Reading all text files in directory

    private void btnReadAllFiles_Click(object sender, EventArgs e)
        {
                string AllFileData = "" ;
                string[] x = Directory.GetFiles(@"d:\Files");
                foreach (string s in x)
                {
                    FileInfo f = new FileInfo(s);
                    if (f.Extension == ".txt")
                    {
                        StreamReader sr = new StreamReader(s);
                        AllFileData += sr.ReadToEnd();
                    }
                }
               textBox1.Text = AllFileData;
        }