Display logged in user name in a Title bar in C# winform app

Put this static field in your login form:

  internal static string UserName = "";

Now change your checkvaliduser method slightly:

public bool checkvaliduser (string a, string b) 
{ 
   string ambi1 = "SELECT * FROM Pword where Usr = '" + a + "' and Pwd = '" + b + "'";    OleDbCommand cmd = new OleDbCommand(ambi1, conn); 
   conn.Open(); 
   OleDbDataReader reader = cmd.ExecuteReader(); 
   if (reader.Read()) 
   { 
      conn.Close(); 
      checklogin = true;
      UserName = a; 
      return true; 
   } 
   else 
   {
      conn.Close();
      return false; 
   }  
}

Suppose, the class name of the login form is LogIn.

The code to write the username to the title bar of each form will now become:

   string user = "User: " + LogIn.UserName;

   foreach(Form f in Application.OpenForms) f.Text = user;

Notice that the static field will still be available after the LogIn form has been closed.

Notice also that if all your 4 forms are not opened at once, you can set the title bar text as you open them individually with code such as this following:

SomeForm f = new SomeForm();
f.Text = "User: " + LogIn.UserName;
f.Show();