How to decide between using if/else vs try/catch?

Just to put the topic to rest (yes, the question was posed 8 months ago, but the internet always knows!), I decided to run a little test for which is more efficient if you're pretty sure you won't have an exception -- e.g., the "else" part is going to only happen 0.001% of the time. As it turns out, if you never have to catch/else anything, the try-catch is about 4% faster (on my machine, anyway). Here's the code and the accompanying results:

CASE 1: if-else:

var startTime = DateTime.Now;
int bazillion = 100000000;
int[] list = {1,2,3,4,5,6,7,8,9,10,11,12,13,14,15,16,17,18,19,20};
for (int i = 0; i < bazillion; i++)
{
    for (int k = 0; k < list.Length; k++)
    {
        if (k >= 0)
        {
            list[k] = i;
        }
        else
        {
            // do nothing.
        }
    }
}
var executionTime = DateTime.Now - startTime;
Debug.WriteLine (executionTime.TotalMilliseconds);

Execution times (milliseconds) on 5 runs: 7441.4256, 7392.4228, 7545.4316, 7531.4308, 7323.4188.
Average = 7446.82592 milliseconds

CASE 2: try-catch:

var startTime = DateTime.Now;
int bazillion = 100000000;
int[] list = { 1, 2, 3, 4, 5, 6, 7, 8, 9, 10, 11, 12, 13, 14, 15, 16, 17, 18, 19, 20 };
for (int i = 0; i < bazillion; i++)
{
    for (int k = 0; k < list.Length; k++)
    {
        try
        {
            list[k] = i;
        }
        catch (Exception e)
        {
            // do nothing
        }
    }
}
var executionTime = DateTime.Now - startTime;
Debug.WriteLine(executionTime.TotalMilliseconds);

Execution times (milliseconds) on 5 runs: 7258.4152, 7137.4083, 7070.4044, 7052.4033, 7120.4073 Average = 7127.8077 milliseconds

Conclusion (based on this rather simplistic example; actual mileage may vary, etc):
In terms of sheer numbers, if you're pretty dang sure that the exception/else case will not occur, try-catch is about 4% faster than having to execute the "if" clause every single time.

You should always use try/catch when you work with files, because the state of a file can change outside of your program.

Consider the following code bit:

if(File.Exists("file.txt"))
    File.Delete("file.txt")

The file might have been deleted by another process right after the if statement, before the Delete() call. When you try to delete it, an exception is raised.

When working with files there are also a lot more things to consider, that you might not be able to catch with ifs, for example the file is on a network connection that got unavailable, access rights that change, hard disk failure etc.

These things are outside the control of your program, so you should have exception handlers in place.

If you think the operation should normally succeed, then try/catch can be easier to read. Especially, if there are many reasons for failure (multiple catch blocks).

Otherwise, if it sometimes succeeds and sometimes fails - and does so for a specific reason, use if/else (this is known as structured exception handling).

Some people point out how exception handling with try/catch can be time consuming. I tend to read advice like that along the following lines: Don't do this in a tight inner loop, if your profiling indicates a performance problem. When coding your first draft, don't bother thinking about optimizing at this level at all!

You should never use try/catch for flow control.

Generating an exception is an extremely expensive action. If/else is much faster and cleaner.