Exception Codes, or Detecting a "File Already Exists" Type Exception

Edit: there is another Hresult that is used when file already exists: 0x800700B7 (-2147024713) "Cannot create a file when that file already exists". Updated the code sample.

When you try to create a new file and it already exists IOException will have Hresult = 0x80070050 (-2147024816).

So you code could look like this:

try
{
    using (var stream = new FileStream("C:\\Test.txt", FileMode.CreateNew))
    using (var writer = new StreamWriter(stream))
    {
        //write file
    }
}
catch (IOException e)
{
    if (e.HResult == -2147024816 || 
        e.HResult == -2147024713)
    {
        // File already exists.
    }
}

To modify @jgauffin, in C# 6, you can use the File.Exists inside of the when clause to avoid entering the catch block and thus behaving more like an actual dedicated exception:

try
{
    using (var stream = new FileStream("C:\\Test.txt", FileMode.CreateNew))
    using (var writer = new StreamWriter(stream))
    {
        //write file
    }
}
catch (IOException e) when (File.Exists(@"C:\Text.text"))
{
   //...
}

You can place this condition in your catch statement for IOException: if(ex.Message.Contains("already exists")) { ... }. It is a hack, but it will work for all cases that a file exists, even temporary files and such.

It's not 100% foolproof (there are other reasons for an IOException), but you can at least exclude all derived exception types:

try
{
    ...
}
catch(IOException e)
{
    if (e is UnauthorizedAccessException) throw;
    if (e is DirectoryNotFoundException) throw;
    if (e is PathTooLongException) throw;
    // etc for other exceptions derived from IOException

    ... assume file exists
}

or the equivalent:

try
{
    ...
}
catch(UnauthorizedAccessException)
{
    throw;
}
catch(DirectoryNotFoundException)
{
    throw;
}
catch(PathTooLongException)
{
    throw;
}
catch(IOException e)
{
    ... assume file exists
}

As for the linked question, I'd just check for existence, prompt the user to overwrite, then use OpenOrCreate to overwrite if it exists. I think most apps work this way even if there is a theoretical risk of overwriting a file that's created just at the wrong moment.

In C# 6 and later:

const int WARN_WIN32_FILE_EXISTS = unchecked((int)0x80070050);

try
{
    ...
}
catch (IOException e) when (e.HResult == WARN_WIN32_FILE_EXISTS)
{
    ...
}

... or just when (e.HResult == -2147024816), if you're going for "quick and impenetrable". ;-)

(FWIW, the Windows-centric error code has been faithfully copied by Mono and also works on Mac/Linux.)

You can't. Unfortunatly IOExceptions are not further specified for some reason beyond my comprehension in the .NET framework.

But in case of creating a new file it is common practice to check if the file exists first. Like so:

       try
        {
            if (File.Exists("C:\\Test.txt"))
            {
                //write file

                using (var stream = new FileStream("C:\\Test.txt", FileMode.CreateNew))
                using (var writer = new StreamWriter(stream))
                {
                    //The actual writing of file

                }

            }
        }
        catch (IOException ex)
        {
            //how do I know this is because a file exists?
            Debug.Print(ex.Message);
        }

Perhaps not the answer you were looking for. But, c'est ca.