do exceptions reduce performance?

Question

My application traverses a directory tree and in each directory it tries to open a file with a particular name (using File.OpenRead()). If this call throws FileNotFoundException then it knows that the file does not exist. Would I rather have a File.Exists() call before that to check if file exists? Would this be more efficient?

Answer 1

Update

I ran these two methods in a loop and timed each:

void throwException()
{
    try
    {
        throw new NotImplementedException();
    }
    catch
    {
    }
}

void fileOpen()
{
    string filename = string.Format("does_not_exist_{0}.txt", random.Next());
    try
    {
        File.Open(filename, FileMode.Open);
    }
    catch
    {
    }
}

void fileExists()
{
    string filename = string.Format("does_not_exist_{0}.txt", random.Next());
    File.Exists(filename);
}

Random random = new Random();

These are the results without the debugger attached and running a release build :

Method          Iterations per second
throwException                  10100
fileOpen                         2200
fileExists                      11300

The cost of a throwing an exception is a lot higher than I was expecting, and calling FileOpen on a file that doesn't exist seems much slower than checking the existence of a file that doesn't exist.

In the case where the file will often not be present it appears to be faster to check if the file exists. I would imagine that in the opposite case - when the file is usually present you will find it is faster to catch the exception. If performance is critical to your application I suggest that you benchmark both apporaches on realistic data.

As mentioned in other answers, remember that even in you check for existence of the file before opening it you should be careful of the race condition if someone deletes the file after your existence check but just before you open it. You still need to handle the exception.