C# OpenFileDialog with multiselect OFN_ALLOWMULTISELECT lpstrFile

OpenFileDialog.cs

var ofn = new NativeMethods.OpenFileName();
ofn.lStructSize = Marshal.SizeOf(ofn);
if (Filter != null)
    ofn.lpstrFilter = Filter.Replace('|', '\0') + "\0\0";
var maxFilePathLength = 250;
var maxFilesForMultiselect = 100;
ofn.nMaxFile = 1 * maxFilePathLength;
if (AllowMultiSelect)
{
    ofn.Flags |= (
        NativeMethods.OpenFileName.DialogFlags.OFN_ALLOWMULTISELECT |
        NativeMethods.OpenFileName.DialogFlags.OFN_EXPLORER |
        NativeMethods.OpenFileName.DialogFlags.OFN_PATHMUSTEXIST |
        NativeMethods.OpenFileName.DialogFlags.OFN_FILEMUSTEXIST
        );
    ofn.nMaxFile = maxFilesForMultiselect * maxFilePathLength;
}
try
{
    ofn.lpstrFileTitle = new string(new char[ofn.nMaxFile]);
    ofn.nMaxFileTitle = ofn.nMaxFile;
    ofn.lpstrFile = Marshal.AllocHGlobal(ofn.nMaxFile * Marshal.SystemDefaultCharSize);
    // Initialize buffer with NULL bytes
    for (int i = 0; i < ofn.nMaxFile * Marshal.SystemDefaultCharSize; i++)
        Marshal.WriteByte(ofn.lpstrFile, i, 0);
    ofn.lpstrInitialDir = InitialDirectory;
    ofn.lpstrTitle = Text;
    ofn.lpstrDefExt = "txt";
    if (NativeMethods.GetOpenFileName(ofn))
    {
        DialogResult = DialogResult.OK;
        List<string> selectedFilesList = new List<string>();
        IntPtr filePointer = ofn.lpstrFile;
        long pointer = (long)filePointer;
        string file = Marshal.PtrToStringAuto(filePointer);
        // Retrieve file names
        while (file.Length > 0)
        {
            selectedFilesList.Add(file);
            pointer += file.Length * Marshal.SystemDefaultCharSize + Marshal.SystemDefaultCharSize;
            filePointer = (IntPtr)pointer;
            file = Marshal.PtrToStringAuto(filePointer);
        }
        if (selectedFilesList.Count == 1)
        {
            FileName = selectedFilesList[0];
            FileNamesMultiSelect = new string[] { FileName };
        }
        else
        {
            FileNamesMultiSelect = new string[selectedFilesList.Count - 1];
            for (int i = 0; i < FileNamesMultiSelect.Length; i++)
            {
                FileNamesMultiSelect[i] = Path.Combine(selectedFilesList[0], selectedFilesList[i + 1]);
            }
            FileName = FileNamesMultiSelect[0];
        }
    }
}
finally
{
    Marshal.FreeHGlobal(ofn.lpstrFile);
}