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Explicit Interface Implementation

Why do you need explicit interface implmenation?
1. Prevent the external "interface" of the class from being polluted with interface implementations. In other words, to avoid seeing long method lists when you press a "." in VS.NET
2. Avoid name collision when there more than one of the interfaces being implemented have a method with exactly the same signature.
3. Simulate covariant return types. Allow overrides in derived classes which return a derived type instead of a base type.
Ex: IEnmerable.GetEnumerator implementations, where the generic object returning version is hidden, in preference to the type specific version.
4. When you have to implement a public class, and you have to implement an interface in that class that is ‘internal’.
Design Guidelines (from Brad Abrams)
  • Do not use explicit members as a security boundary. They can be called by any client who cast an instance to the interface
  • Do use explicit members to hide implementation details (Option 4)
  • Do use explicit members to approximate private interface implementations.  If you need to implement an interface for only infrastructure reasons and you NEVER expect developers to directly call methods on that interface from this type then implement the members explicitly to “hide” them from public view. Ex: IConvertible implemenation in the base CLR data types. (Option 1)
  • Do expose an alternative way to access any explicitly implemented members that subclasses are allowed to override
    public class FixedBase : ITest
        void ITest.SomeMethod()
        virtual protected void SomeMethod()
            // …
    public class Derived : FixedBase, ITest
        override protected void SomeMethod()
            // …
            // This would cause recursion and a stack overflow.
            // ((ITest)this).SomeMethod();
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Categories: .NET Framework
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