How to display messagebox in ASP.NET using C#

add script code in your page:

<script type="text/javascript">
        function Showalert() {
            alert('Call JavaScript function from codebehind');
        }
</script>

here is code for display the alert message... It is similar to msgbox.

<pre>
<div>
<asp:Button ID="btnSubmit" runat="server" Text="Show alert" onclick="btnSubmit_Click"/>
<asp:Button ID="btnClick" runat="server" Text="Call JavaScript Function" onclick="btnClick_Click" />
</div>

I hope this is help for You.

You can add reference of System.Window.Form, that would give to add MessageBox in your asp.net application, it is good when you are in debug mode mean only you want to test your website. But problems come when you host you host your website, mean production phase.
Better you you javascript.

Response.Write(@"<script language='javascript'>alert('Details saved successfully')</script>");

There are no of ways to do this.
you van use javascript code inside c# code as

string display = "Message Pop-up!";
            ClientScript.RegisterStartupScript(this.GetType(), "myalert", "alert('" + display + "');", true);

or use modalpopup extender
as
http://forums.asp.net/t/1900752.aspx/1?Display+success+Message+box+in+asp+net+using+c+[^]

Response.Write("<script type=\"text/javascript\">alert('Your Message');</script>");

Dim myStringVariable1 As String = string.Empty
myStringVariable1 = ("Policy Number:" + ("  "  _
            + (txtPolNo.Text.ToString + ("  " + ("with Issuance office:" + ("  "  _
            + (dropIssOff.Text.ToString + ("  " + "was not found on the database. Please make sure that your inputs are correct."))))))))
ClientScript.RegisterStartupScript(Me.GetType, "myalert", ("alert('"  _
                + (myStringVariable1 + "');")), true)