Do I really must declare B as an interface and implement the exact same 10 lines of code for each X that needs the behavIoUr of B?

是的,不是.你需要让B成为一个界面.但是,不应在所有接口实现中复制常见的方法实现.相反,他们应该进入接口B的类扩展：

public interface B {
    void MethodX();
    void MethodY();
}
public static class ExtensionsB {
    public static void MethodZ(this B b) {
        // Common implementations go here
    }
}

扩展方法提供了一种“水平”共享实现的方法,而不会让您的类继承自第二个类.扩展方法的行为就像它们是类的常规方法一样：

class X : A,B {
    public void MethodX() {...}
    public void MethodY() {...}
}
public static void Main(string[] args) {
    var x = new X();
    x.SomeMethodFromA();
    x.MethodX(); // Calls method from X
    x.MethodY(); // Calls method from X
    x.MethodZ(); // Calls method from ExtensionsB
}