For vs foreach in c #

For vs foreach in c #

Today, we occasionally recommend that you do not use for. The readability is too poor and the performance is low, but I personally think it depends on the specific situation. After all, for keywords in most languages, readability cannot be improved due to the substitution of a keyword, most of which depends on the design and coding habits.
As for the performance, I plan to write some code to test them separately.

var arr = Enumerable.Range(0, 100000000).ToList();var sw = Stopwatch.StartNew();for(var i = 0;i < arr.Count; i++){}sw.Stop();Console.WriteLine("for loop takes : " + sw.ElapsedTicks);sw = Stopwatch.StartNew();foreach(var x in arr){}sw.Stop();Console.WriteLine("for loop takes : " + sw.ElapsedTicks);

First look at the generated IL


For Loop:

...IL_0018:  ldc.i4.0    IL_0019:  stloc.2     // iIL_001A:  br.s        IL_0022IL_001C:  nop         IL_001D:  nop         IL_001E:  ldloc.2     // iIL_001F:  ldc.i4.1    IL_0020:  add         IL_0021:  stloc.2     // iIL_0022:  ldloc.2     // iIL_0023:  ldloc.0     // arrIL_0024:  callvirt    System.Collections.Generic.List
 
  .get_CountIL_0029:  clt         IL_002B:  stloc.s     04 // CS$4$0000IL_002D:  ldloc.s     04 // CS$4$0000IL_002F:  brtrue.s    IL_001C...
 

Except that callvirt in IL_0024 triggers the method virtual Table query, there are almost no high-consumption commands. Let's take a look at the foreach IL section:

...IL_005A:  ldloc.0     // arrIL_005B:  callvirt    System.Collections.Generic.List
 
  .GetEnumeratorIL_0060:  stloc.s     05 // CS$5$0001IL_0062:  br.s        IL_006EIL_0064:  ldloca.s    05 // CS$5$0001IL_0066:  call        System.Collections.Generic.List
  
   +Enumerator.get_CurrentIL_006B:  stloc.3     // xIL_006C:  nop         IL_006D:  nop         IL_006E:  ldloca.s    05 // CS$5$0001IL_0070:  call        System.Collections.Generic.List
   
    +Enumerator.MoveNextIL_0075:  stloc.s     04 // CS$4$0000IL_0077:  ldloc.s     04 // CS$4$0000IL_0079:  brtrue.s    IL_0064IL_007B:  leave.s     IL_008CIL_007D:  ldloca.s    05 // CS$5$0001IL_007F:  constrained. System.Collections.Generic.List<>.EnumeratorIL_0085:  callvirt    System.IDisposable.DisposeIL_008A:  nop         IL_008B:  endfinally  IL_008C:  nop   ...
   
  
 

First, we can see a lot of method calls and call the MoveNext method every time within the loop body. Finally, we can see that there is end finally, which means that this loop contains a layer of try-finally. From the perspective of IL, foreach is much slower than.


Now let's look at the execution result of StopWatch.

for loop takes : 764538for loop takes : 1311252

For is almost doubled.


Conclusion: The keywords provided by the programming language exist to solve a certain problem. It is necessary to judge the specific business scenario before deciding. In terms of performance, we must speak out the data.