如何在C＃中使用派生返回类型覆盖方法？

我想用派生类类型覆盖虚方法 . 目前最好的方法是什么？到目前为止，我发现了两种方法：

• 对每个派生类型使用 abstract base class ;用 protected 方法桥接 .

• 将 protected 实现与 public 访问器一起使用 .

基本情况（未实施解决方案， Clone 始终返回基本类型 A1 ）：

public class A1
    {
        public int X1 { get; set; }
        public A1(int x1) { this.X1 = x1; }
        public virtual A1 Clone() { return new A1(X1); }
    }
    public class A2 : A1
    {
        public int X2 { get; set; }
        public A2(int x1, int x2) : base(x1)  { this.X2 = x2; }
        public override A1 Clone() { return new A2(X1, X2); }  //can't explicitly return A2
    }
    public class A3 : A2
    {
        public int X3 { get; set; }
        public A3(int x1, int x2, int x3) : base(x1, x2) { this.X3 = x3; }
        public override A1 Clone() { return new A3(X1, X2, X3); }  //can't explicitly return A3
    }

解决方案＃1（对于具有 protected 网桥的每个派生类型使用 abstract 基类）：

public class B1
    {
        public int X1 { get; set; }
        public B1(int x1) { this.X1 = x1; }
        public virtual B1 Clone() { return new B1(X1); }
    }
    public abstract class B2_Base : B1
    {
        public B2_Base(int x1) : base(x1) { }
        public sealed override B1 Clone() { return this.CloneAsB1(); }
        protected abstract B1 CloneAsB1();
    }
    public class B2 : B2_Base
    {
        public int X2 { get; set; }
        public B2(int x1, int x2) : base(x1) { this.X2 = x2; }
        protected sealed override B1 CloneAsB1() { return this.Clone(); }
        public new virtual B2 Clone() { return new B2(X1, X2); }  //CAN explicitly return B2
    }
    public abstract class B3_Base : B2
    {
        public B3_Base(int x1, int x2) : base(x1, x2) { }
        public sealed override B2 Clone() { return this.CloneAsB2(); }
        protected abstract B2 CloneAsB2();
    }
    public class B3 : B3_Base
    {
        public int X3 { get; set; }
        public B3(int x1, int x2, int x3) : base(x1, x2) { this.X3 = x3; }
        protected sealed override B2 CloneAsB2() { return this.Clone(); }
        public new virtual B3 Clone() { return new B3(X1, X2, X3); }  //CAN explicitly return B3
    }

解决方案＃2（使用带 public 访问器的 protected 实现）：

public class C1
    {
        public int X1 { get; set; }
        public C1(int x1) { this.X1 = x1; }
        public C1 Clone() { return this.CloneImplementation(); }
        protected virtual C1 CloneImplementation() { return new C1(X1); }
    }
    public class C2 : C1
    {
        public int X2 { get; set; }
        public C2(int x1, int x2) : base(x1) { this.X2 = x2; }
        public new C2 Clone() { return this.CloneImplementation() as C2; }  //trusts CloneImplementation to return a C2
        protected override C1 CloneImplementation() { return new C2(X1, X2); }
    }
    public class C3 : C2
    {
        public int X3 { get; set; }
        public C3(int x1, int x2, int x3) : base(x1, x2) { this.X3 = x3; }
        public new C3 Clone() { return this.CloneImplementation() as C3; }  //trusts CloneImplementation to return a C3
        protected override C1 CloneImplementation() { return new C3(X1, X2, X3); }
    }

据我所知，解决方案＃1是最严格的方法，但它需要 abstract base class 为每个派生的 class 想要替换基础 class 的返回类型 .

解决方案＃2 's simpler and easier to understand, but it has a small break in internal type safety. Specifically, each derived type' s public accessor信任其 protected 方法将返回正确的类型 . 所以可以有一个内部类型断开，例如：

public class C2 : C1
    {
        public int X2 { get; set; }
        public C2(int x1, int x2) : base(x1) { this.X2 = x2; }
        public new C2 Clone() { return this.CloneImplementation() as C2; }  //trusts CloneImplementation to return a C2
        protected override C1 CloneImplementation() { return new C1(X1); }
    }

是否有一个正确的（普遍接受的）最佳实践来覆盖派生类型的方法？