C＃中从正数到负数的最佳做法

Question

There are 2 common ways to convert positive number to negative and vice versa:

var a = -a;

and

var a = (-1)*a;

Second is preferred, as I know, but why? And is there any other best practice in converting the sign of the number (int, float, double, etc.) ?

EDIT: is there any difference in unary minus operation and multiplication to -1 ?

Answer 1

On site http://tryroslyn.azurewebsites.net/ you can see code generated by compiler.

And for:

using System;
public class C {
    public int M() {
        int a = -2;

        a = -a;

        return a;               
    }

    public int M1() {
        int a = 3;

        a = (-1) * a;

        return a;
    }
}

Compiler generates:

.class private auto ansi '<Module>'
{
} // end of class <Module>

.class public auto ansi beforefieldinit C
    extends [mscorlib]System.Object
{
    // Methods
    .method public hidebysig 
        instance int32 M () cil managed 
    {
        // Method begins at RVA 0x2050
        // Code size 4 (0x4)
        .maxstack 8

        IL_0000: ldc.i4.s -2
        IL_0002: neg
        IL_0003: ret
    } // end of method C::M

    .method public hidebysig 
        instance int32 M1 () cil managed 
    {
        // Method begins at RVA 0x2058
        // Code size 8 (0x8)
        .maxstack 2
        .locals init (
            [0] int32
        )

        IL_0000: ldc.i4.3
        IL_0001: stloc.0
        IL_0002: ldc.i4.m1
        IL_0003: ldloc.0
        IL_0004: mul
        IL_0005: stloc.0
        IL_0006: ldloc.0
        IL_0007: ret
    } // end of method C::M1

    .method public hidebysig specialname rtspecialname 
        instance void .ctor () cil managed 
    {
        // Method begins at RVA 0x206c
        // Code size 7 (0x7)
        .maxstack 8

        IL_0000: ldarg.0
        IL_0001: call instance void [mscorlib]System.Object::.ctor()
        IL_0006: ret
    } // end of method C::.ctor

} // end of class C

As you see code for method M i much simplier and shorter. Then -a is better way.

Answer 2

I don't understand why you think that the second would be the preferred method as the first one is a lot simpler and I use this method all the time. The second method is also a very common one but is not used as you would want to write the least amount of Code, but if you are planning to make everything clear... then I would prefer the Second method. You could also use Math.abs(x) if you wanted to but I would definitely prefer the first method. If you want to find out more about Math.abs, then you can find a lot of tutorials through google. Hope this resolved your question in a way. :)