should do:
var test = ((IWorkItemControl)x).Properties; // accessible through the interface

There are several use cases for EII. For example, you want to provide a Close method for your class to free up acquired resources but you still want to implement IDisposable. You could do:

class Test : IDisposable {
    public void Close() {
        // Frees up resources
    }
    void IDisposable.Dispose() {
        Close();
    }
}

This way, the consumers of class can only call Close directly (and they won't even see Dispose in Intellisense list) but you can still use the Test class wherever an IDisposable is expected (e.g. in a using statement).

Another use case for EII is providing different implementations of an identically named interface member for two interfaces:

interface IOne {
   bool Property { get; }
}

interface ITwo {
   string Property { get; }
}

class Test : IOne, ITwo {
   bool IOne.Property { ... }
   string ITwo.Property { ... }
}

As you see, without EII it's not even possible to implement both interfaces of this example in a single class (as the properties differ just in return type). In other cases, you might want to intentionally provide different behavior for individual views of a class through different interfaces.

Answer 2

All elements of an interface must be public. After all, an interface is the public view of an object.

Since Properties is an element of an interface IWorkItemControl, it is already public, and you cannot specify its access level, even to redundantly specify that it is public.