GOTO和DO的区别

本文关键字：区别 DO GOTO | 更新日期: 2023-09-27 18:04:57

以下两个c#代码段在执行上有区别吗?

do
{
    Console.WriteLine(x.ToString());
    ++x;
} 
while (x < 7);

和

label:
{
    Console.WriteLine(x.ToString());
    ++x;
}
if (x < 7) goto label;

我想弄明白为什么情况如此糟糕。谢谢。

编辑:如果我添加括号，代码段非常相似。

EDIT2:在Visual Studio中，我点击了Go to Disassembly，我得到了以下代码:

            do
            {
00000037  nop 
                Console.WriteLine(x.ToString());
00000038  lea         ecx,[ebp-40h] 
0000003b  call        63129C98 
00000040  mov         dword ptr [ebp-48h],eax 
00000043  mov         ecx,dword ptr [ebp-48h] 
00000046  call        63148168 
0000004b  nop 
                ++x;
0000004c  inc         dword ptr [ebp-40h] 
            } 
0000004f  nop 
            while (x < 7);
00000050  cmp         dword ptr [ebp-40h],7 
00000054  setl        al 
00000057  movzx       eax,al 
0000005a  mov         dword ptr [ebp-44h],eax 
0000005d  cmp         dword ptr [ebp-44h],0 
00000061  jne         00000037

和

            label:
            {
                Console.WriteLine(x.ToString());
00000069  lea         ecx,[ebp-40h] 
0000006c  call        63129C98 
00000071  mov         dword ptr [ebp-4Ch],eax 
00000074  mov         ecx,dword ptr [ebp-4Ch] 
00000077  call        63148168 
0000007c  nop 
                ++x;
0000007d  inc         dword ptr [ebp-40h] 
            }
00000080  nop 
            if (x < 7) goto label;
00000081  cmp         dword ptr [ebp-40h],7 
00000085  setge       al 
00000088  movzx       eax,al 
0000008b  mov         dword ptr [ebp-44h],eax 
0000008e  cmp         dword ptr [ebp-44h],0 
00000092  jne         00000097 
00000094  nop 
00000095  jmp         00000068

区别是无条件跳转

GOTO和DO的区别

不，我甚至认为while在后面是这样实现的。

使用goto的缺点是它鼓励在代码中来回切换(也被称为"意大利面条代码":它是一团乱)。这会让你的代码难以阅读、调试和分析，还会带来bug，因为你无法真正理解到底发生了什么。

while的好处是，你可以理解它，编译器可以理解它，所以它可以给你很好的警告。

让我们看一下IL:

.method private hidebysig 
    instance void While () cil managed 
{
    // Method begins at RVA 0x2050
    // Code size 31 (0x1f)
    .maxstack 2
    .locals init (
        [0] int32 x,
        [1] bool CS$4$0000
    )
    IL_0000: nop
    IL_0001: ldc.i4.0
    IL_0002: stloc.0
    // loop start (head: IL_0003)
        IL_0003: nop
        IL_0004: ldloca.s x
        IL_0006: call instance string [mscorlib]System.Int32::ToString()
        IL_000b: call void [mscorlib]System.Console::WriteLine(string)
        IL_0010: nop
        IL_0011: ldloc.0
        IL_0012: ldc.i4.1
        IL_0013: add
        IL_0014: stloc.0
        IL_0015: nop
        IL_0016: ldloc.0
        IL_0017: ldc.i4.7
        IL_0018: clt
        IL_001a: stloc.1
        IL_001b: ldloc.1
        IL_001c: brtrue.s IL_0003
    // end loop
    IL_001e: ret
} // end of method Program::While
.method private hidebysig 
    instance void Goto () cil managed 
{
    // Method begins at RVA 0x207c
    // Code size 34 (0x22)
    .maxstack 2
    .locals init (
        [0] int32 x,
        [1] bool CS$4$0000
    )
    IL_0000: nop
    IL_0001: ldc.i4.0
    IL_0002: stloc.0
    // loop start (head: IL_0003)
        IL_0003: ldloca.s x
        IL_0005: call instance string [mscorlib]System.Int32::ToString()
        IL_000a: call void [mscorlib]System.Console::WriteLine(string)
        IL_000f: nop
        IL_0010: ldloc.0
        IL_0011: ldc.i4.1
        IL_0012: add
        IL_0013: stloc.0
        IL_0014: ldloc.0
        IL_0015: ldc.i4.7
        IL_0016: clt
        IL_0018: ldc.i4.0
        IL_0019: ceq
        IL_001b: stloc.1
        IL_001c: ldloc.1
        IL_001d: brtrue.s IL_0021
        IL_001f: br.s IL_0003
    // end loop
    IL_0021: ret
} // end of method Program::Goto

ILSpy事件将goto标记为循环。当将其反编译为c#时，它甚至将两个循环显示为do while。

但是有不同的!while循环的作用域是:

        int x = 0;
        do
        {
            string z = "TEST";
            Console.WriteLine(x.ToString());
            ++x;
        }
        while (x < 7);
        Console.WriteLine(z); // invalid!

但这是有效的:

        int x = 0;
    label:
        string z = "TEST";
        Console.WriteLine(x.ToString());
        ++x;
        if (x < 7) goto label;
        Console.WriteLine(z);

你应该使用它吗?不!

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