Do try/catch blocks hurt performance when exceptions are not thrown?
During a code review with a Microsoft employee we came across a large section of code inside a try{} block. She and an IT representative suggested this can have effects on performance of the code. In fact, they suggested most of the code should be outside of try/catch blocks, and that only important sections should be checked. The Microsoft employee added and said an upcoming white paper warns against incorrect try/catch blocks.
I've looked around and found it can affect optimizations, but it seems to only apply when a variable is shared between scopes.
I'm not asking about maintainability of the code, or even handling the right exceptions (the code in question needs re-factoring, no doubt). I'm also not referring to using exceptions for flow control, this is clearly wrong in most cases. Those are important issues (some are more important), but not the focus here.
How do try/catch blocks affect performance when exceptions are not thrown?
EDIT: I'm adding a bounty. There are interesting responses, but I would like to get some more input.
Check it.
static public void Main(string[] args)
{
Stopwatch w = new Stopwatch();
double d = 0;
w.Start();
for (int i = 0; i < 10000000; i++)
{
try
{
d = Math.Sin(1);
}
catch (Exception ex)
{
Console.WriteLine(ex.ToString());
}
}
w.Stop();
Console.WriteLine(w.Elapsed);
w.Reset();
w.Start();
for (int i = 0; i < 10000000; i++)
{
d = Math.Sin(1);
}
w.Stop();
Console.WriteLine(w.Elapsed);
}
Output:
00:00:00.4269033 // with try/catch
00:00:00.4260383 // without.
In milliseconds:
449
416
New code:
for (int j = 0; j < 10; j++)
{
Stopwatch w = new Stopwatch();
double d = 0;
w.Start();
for (int i = 0; i < 10000000; i++)
{
try
{
d = Math.Sin(d);
}
catch (Exception ex)
{
Console.WriteLine(ex.ToString());
}
finally
{
d = Math.Sin(d);
}
}
w.Stop();
Console.Write(" try/catch/finally: ");
Console.WriteLine(w.ElapsedMilliseconds);
w.Reset();
d = 0;
w.Start();
for (int i = 0; i < 10000000; i++)
{
d = Math.Sin(d);
d = Math.Sin(d);
}
w.Stop();
Console.Write("No try/catch/finally: ");
Console.WriteLine(w.ElapsedMilliseconds);
Console.WriteLine();
}
New results:
try/catch/finally: 382
No try/catch/finally: 332
try/catch/finally: 375
No try/catch/finally: 332
try/catch/finally: 376
No try/catch/finally: 333
try/catch/finally: 375
No try/catch/finally: 330
try/catch/finally: 373
No try/catch/finally: 329
try/catch/finally: 373
No try/catch/finally: 330
try/catch/finally: 373
No try/catch/finally: 352
try/catch/finally: 374
No try/catch/finally: 331
try/catch/finally: 380
No try/catch/finally: 329
try/catch/finally: 374
No try/catch/finally: 334
After seeing all the stats for with try/catch and without try/catch, curiosity forced me to look behind to see what is generated for both the cases. Here is the code:
C#:
private static void TestWithoutTryCatch(){
Console.WriteLine("SIN(1) = {0} - No Try/Catch", Math.Sin(1));
}
MSIL:
.method private hidebysig static void TestWithoutTryCatch() cil managed
{
// Code size 32 (0x20)
.maxstack 8
IL_0000: nop
IL_0001: ldstr "SIN(1) = {0} - No Try/Catch"
IL_0006: ldc.r8 1.
IL_000f: call float64 [mscorlib]System.Math::Sin(float64)
IL_0014: box [mscorlib]System.Double
IL_0019: call void [mscorlib]System.Console::WriteLine(string,
object)
IL_001e: nop
IL_001f: ret
} // end of method Program::TestWithoutTryCatch
C#:
private static void TestWithTryCatch(){
try{
Console.WriteLine("SIN(1) = {0}", Math.Sin(1));
}
catch (Exception ex){
Console.WriteLine(ex);
}
}
MSIL:
.method private hidebysig static void TestWithTryCatch() cil managed
{
// Code size 49 (0x31)
.maxstack 2
.locals init ([0] class [mscorlib]System.Exception ex)
IL_0000: nop
.try
{
IL_0001: nop
IL_0002: ldstr "SIN(1) = {0}"
IL_0007: ldc.r8 1.
IL_0010: call float64 [mscorlib]System.Math::Sin(float64)
IL_0015: box [mscorlib]System.Double
IL_001a: call void [mscorlib]System.Console::WriteLine(string,
object)
IL_001f: nop
IL_0020: nop
IL_0021: leave.s IL_002f //JUMP IF NO EXCEPTION
} // end .try
catch [mscorlib]System.Exception
{
IL_0023: stloc.0
IL_0024: nop
IL_0025: ldloc.0
IL_0026: call void [mscorlib]System.Console::WriteLine(object)
IL_002b: nop
IL_002c: nop
IL_002d: leave.s IL_002f
} // end handler
IL_002f: nop
IL_0030: ret
} // end of method Program::TestWithTryCatch
I'm not an expert in IL but we can see that an local exception object is created on fourth line .locals init ([0] class [mscorlib]System.Exception ex) after that things are pretty same as for method without try/catch till the line seventeen IL_0021: leave.s IL_002f . If an exception occurs the control jumps to line IL_0025: ldloc.0 otherwise we jump to label IL_002d: leave.s IL_002f and function returns.
I can safely assume that if no exceptions occur then it is the overhead of creating local variables to hold exception objects only and a jump instruction.
不可以。如果try / finally块排除的微小优化实际上会对程序产生可衡量的影响，那么您可能不应该首先使用.NET。
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