Continuing along through the .NET Exception Handling series, today we're going to examine the ever-popular System.IO.FileNotFoundException . The System.IO.FileNotFoundException is common because, as the name suggests, it primarily rears its head when attempting to access a file that doesn't exist.

In this article, we'll dive into where System.IO.FileNotFoundException fits into the .NET exception hierarchy, look at when System.IO.FileNotFoundExceptions typically appear, and see how to handle them if you run into one yourself. Let's get crackin'!

The Technical Rundown

When Should You Use It?

The most common method of accidentally throwing System.IO.FileNotFoundException is when manipulating a file that doesn't exist, either due to an incorrect path or otherwise. For example, let's first take a simple code snippet where we've defined two methods, WriteLineToFile() and ReadLineFromFile() , both of which perform the function their name describes. We've also ensured we aren't producing any exceptions by using a try-catch block and catching FileNotFoundExceptions :

using System;

using System.IO;
namespace ConsoleApplication

    public class FileNotFoundExceptionExample

        public static void Main(string[] args)

            WriteLineToFile(@"names.txt", "Jane Doe");

            ReadLineFromFile(@"names.txt");

        private static void ReadLineFromFile(string fileName)

            FileStream fs = null;

            string line = null;
                // Opening file stream

                fs = new FileStream(fileName, FileMode.Open);

                using (StreamReader reader = new StreamReader(fs))

                    // Read first line

                    line = reader.ReadLine();

                    Console.WriteLine($"Reading first line: {line}");

            catch(FileNotFoundException exception)

                LogException(exception);

        private static void WriteLineToFile(string fileName, string line)

            FileStream fs = null;
                // Opening file stream

                fs = new FileStream(fileName, FileMode.Append);

                using (StreamWriter writer = new StreamWriter(fs))

                    // Write line

                    writer.WriteLine(line);

                    Console.WriteLine($"Writing new line: {line}");

            catch(FileNotFoundException exception)

                LogException(exception);

        private static void LogException(Exception exception, bool expected = true)

            Console.WriteLine($"[{(expected ? "EXPECTED" : "UNEXPECTED")}] {exception.ToString()}: {exception.Message}");

Since we're taking advantage of the using code block within both our methods, in which to use our StreamWriterand StreamReader objects, respectively, we don't need to worry about calling Flush() or Dispose() methods for our reader/writer; .NET does this for us at the end of our using block. Therefore, the meat is simply in creating a new FileStream for the passed in fileName (names.txt in this case), attaching it to our respective reader or writer, then writing a new passed in line value or reading the first line of the existing file. In both cases, we output the line to the console.
 
As expected, our console output confirms that we did write our new line, and then in the call to ReadLineFromFile(), we were able to retrieve the line:
 
Writing new line: Jane Doe

Reading first line: Jane Doe

Now, what happens if we add a second call to ReadLineFromFile() within our Main() method, but we pass an invalid file name to it? Let's keep everything else we have in the previous code snippet, but our Main() method block now looks like this:
 
public static void Main(string[] args)

    WriteLineToFile(@"names.txt", "Jane Doe");

    ReadLineFromFile(@"names.txt");

    ReadLineFromFile(@"invalid.txt");

As we might've expected, running our code now behaves fine, up until it hits that second call to ReadLineFromFile(). Since the invalid.txt file doesn't exist, we catch a System.IO.FileNotFoundException:
 
[EXPECTED] System.IO.FileNotFoundException: Could not find file 'g:\dev\work\Airbrake.io\invalid.txt'.

File name: 'g:\dev\work\Airbrake.io\invalid.txt'

   at System.IO.Win32FileStream..ctor(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options, FileStream parent)

   at System.IO.Win32FileSystem.Open(String fullPath, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options, FileStream parent)

   at System.IO.FileStream.Init(String path, FileMode mode, FileAccess access, FileShare share, Int32 bufferSize, FileOptions options)

   at System.IO.FileStream..ctor(String path, FileMode mode)

   at ConsoleApplication.FileNotFoundExceptionExample.ReadLineFromFile(String fileName) in g:\dev\work\Airbrake.io\Exceptions\.NET\System.IO.FileNotFoundException\System.IO.FileNotFoundException.cs:line 24: Could not find file 'g:\dev\work\Airbrake.io\invalid.txt'.

As it happens, the solution in this case is rather simple. While arguably not a smart practice in production code, we can change the System.IO.FileMode in which our FileStream opens our file within the ReadLineFromFile() method. Originally, we were using FileMode.Open, which simply attempts to open the specified file path, and if it doesn't exist, we throw a System.IO.FileNotFoundException. Instead, if we want to avoid a System.IO.FileNotFoundException in this case, we can change it to FileMode.OpenOrCreate, like so:
 
fs = new FileStream(fileName, FileMode.OpenOrCreate);
 
While this isn't the best long-term solution (we still produce a console output with no line text to pass to it), at the least we're no longer producing our System.IO.FileNotFoundException.
 
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