C#基础 继承和实例化

有代码如下，问输出的是多少：

class Program
{
    static void Main(string[] args)
    {
        B b = new B();
        Console.ReadKey();
    }
    class A
    {
        public A()
        {
            PrintFields();
        }
        public virtual void PrintFields() { }
    }
    class B : A
    {
        int x = ;
        int y;
        public B()
        {
            y = -;
        }
        public override void PrintFields()
        {
            Console.WriteLine("x={0},y={1}", x, y);
        }
    }
}

结果：x=1;y=0;

刚开始有点不理解，觉得输出是x=1;y=-1;然后反编译看了下IL代码，然而。。还是没看出来，IL代码如下：

.class private auto ansi beforefieldinit Program
    extends [mscorlib]System.Object
{
    .method public hidebysig specialname rtspecialname instance void .ctor() cil managed
    {
        .maxstack
        L_0000: ldarg.0
        L_0001: call instance void [mscorlib]System.Object::.ctor()
        L_0006: nop
        L_0007: ret
    }

    .method private hidebysig static void Main(string[] args) cil managed
    {
        .entrypoint
        .maxstack
        .locals init (
            [] class ConsoleApplication1.Program/B b)
        L_0000: nop
        L_0001: newobj instance void ConsoleApplication1.Program/B::.ctor()
        L_0006: stloc.0
        L_0007: call valuetype [mscorlib]System.ConsoleKeyInfo [mscorlib]System.Console::ReadKey()
        L_000c: pop
        L_000d: ret
    }

    .class auto ansi nested private beforefieldinit A
        extends [mscorlib]System.Object
    {
        .method public hidebysig specialname rtspecialname instance void .ctor() cil managed
        {
            .maxstack
            L_0000: ldarg.0
            L_0001: call instance void [mscorlib]System.Object::.ctor()
            L_0006: nop
            L_0007: nop
            L_0008: ldarg.0
            L_0009: callvirt instance void ConsoleApplication1.Program/A::PrintFields()
            L_000e: nop
            L_000f: ret
        }

        .method public hidebysig newslot virtual instance void PrintFields() cil managed
        {
            .maxstack
            L_0000: nop
            L_0001: ret
        }

    }

    .class auto ansi nested private beforefieldinit B
        extends ConsoleApplication1.Program/A
    {
        .method public hidebysig specialname rtspecialname instance void .ctor() cil managed
        {
            .maxstack
            L_0000: ldarg.0
            L_0001: ldc.i4.1
            L_0002: stfld int32 ConsoleApplication1.Program/B::x
            L_0007: ldarg.0
            L_0008: call instance void ConsoleApplication1.Program/A::.ctor()
            L_000d: nop
            L_000e: nop
            L_000f: ldarg.0
            L_0010: ldc.i4.m1
            L_0011: stfld int32 ConsoleApplication1.Program/B::y
            L_0016: ret
        }

        .method public hidebysig virtual instance void PrintFields() cil managed
        {
            .maxstack
            L_0000: nop
            L_0001: ldstr "x={0},y={1}"
            L_0006: ldarg.0
            L_0007: ldfld int32 ConsoleApplication1.Program/B::x
            L_000c: box int32
            L_0011: ldarg.0
            L_0012: ldfld int32 ConsoleApplication1.Program/B::y
            L_0017: box int32
            L_001c: call void [mscorlib]System.Console::WriteLine(string, object, object)
            L_0021: nop
            L_0022: ret
        }

        .field private int32 x

        .field private int32 y

    }
}

IL代码

最后打断点，找到了原因

原因分析：

B b = new B();

执行顺序：

1、int x = 1;int y;给x赋值1，给y赋值默认为0（int类型）

2、public B()，然后关键点来了

3、public A()，执行A里面的PrintFields();但是A里面的这个方法是个虚方法，会调用B里面重写的方法

4、public override void PrintFields()这个是B里面的然后Console.WriteLine("x={0},y={1}", x, y);结果当然就是x=1;y=0;

5、然后才回到了B的public B()，接着给y赋值y = -1;这个时候y才变了，如果这个时候打印y的值才是-1，这个时候B的实例化就完成了

关键点：实例化类的时候，会先执行继承的父类的构造函数，如果父类构造函数含有虚方法又会回调子类的重写方法，之后才回到初始类的构造函数