How can I display a messagebox in ASP.NET?

@freelancer If you are using ScriptManager then try this code for message..

string script = "alert(\"Hello!\");";
ScriptManager.RegisterStartupScript(this, GetType(), 
                      "ServerControlScript", script, true);

Make a method of MsgBox in your page.

public void MsgBox(String ex, Page pg,Object obj) 
    string s = "<SCRIPT language='javascript'>alert('" + ex.Replace("\r\n", "\\n").Replace("'", "") + "'); </SCRIPT>";
    Type cstype = obj.GetType();
    ClientScriptManager cs = pg.ClientScript;
    cs.RegisterClientScriptBlock(cstype, s, s.ToString());

and when you want to use msgbox just put this line

MsgBox("! your message !", this.Page, this);