What exception should I throw for a failed file parsing?

Question

I have a method parsing a file. However, this parsing could fail anytime, depending on various conditions (not-so-cautious user playing with the file, for example).

public string ParseDatFile(string datFile)
{
    string[] deezLines = File.ReadAllLines(datFile);

    // We're searching for an essential data inside the file.
    bool daEssentialDataFound = false;
    foreach (string datLine in deezLines)
    {
        if (datLine.Contains("daEssentialData"))
        {
            daEssentialDataFound = true;
            break;
        }
    }

    if (!daEssentialDataFound)
        throw new WhatShouldIThrowException("yo dood where's da essential data in " + datFile + "?");

    DoStuffWith(deezLines);
}

Is there an exception I could use in such a scenario? I thought about:

• FormatException: not really clear about the issue,
• Custom exception: I do not have any special treatment for the exception I'm throwing, so I'd rather avoid using a custom exception, even though that's always a way to settle things down.

Answer 1

FileFormatException should be fine :

The exception that is thrown when an input file or a data stream that is supposed to conform to a certain file format specification is malformed.

You can optionally provide the uri and a descriptive error message.

---

If you don't want to reference WindowsBase then you may create your own exception specific to your format. Based on the fact there is an XmlException thrown by XmlReader.Read.