Use System.IO.StreamReader/StreamWriter to read/write the text files.
For example...
using System.IO;
DirectoryInfo di = new DirectoryInfo("<path to containing directory>");
FileInfo[] files = di.GetFiles();
string data = "";
foreach (FileInfo fi in files)
{
if (fi.Name.ToUpper().StartsWith("A_L_PRTD"))
{
StreamReader rd = new StreamReader(fi.FullName);
data += rd.ReadToEnd();
rd.Close();
rd.Dispose();
File.Delete(fi.FullName);
}
}
StreamWriter wr = new StreamWriter(string.Format("L_PRTD_{0}.txt", DateTime.Now.ToString("ddMMyyyy")));
wr.Write(data);
wr.Close();
wr.Dispose();