Avoid empty interfaces

Cause

The interface does not declare any members or implement two or more other interfaces.

Rule Description

Interfaces define members that provide a behavior or usage contract. The functionality described by the interface can be adopted by any type, regardless of where the type appears in the inheritance hierarchy. A type implements an interface by providing implementations for the interface's members. An empty interface does not define any members, and as such, does not define a contract that can be implemented.

If your design includes empty interfaces that types are expected to implement, you are probably using an interface as a marker, or a way of identifying a group of types. If this identification will occur at runtime, the correct way to accomplish this is to use a custom attribute. Use the presence or absence of the attribute, or the attribute's properties, to identify the target types. If the identification must occur at compile time, then using an empty interface is acceptable.

How to Fix Violations

Remove the interface or add members to it. If the empty interface is being used to label a set of types, replace the interface with a custom attribute.

When to Exclude Warnings

It is safe to exclude a warning from this rule if the interface is used to identify a set of types at compile-time.

Example

The following example shows an empty interface.

using System;

namespace DesignLibrary
{
   public interface IBadInterface  // Violates rule
   {
   }
}

The following example shows an empty interface.