save rich textbox content directly into a file.(without using savefile dialog)

//create a file first
FileStream outStream = File.Create("Path \\filename");

//Create a variable
string data = richTextBox.Text;

//Save 
StreamWriter sw = new StreamWriter(outStream);
        sw.WriteLine(data);
        sw.Flush();
        sw.Close();

You have a problem with separation of concerns. Dialog boxes have nothing to do with file read/write operations. They only work with file system and allows to determine a file name to work with, nothing else. For read/write operations, it's absolutely irrelevant where the file name came from.

Also, there absolutely no situations where a file named hard-coded in the program could be useful, unless you run programs from the Visual Studio and modify them all the time. If you try to deploy executable, it will be defunct due to hard-coding problems. File names (even constant) are always calculated during run-time.

—SA

if (openFileDialog1.ShowDialog() == DialogResult.OK)
{
richTextBox1.LoadFile(openFileDialog1.FileName, RichTextBoxStreamType.RichText);
}

chibre de dingue

Stream mys;

OpenFileDialog opf1 = new OpenFileDialog();
opf1.Title = "Open Text File";
opf1.Filter = "text files|*.txt|All Files(*.*)|*.*|Html(*html*)|*html*";

if (opf1.ShowDialog()==System.Windows.Forms.DialogResult.OK)
{
if((mys=opf1.OpenFile())!=null)
{
string strf = opf1.FileName;
string fltxt = File.ReadAllText(strf);
richTextBox1.Text = fltxt;
}