void Main()
{
  FindFiles(@"C:\Temp\*.txt");
  FindFiles(@"C:\Temp\*.txto");
  FindFiles(@"C:\Temp\*.txtold");
}

public void FindFiles(string fullPath)
{
  fullPath.Dump("FindFiles");

  var data = new WIN32_FIND_DATA();
  var hnd = FindFirstFile(fullPath, out data);
  data.Dump(hnd.ToString());
  
  while (FindNextFile(hnd, out data))
  {
    data.Dump();
  }
}

public const int MAX_PATH = 260;
public const int MAX_ALTERNATE = 14;

[StructLayout(LayoutKind.Sequential, CharSet = CharSet.Unicode)]
public struct WIN32_FIND_DATA
{
  public FileAttributes dwFileAttributes;
  public FILETIME ftCreationTime;
  public FILETIME ftLastAccessTime;
  public FILETIME ftLastWriteTime;
  public uint nFileSizeHigh; //changed all to uint, otherwise you run into unexpected overflow
  public uint nFileSizeLow;  //|
  public uint dwReserved0;   //|
  public uint dwReserved1;   //v
  [MarshalAs(UnmanagedType.ByValTStr, SizeConst = MAX_PATH)]
  public string cFileName;
  [MarshalAs(UnmanagedType.ByValTStr, SizeConst = MAX_ALTERNATE)]
  public string cAlternate;
}

[DllImport("kernel32.dll", BestFitMapping = false, CharSet = CharSet.Auto, ExactSpelling = false, SetLastError = true)]
public static extern IntPtr FindFirstFile(string fileName, out WIN32_FIND_DATA lpFindFileData);

[DllImport("kernel32.dll", BestFitMapping=false, CharSet=CharSet.Auto, ExactSpelling=false, SetLastError=true)]
public static extern bool FindNextFile(IntPtr hndFindFile, out WIN32_FIND_DATA lpFindFileData);