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Compiler Error CS1721

Class 'class' cannot have multiple base classes: 'class_1' and 'class_2'

The most common cause of this error message is attempting to use multiple inheritance. A class in C# may only inherit directly from one class. However, a class can implement any number of interfaces.

Example

The following example shows one way in which CS1721 is generated:

// CS1721.cs
public class A {}
public class B {}
public class MyClass : A, B {}   // CS1721

To correct this error

The following are different ways to correct this error:

  • Make class B inherit from A, and MyClass inherit from B:

    public class A {}
public class B : A {}
public class MyClass : B {}

  • Declare B as an interface. Make MyClass inherit from the interface B, and the class A:

    public class A {}
public interface B {}
public class MyClass : A, B {}

See also