params collections in C#13 and error CS4007

[Aniobodo]

public static IEnumerable<string> EnumerateFiles(string folderPath, params ReadOnlySpan<string> searchPatterns)
{
    foreach (string searchPattern in searchPatterns)
    { foreach (string filePath in Directory.EnumerateFiles(folderPath, searchPattern)) yield return filePath; }
}

Error CS4007 is thrown at build time for the method above. The message implies that the instance of the type “System.ReadOnlySpan” cannot be retained beyond the limits “await” or “yield”.
This is a serious limitation on the params collections feature of C# 13.
So, I wonder if there is a safe way out of the limitation.

[333fred]

Not if you want to use a generated iterator. You need either evaluate everything up front, and return all the results at once, or pay for the search patterns alloe. Imo, you're already paying for the iterator allocation, so the additional memory for the patterns really isn't much more.

[colejohnson66]

To elaborate: it's a memory-safety issue. Consider this code:

public void DoStuff()
{
    IEnumerable<string> files = GetFiles();
    // process 'files'
}
public IEnumerable<string> GetFiles()
{
    return EnumerateFiles("<folder>", "<search-pattern-1>", "<search-pattern-2>", "<search-pattern-3>");
}

The implicitly created ReadOnlySpan<string> in GetFiles is only valid inside GetFiles. Returning it further up the stack into DoStuff would cause the span to be pointing at garbage on the stack, which is a memory-safety no-no. It's the same problem as this:

public ref int GetInt()
{
    int i = 42;
    return ref i;
}

[timcassell]

You can use ArrayPool to rent an array for use in the iterator. Though as @333fred mentioned, you're already allocating for the iterator, so it's maybe not worth it, as you can just allocate an array.