Can fields be declared inside interfaces?

In C#, you cannot declare fields (instance variables) inside interfaces. An interface can only contain declarations of methods, properties, events, and indexers without implementation (up to C# 8.0).

This is because an interface defines a contract, not a state.

Starting with C# 8.0, interfaces can contain default method implementations, but fields are still prohibited.

Example of an interface:

public interface IExample
{
    int Id { get; set; }
    void DoWork();
}

If you need to store state, it is implemented in classes that implement the interface.