[DllImport] Improve pinvoke for `GetDroppedFiles()` and similar.

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• [x ] The issue is specific to Raylib-cs and not raylib
• [ x] I checked there is no similar issue already reported
• [ x] My code has no errors or misuse of Raylib-cs

Issue description

In raylib.h the definition is:

RLAPI char **GetDroppedFiles(int *count);  

Currently in raylib-cs the following is used:

        [DllImport(nativeLibName, CallingConvention = CallingConvention.Cdecl)]
        public static extern IntPtr GetDroppedFiles(ref int count);

I suggest using a call more precicely matching raylib, plus a wrapper to make it easier for C# devs to use, as otherwise the C# dev needs to do this wrapper themselves.

[DllImport(nativeLibName, CallingConvention = CallingConvention.Cdecl)]
public static extern byte** GetDroppedFiles(int* count);

public static ReadOnlySpan<string> GetDroppedFiles()
{
    int count;
    var buffer = GetDroppedFiles(&count);
    var files  = new string[count];

    for (int i = 0; i < count; i++)
    {
        files[i] = Marshal.PtrToStringUTF8((IntPtr)buffer[i]);
    }

    return files;
}

Environment

N/A Win10 x64, dotnet6

[DaZombieKiller]

Not sure about returning ReadOnlySpan though.

Yeah, it might be better if it just returned a string[] directly. It would make it more obvious that it’s an allocating API, and you can always assign the result to a ReadOnlySpan<string> yourself. Ideally, Raylib would have an additional GetDroppedFilesCount API (assuming multiple calls to GetDroppedFiles aren’t allowed), so you could preallocate a buffer yourself and have the API write into it (which would be translated into something like a Span<string> parameter in the C# binding, or something like Span<Utf8CString> where Utf8CString is just a struct wrapping a byte* if you wanted absolutely minimal allocations.)

There is also the option of using the Utf8CString concept with the existing API:

[DllImport(nativeLibName, CallingConvention = CallingConvention.Cdecl)]
public static extern byte** GetDroppedFiles(int* count);

public static ReadOnlySpan<Utf8CString> GetDroppedFiles()
{
    int count;
    var buffer = GetDroppedFiles(&count);
    return new(buffer, count);
}

// Minimal implementation, real deal would have more code
// to make it more convenient to work with alongside "string"
public readonly struct Utf8CString
{
    readonly byte* address;
    public override string ToString() => Marshal.PtrToStringUTF8((IntPtr)address);
}

[jasonswearingen]

I think that requiring the user to write glue code is the best “smell test”. so in your or the examples use of raylib, if there are areas that need similar workflows. Though I don’t think there are many like that. I think the biggest pain was needing to cast everything to IntPtr which seems to be resolved in the 4.0 changes!