Which code snippet will give better performance? The below code segments were written in C#.
1.
for(int counter=0; counter<list.Count; counter++)
{
list[counter].DoSomething();
}
2.
foreach(MyType current in list)
{
current.DoSomething();
}
Well, it partly depends on the exact type of list
. It will also depend on the exact CLR you're using.
Whether it's in any way significant or not will depend on whether you're doing any real work in the loop. In almost all cases, the difference to performance won't be significant, but the difference to readability favours the foreach
loop.
I'd personally use LINQ to avoid the "if" too:
foreach (var item in list.Where(condition))
{
}
EDIT: For those of you who are claiming that iterating over a List<T>
with foreach
produces the same code as the for
loop, here's evidence that it doesn't:
static void IterateOverList(List<object> list)
{
foreach (object o in list)
{
Console.WriteLine(o);
}
}
Produces IL of:
.method private hidebysig static void IterateOverList(class [mscorlib]System.Collections.Generic.List`1<object> list) cil managed
{
// Code size 49 (0x31)
.maxstack 1
.locals init (object V_0,
valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<object> V_1)
IL_0000: ldarg.0
IL_0001: callvirt instance valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<!0> class [mscorlib]System.Collections.Generic.List`1<object>::GetEnumerator()
IL_0006: stloc.1
.try
{
IL_0007: br.s IL_0017
IL_0009: ldloca.s V_1
IL_000b: call instance !0 valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<object>::get_Current()
IL_0010: stloc.0
IL_0011: ldloc.0
IL_0012: call void [mscorlib]System.Console::WriteLine(object)
IL_0017: ldloca.s V_1
IL_0019: call instance bool valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<object>::MoveNext()
IL_001e: brtrue.s IL_0009
IL_0020: leave.s IL_0030
} // end .try
finally
{
IL_0022: ldloca.s V_1
IL_0024: constrained. valuetype [mscorlib]System.Collections.Generic.List`1/Enumerator<object>
IL_002a: callvirt instance void [mscorlib]System.IDisposable::Dispose()
IL_002f: endfinally
} // end handler
IL_0030: ret
} // end of method Test::IterateOverList
The compiler treats arrays differently, converting a foreach
loop basically to a for
loop, but not List<T>
. Here's the equivalent code for an array:
static void IterateOverArray(object[] array)
{
foreach (object o in array)
{
Console.WriteLine(o);
}
}
// Compiles into...
.method private hidebysig static void IterateOverArray(object[] 'array') cil managed
{
// Code size 27 (0x1b)
.maxstack 2
.locals init (object V_0,
object[] V_1,
int32 V_2)
IL_0000: ldarg.0
IL_0001: stloc.1
IL_0002: ldc.i4.0
IL_0003: stloc.2
IL_0004: br.s IL_0014
IL_0006: ldloc.1
IL_0007: ldloc.2
IL_0008: ldelem.ref
IL_0009: stloc.0
IL_000a: ldloc.0
IL_000b: call void [mscorlib]System.Console::WriteLine(object)
IL_0010: ldloc.2
IL_0011: ldc.i4.1
IL_0012: add
IL_0013: stloc.2
IL_0014: ldloc.2
IL_0015: ldloc.1
IL_0016: ldlen
IL_0017: conv.i4
IL_0018: blt.s IL_0006
IL_001a: ret
} // end of method Test::IterateOverArray
Interestingly, I can't find this documented in the C# 3 spec anywhere...