I want to add /show some message with a OK button in c# asp.net without using the windows.forms name space.
One easy solution is to register some JavaScript code in your page:
string script = "alert(\"Hello!\");";
ScriptManager.RegisterStartupScript(this, GetType(),ServerControlScript", script, true);
Use:
string display = "Pop-up!";
ClientScript.RegisterStartupScript(this.GetType(), "yourMessage", "alert('" + display + "');", true);