【C#】抽象类和接口的区别

namespace cs {
    public partial class Program {
        public abstract class A {
            // static members
            // abstract and virtual modifiers are not allowed on fields
            public static int P1SF;
            protected internal static int PISF;
            internal static int InSF;
            protected static int P2SF;
            private static int P3SF;
            // You cannot use abstract / virtual modifier on static properties of an abstract class
            public static int P1S_P { get; set; }
            protected internal static int PIS_P { get; set; }
            internal static int InS_P { get; set; }
            protected static int P2S_P { get; set; }
            private static int P3S_P { get; set; }
            // You cannot use abstract / virtual modifier on static methods of an abstract class
            public static void P1S_M() { }
            protected internal static void PIS_M() { }
            internal static void InS_M() { }
            protected static void P2S_M() { }
            private static void P3S_M() { }
            // instance members
            // abstract and virtual modifiers are not allowed on fields
            public int P1_F;
            protected internal int PI_F;
            internal int In_F;
            protected int P2_F;
            private int P3_F;
            public int P1__P { get; set; }
            protected internal int PI__P { get; set; }
            internal int In__P { get; set; }
            protected int P2__P { get; set; }
            private int P3__P { get; set; }
            public abstract int P1_AP { get; set; }
            protected internal abstract int PI_AP { get; set; }
            internal abstract int In_AP { get; set; }
            protected abstract int P2_AP { get; set; }
            // abstract members cannot be private
            public virtual int P1_VP { get; set; }
            protected internal virtual int PI_VP { get; set; }
            internal virtual int In_VP { get; set; }
            protected virtual int P2_VP { get; set; }
            // virtual members cannot be private
            public void P1__M() { }
            protected internal void PI__M() { }
            internal void In__M() { }
            protected void P2__M() { }
            private void P3__M() { }
            public abstract void P1_AM();
            protected internal abstract void PI_AM();
            internal abstract void In_AM();
            protected abstract void P2_AM();
            // abstract members cannot be private
            public virtual void P1_VM() { }
            protected internal virtual void PI_VM() { }
            internal virtual void In_VM() { }
            protected virtual void P2_VM() { }
            // virtual members cannot be private
        }
        public interface I {
            // static members
            // abstract and virtual modifiers are not allowed on fields
            public static int P1SF;
            protected internal static int PISF;
            internal static int InSF;
            protected static int P2SF;
            private static int P3SF;
            // In C# 11 (.NET 7), you can use abstract / virtual modifier on static properties of an interface
            public static int P1S_P { get; set; }
            protected internal static int PIS_P { get; set; }
            internal static int InS_P { get; set; }
            protected static int P2S_P { get; set; }
            private static int P3S_P { get; set; }
            public static abstract int P1SAP { get; set; }
            protected internal static abstract int PISAP { get; set; }
            internal static abstract int InSAP { get; set; }
            protected static abstract int P2SAP { get; set; }
            public static virtual int P1SVP { get; set; }
            protected internal static virtual int PISVP { get; set; }
            internal static virtual int InSVP { get; set; }
            protected static virtual int P2SVP { get; set; }
            // In C# 11 (.NET 7), you can use abstract / virtual modifier on static methods of an interface
            public static void P1S_M() { }
            protected internal static void PIS_M() { }
            internal static void InS_M() { }
            protected static void P2S_M() { }
            private static void P3S_M() { }
            public static abstract void P1SAM();
            protected internal static abstract void PISAM();
            internal static abstract void InSAM();
            protected static abstract void P2SAM();
            public static virtual void P1SVM() { }
            protected internal static virtual void PISVM() { }
            internal static virtual void InSVM() { }
            protected static virtual void P2SVM() { }
            // instance members
            // interfaces cannot have instance fields
            public int P1__P { get; set; }
            protected internal int PI__P { get; set; }
            internal int In__P { get; set; }
            protected int P2__P { get; set; }
            private int P3__P { get { return 0; } set { } }
            public abstract int P1_AP { get; set; }
            protected internal abstract int PI_AP { get; set; }
            internal abstract int In_AP { get; set; }
            protected abstract int P2_AP { get; set; }
            // abstract members cannot be private
            public virtual int P1_VP { get { return 0; } set { } }
            protected internal virtual int PI_VP { get { return 0; } set { } }
            internal virtual int In_VP { get { return 0; } set { } }
            protected virtual int P2_VP { get { return 0; } set { } }
            // virtual members cannot be private
            public void P1__M() { }
            protected internal void PI__M() { }
            internal void In__M() { }
            protected void P2__M() { }
            private void P3__M() { }
            public abstract void P1_AM();
            protected internal abstract void PI_AM();
            internal abstract void In_AM();
            protected abstract void P2_AM();
            // abstract members cannot be private
            public virtual void P1_VM() { }
            protected internal virtual void PI_VM() { }
            internal virtual void In_VM() { }
            protected virtual void P2_VM() { }
            // virtual members cannot be private
        }
        public class C : A {
            public override int P1_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            protected override int P2_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            protected internal override int PI_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            internal override int In_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            public override void P1_AM() => throw new NotImplementedException();
            protected override void P2_AM() => throw new NotImplementedException();
            protected internal override void PI_AM() => throw new NotImplementedException();
            internal override void In_AM() => throw new NotImplementedException();
        }
        public class D : I {
            static int I.P1SAP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            static int I.PISAP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            static int I.InSAP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            static int I.P2SAP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.P1__P { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.PI__P { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.In__P { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.P2__P { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.P1_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.PI_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.In_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.P2_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            static void I.InSAM() => throw new NotImplementedException();
            static void I.P1SAM() => throw new NotImplementedException();
            static void I.P2SAM() => throw new NotImplementedException();
            static void I.PISAM() => throw new NotImplementedException();
            void I.In_AM() => throw new NotImplementedException();
            void I.P1_AM() => throw new NotImplementedException();
            void I.P2_AM() => throw new NotImplementedException();
            void I.PI_AM() => throw new NotImplementedException();
        }
        public class E : A, I {
            public override int P1_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            protected override int P2_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            protected internal override int PI_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            internal override int In_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            public override void P1_AM() => throw new NotImplementedException();
            protected override void P2_AM() => throw new NotImplementedException();
            protected internal override void PI_AM() => throw new NotImplementedException();
            internal override void In_AM() => throw new NotImplementedException();
            static int I.P1SAP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            static int I.PISAP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            static int I.InSAP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            static int I.P2SAP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.PI__P { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.In__P { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.P2__P { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.PI_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.In_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            int I.P2_AP { get => throw new NotImplementedException(); set => throw new NotImplementedException(); }
            static void I.P1SAM() => throw new NotImplementedException();
            static void I.InSAM() => throw new NotImplementedException();
            static void I.P2SAM() => throw new NotImplementedException();
            static void I.PISAM() => throw new NotImplementedException();
            void I.In_AM() => throw new NotImplementedException();
            void I.P2_AM() => throw new NotImplementedException();
            void I.PI_AM() => throw new NotImplementedException();
        }
        public static void Main(string[] args) {
            C c = new();
            D d = new();
            E e = new();
            Console.WriteLine(c);
            Console.WriteLine(d);
            Console.WriteLine(e);
        }
    }
}

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相同点

• abstract 和 virtual 关键字均不可修饰字段，但可以修饰属性和方法。
• abstract 和 virtual 成员不能是 private 访问控制。
• 对于每个成员，abstract 和 virtual 关键字只能使用一个，或者都不使用。
• 均不可实例化。

不同点

• abstract 和 virtual 关键字均不可修饰抽象类的静态属性和方法，但自 C# 11 起，可以修饰接口的静态属性和方法。故而，接口的非 private 静态抽象属性和非 private 静态抽象方法均必须实现，否则编译不通过。
• 在抽象类中，不被 abstract 修饰但省略了方法体的方法会令编译报错（要求添加 abstract，extern 或 partial 关键字）。而在接口中，不被 abstract 修饰但省略了方法体（不具有默认实现）的方法本身不会令编译报错，但必须提供这些方法的实现。
• 在抽象类中，实现 abstract 属性和 abstract 方法时，需要使用 override 关键字。但在接口中，实现属性和方法时，不需要使用 override 关键字，即便该方法被 abstract 修饰。
• 抽象类可以含有实例字段，但接口不可含有实例字段。
• 抽象类的非 private 一般实例属性（非 abstract 且非 virtual）不要求必须实现，而接口的非 private 一般实例属性必须提供实现，否则编译不通过。
• 抽象类的 private 一般实例属性及 virtual 的实例属性可以使用默认的 get accessor 和 set accessor，但接口的 private 一般实例属性及 virtual 的实例属性不可使用默认的 get accessor 和 set accessor。
• 一个类只能继承单一的父类（包括抽象类），但可以实现多个接口。
• 虽然在很多时候将抽象类和接口互换并不影响程序的行为，然而习惯上，抽象类及其子类描述一个实体，例如System.IO.Stream, Microsoft.AspNetCore.Mvc.Controller；而接口要求其实现类具有某个行为或性质，例如：System.Collections.IEnumerable, System.IComparable。