You'll want to check out the Path class. Remember to reference System.IO
OpenFileDialog op = new OpenFileDialog();
op.Title = "open";
op.Filter = "Text Document(*.txt)|*.txt|All Files(*.*)|*.*";
if (op.ShowDialog() == DialogResult.OK)
{
richTextBox1.LoadFile(op.FileName, RichTextBoxStreamType.PlainText);
button1.Text = Path.GetFileNameWithoutExtension(op.FileName);
}