How expensive are exceptions in C#?

Question

How expensive are exceptions in C#? It seems like they are not incredibly expensive as long as the stack is not deep; however I have read conflicting reports.

Is there definitive report that hasn't been rebutted?

Answer 1

Having read that exceptions are costly in terms of performance I threw together a simple measurement program, very similar to the one Jon Skeet published years ago. I mention this here mainly to provide updated numbers.

It took the program below 29914 milliseconds to process one million exceptions, which amounts to 33 exceptions per millisecond. That is fast enough to make exceptions a viable alternative to return codes for most situations.

Please note, though, that with return codes instead of exceptions the same program runs less than one millisecond, which means exceptions are at least 30,000 times slower than return codes. As stressed by Rico Mariani these numbers are also minimum numbers. In practice, throwing and catching an exception will take more time.

Measured on a laptop with Intel Core2 Duo T8100 @ 2,1 GHz with .NET 4.0 in release build not run under debugger (which would make it way slower).

This is my test code:

static void Main(string[] args)
{
    int iterations = 1000000;
    Console.WriteLine("Starting " + iterations.ToString() + " iterations...\n");

    var stopwatch = new Stopwatch();

    // Test exceptions
    stopwatch.Reset();
    stopwatch.Start();
    for (int i = 1; i <= iterations; i++)
    {
        try
        {
            TestExceptions();
        }
        catch (Exception)
        {
            // Do nothing
        }
    }
    stopwatch.Stop();
    Console.WriteLine("Exceptions: " + stopwatch.ElapsedMilliseconds.ToString() + " ms");

    // Test return codes
    stopwatch.Reset();
    stopwatch.Start();
    int retcode;
    for (int i = 1; i <= iterations; i++)
    {
        retcode = TestReturnCodes();
        if (retcode == 1)
        {
            // Do nothing
        }
    }
    stopwatch.Stop();
    Console.WriteLine("Return codes: " + stopwatch.ElapsedMilliseconds.ToString() + " ms");

    Console.WriteLine("\nFinished.");
    Console.ReadKey();
}

static void TestExceptions()
{
    throw new Exception("Failed");
}

static int TestReturnCodes()
{
    return 1;
}

Answer 2

I guess I'm in the camp that if performance of exceptions impacts your application then you're throwing WAY too many of them. Exceptions should be for exceptional conditions, not as routine error handling.

That said, my recollection of how exceptions are handled is essentially walking up the stack finding a catch statement that matches the type of the exception thrown. So performance will be impacted most by how deep you are from the catch and how many catch statements you have.

Answer 3

In my case, exceptions were very expensive. I rewrote this:

public BlockTemplate this[int x,int y, int z]
{
    get
    {
        try
        {
            return Data.BlockTemplate[World[Center.X + x, Center.Y + y, Center.Z + z]];
        }
        catch(IndexOutOfRangeException e)
        {
            return Data.BlockTemplate[BlockType.Air];
        }
    }
}

Into this:

public BlockTemplate this[int x,int y, int z]
{
    get
    {
        int ix = Center.X + x;
        int iy = Center.Y + y;
        int iz = Center.Z + z;
        if (ix < 0 || ix >= World.GetLength(0)
            || iy < 0 || iy >= World.GetLength(1)
            || iz < 0 || iz >= World.GetLength(2)) 
            return Data.BlockTemplate[BlockType.Air];
        return Data.BlockTemplate[World[ix, iy, iz]];
    }
}

And I noticed a good speed increase of about 30 seconds. This function gets called at least 32,000 times at startup. The code isn't as clear as to what the intention is, but the cost savings were huge.