An interface and an abstract class with all abstract methods both provide a blueprint for implementing classes, but there are some key differences between the two:

Inheritance: An interface can be implemented by multiple classes, whereas an abstract class can only be inherited by a single class.

Accessibility: Members of an interface are public by default, while members of an abstract class can have any accessibility level.

Implementation: In an interface, the implementing class must provide implementations for all methods, while in an abstract class, some or all methods can be left abstract and implemented by subclasses.

Properties and fields: Interfaces can only have method signatures and events, while abstract classes can have fields, properties, and instance variables.

Constructors: An interface cannot have a constructor, while an abstract class can have a constructor.

In general, you should use an interface if you want to specify a contract that can be implemented by multiple classes, and use an abstract class if you want to create a base class that provides some default behavior.