[No0000B7]If else 與 三元表達式? : 效率對比

先看 if else 一段代碼

using System;

class Program
{
    private static void Main()
    {
        int i = 0;

        if (i == 0) i = -1;
        else i = -2;

        Console.WriteLine(i);
    }
}

輸出 -1

用IL DASM （"C:\Program Files (x86)\Microsoft SDKs\Windows\v10.0A\bin\NETFX 4.6.1 Tools\ildasm.exe"vs2015 up3，項目框架.NET Framework 4.5.2）打開

.method private hidebysig static void Main() cil managed
{
    .entrypoint
    // 代碼大小 26 (0x1a)
    .maxstack 2
    .locals init ([0] int32 i,
    [1] bool V_1)
    IL_0000: nop
    IL_0001: ldc.i4.0
    IL_0002: stloc.0
    IL_0003: ldloc.0
    IL_0004: ldc.i4.0
    IL_0005: ceq
    IL_0007: stloc.1
    IL_0008: ldloc.1
    IL_0009: brfalse.s IL_000f
    IL_000b: ldc.i4.m1
    IL_000c: stloc.0
    IL_000d: br.s IL_0012
    IL_000f: ldc.i4.s -2
    IL_0011: stloc.0
    IL_0012: ldloc.0
    IL_0013: call void [mscorlib]System.Console::WriteLine(int32)
    IL_0018: nop
    IL_0019: ret
} // end of method Program::Main

在看三元表達式? :一段代碼

using System;
class Program
{
    private static void Main()
    {
        int i = 0;

        i = i == 0 ? -1 : -2;

        Console.WriteLine(i);
    }
}

輸出 -1

.method private hidebysig static void Main() cil managed
{
    .entrypoint
    // 代碼大小 20 (0x14)
    .maxstack 1
    .locals init ([0] int32 i)
    IL_0000: nop
    IL_0001: ldc.i4.0
    IL_0002: stloc.0
    IL_0003: ldloc.0
    IL_0004: brfalse.s IL_000a
    IL_0006: ldc.i4.s -2
    IL_0008: br.s IL_000b
    IL_000a: ldc.i4.m1
    IL_000b: stloc.0
    IL_000c: ldloc.0
    IL_000d: call void [mscorlib]System.Console::WriteLine(int32)
    IL_0012: nop
    IL_0013: ret
} // end of method Program::Main

明顯，執行效率不一樣。三元表達式? :執行效率更高。

using System;
using System.Diagnostics;

class Program
{
    private static void Main()
    {
        int i = 0;

        Stopwatch stopwatch = new Stopwatch();
        stopwatch.Start();
        for (int j = 0; j < 100000000; j++)
        {
            if (i == 0) i = -1;
            else i = -2;
        }
        stopwatch.Stop();

        Console.WriteLine(stopwatch.ElapsedMilliseconds);

        stopwatch.Reset();

        stopwatch.Start();
        for (int j = 0; j < 100000000; j++)
        {
            i = i == 0 ? -1 : -2;
        }
        stopwatch.Stop();

        Console.WriteLine(stopwatch.ElapsedMilliseconds);
    }
}