Read & Write in file.

gistfile1.cs

// Example #1 
// Read the file as one string. 
string text = System.IO.File.ReadAllText(@"C:\text.txt");

textBox1.Text = text;

/** *************************************** **/

// Example #2 
// Write to the file from string.
string txt = "محمد";

System.IO.File.WriteAllText(@"C:\text.txt", txt);