GOTO 与 DO WHILE 的区别

Question

以下 2 个 C# 代码片段在执行上有差异吗？

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

和

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

我想弄清楚为什么 goto 这么糟糕。谢谢

编辑：如果我添加方括号，片段非常相似。

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

Answer 1

不，我什至认为 while 是像后面那样实现的。

使用 goto 的坏处在于它会鼓励您在代码中来回切换（也称为术语 'spaghetti code'：一团糟）。它使您的代码极难阅读、调试和分析，并且会引入错误，因为您无法真正理解正在发生的事情。

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

Answer 2

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[=10=]00
    )

    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[=10=]00
    )

    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。将其反编译为 C# 时，它甚至将两个循环显示为 do while.

但有不同！ while 循环的范围是：

        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！请参阅答案。

GOTO 与 DO WHILE 的区别

GOTO versus DO WHILE differences

c#

loops

do-while

goto