Exception codes, or detecting a "file already exists" type exception

Question

In trying to answer this question, I was surprised to discover that attempting to create a new file when that file already exists does not throw a unique exception type, it just throws a generic IOException.

I am therefore left wondering how to determine if the IOException is the result of an existing file, or some other IO error.

The exception has an HResult, but this property is protected, and thus unavailable to me.

The only other way I can see is to pattern match the message string which feels awful.

example:

try
{
    using (var stream = new FileStream("C:\\Test.txt", FileMode.CreateNew))
    using (var writer = new StreamWriter(stream))
    {
        //write file
    }
}
catch (IOException e)
{
    //how do I know this is because a file exists?
}

Answer 1

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

Won't work for temp files etc which might have been deleted again.

Here are my exception best practices: https://coderr.io/exception-handling

Answer 2

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.
    }
}

Answer 3

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.

Answer 4

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"))
{
   //...
}