Trying to understand exceptions in C#

Question

I do not really use any try/catches in my code ever but I\'m trying to break that habit and now get in to using exceptions.

I figure the most important place to have

Answer 1

I'm not quite sure why you're not getting your custom error message. That should be happening (unless it's something else throwing a KeyNotFoundException, not the one that you're explicitly throwing).

Also, generally you should put all the code that relies on the file read being successful inside the try, which is often the rest of the body of your method. You no longer would need a return inside your catch block, and subsequent code that doesn't rely on the file read being successful could still execute after a failure.

Example:

public static void Main()
{
    var filename = "whatever";

    try
    {
        personsReader.Read(filename, persons);
        var result = personsReader.DoSomethingAfterReading();
        result.DoSomethingElse();
    }
    catch (KeyNotFoundException e)
    {
        MessageBox.Show(e.Message);
    }
    finally
    {
        personsReader.CloseIfYouNeedTo();
    }

    DoSomeUnrelatedCodeHere();
}

And the reason it's good practice not to catch any old Exception e is because you only want to catch and handle the exceptions you're expecting to get. If you get a different kind of exception that you weren't expecting to get, typically this means that something novel failed in a way you didn't anticipate, and you want this behavior to be noticeable, not just get swept under the rug with all the regular error-handling code.

A lot of production-level systems will have one big try/catch around the entire program that catches any exception and performs logging and cleanup before crashing gracefully. This is complemented by having specific try/catch blocks deeper inside the code that handle expected exceptions in a well-defined manner. For unexpected exceptions, you could always just let the CLR bomb ungracefully and figure out what happened from that.

Here's an example of a novel exception. What if something goes terribly wrong and in this line:

IEnumerable person = xReader.Descendants("Person").Elements();

...you get an OutOfMemoryException? Should you really just display a popup to the user and allow your program to try to carry on like normal, even though there's simply no way it will be able to? And what if, because you failed silently on an OutOfMemoryException, you later attempt to dereference a null reference, and get a NullReferenceException that causes your program to crash? You'll have a devil of a time trying to track down the root cause of why that reference was null.

Best way to suss out a bug is to fail fast and fail noisily.