C＃Switch语句：更高效不使用默认值？

Question

I was creating a C# method in visual studio that contained only a switch statement where each case returned a value. By personal habit, I put something similar to the following code:

private string SwitchMethod(int num)
    {
        switch (num)
        {
            case 0:
                return "result 1";
            case 1:
                return "result 2";
            case 2:
                return "result 3";
        }
        return "no result";
    }

My question is this: Which code will have better performance? The code above or below, or are the same? And why?

I would assume that because of compiler optimizations...they might just be the same...but I really don't know.

private string SwitchMethod(int num)
    {
        switch (num)
        {
            case 0:
                return "result 1";
            case 1:
                return "result 2";
            case 2:
                return "result 3";
            default:
                return "no result";
        }
    }

REVISION: It seems that I should be more specific: When compiled...will less efficient code be generated by one or the other?

I realize that the difference in performance may be insignificant...I'm just curious really.

Answer 1

public static string foo(int num)
        {
            switch (num)
            {
                case 0:
                    return "result 1";
                case 1:
                    return "result 2";
                case 2:
                    return "result 3";
            }
            return "no result";
        }

Becomes:

.method public hidebysig static string  foo(int32 num) cil managed
{
  // Code size       57 (0x39)
  .maxstack  1
  .locals init ([0] string CS$1$0000,
           [1] int32 CS$4$0001)
  IL_0000:  nop
  IL_0001:  ldarg.0
  IL_0002:  stloc.1
  IL_0003:  ldloc.1
  IL_0004:  switch     ( 
                        IL_0017,
                        IL_001f,
                        IL_0027)
  IL_0015:  br.s       IL_002f
  IL_0017:  ldstr      "result 1"
  IL_001c:  stloc.0
  IL_001d:  br.s       IL_0037
  IL_001f:  ldstr      "result 2"
  IL_0024:  stloc.0
  IL_0025:  br.s       IL_0037
  IL_0027:  ldstr      "result 3"
  IL_002c:  stloc.0
  IL_002d:  br.s       IL_0037
  IL_002f:  ldstr      "no result"
  IL_0034:  stloc.0
  IL_0035:  br.s       IL_0037
  IL_0037:  ldloc.0
  IL_0038:  ret
} // end of method Program::foo

Moving the return into a default case:

.method public hidebysig static string  foo(int32 num) cil managed
{
  // Code size       57 (0x39)
  .maxstack  1
  .locals init ([0] string CS$1$0000,
           [1] int32 CS$4$0001)
  IL_0000:  nop
  IL_0001:  ldarg.0
  IL_0002:  stloc.1
  IL_0003:  ldloc.1
  IL_0004:  switch     ( 
                        IL_0017,
                        IL_001f,
                        IL_0027)
  IL_0015:  br.s       IL_002f
  IL_0017:  ldstr      "result 1"
  IL_001c:  stloc.0
  IL_001d:  br.s       IL_0037
  IL_001f:  ldstr      "result 2"
  IL_0024:  stloc.0
  IL_0025:  br.s       IL_0037
  IL_0027:  ldstr      "result 3"
  IL_002c:  stloc.0
  IL_002d:  br.s       IL_0037
  IL_002f:  ldstr      "result 4"
  IL_0034:  stloc.0
  IL_0035:  br.s       IL_0037
  IL_0037:  ldloc.0
  IL_0038:  ret
} // end of method Program::foo

Exactly the same. No performance difference. I changed the "no result" to result 4 just to make sure the code was regenerated. Apparently the C# compiler optimizes it or it just ends up being equivalent.

Answer 2

It is good practice to always include a default case at the base of a switch for situations where none of the other cases are valid.

It is not an efficiency problem because if one of the previous cases is hit - your program will not check any other cases (it is the equivalent of using if/else if/else - where the final else is the default).

Hope this helps.

Answer 3

Instead of a function and a switch statement you would be better off with a generic Dictionary where the key is KofaxEnvironment and the value is the same as what you're returning from the switch. Something like:

Dictionary<KofaxEnvironment, string>

or

Dictionary<int, string>

I also wouldn't worry about performance. Correctness should be your first goal.

But, if you do stick with a switch use a default that throws an exception:

default:
    throw new ArgumentException("Serious programmer error!");

And as for performance, the difference (if any at all) between a switch default and falling through to a return will be negligible.