Many types of Pop up message box in asp.net ?

Through JavaScript :
Default.aspx page code:

function ShowMessage()
 {
          alert("Doesn't have Data");
      }

default.aspx.cs

ScriptManager.RegisterStartupScript(this, this.GetType(), "key", "ShowMessage()", true);
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2nd type on .cs
Default.aspx page code:

ClientScript.RegisterStartupScript(this.Page.GetType(), "", "<script type=\"text/javascript\">alert('Your Mgs');</script>");
third type:
Response.Write("<script>alert('Your Msg')</script>");