Why am I getting FileNotFoundException when using FileMode.OpenOrCreate?

Question

I am developing a simple ASP.NET WebApi application. In a controller I have a POST action that does the following code:

using (var isoStore = IsolatedStorageFile.GetUserStoreForAssembly())
{
    using (var textWriter = new StreamWriter(
        isoStore.OpenFile(filename, FileMode.OpenOrCreate | FileMode.Truncate)))
    {
        // write to file
    }
}

which throws the FileNotFoundException exception on the using line when the file does not exist (it works fine when the file exists).

When I rewrite it as follows:

using (var isoStore = IsolatedStorageFile.GetUserStoreForAssembly())
{
    StreamWriter textWriter;
    if (!isoStore.FileExists(filename))
    {
        textWriter = new StreamWriter(isoStore.CreateFile(filename));
    }
    else
    {
        textWriter = new StreamWriter(
             isoStore.OpenFile(filename, FileMode.Open | FileMode.Truncate));
    }
}

everything works fine, and I can write to the file, regardless whether it existed or not.

Isn't the purpose of OpenOrCreate to cover both cases from the second snippet in a single call?

Answer 1

FileMode enum is not a flag type enum, which means you should not perform bitwise operations on it. FileMode.OpenOrCreate | FileMode.Truncate resolves to FileMode.Truncate which expects a file to be there.

In general OpenOrCreate and Truncate are two different modes of opening a file, so you need to chose one (Create being a reasonable alternative for what you need, credit to @Damien_The_Unbeliever).