Message Box.Show() in ASP.Net

First add asp script manager to the page.

Then just write the below code in your .cs file

ClientScript.RegisterStartupScript(this.GetType(), "msg", "<script>alert('Hello User.')</script>");

The total code is:
.aspx :

    <form id="form2" runat="server">

    <asp:ScriptManager ID="ScriptManager1" runat="server">

    </asp:ScriptManager>

    <div>

    <asp:Button ID="btn" runat="server" Text="Click" onclick="btn_Click" />

    </div>

    </form>

.cs :


 protected void btn_Click(object sender, EventArgs e)

    {

        ClientScript.RegisterStartupScript(this.GetType(), "msg", "<script>alert('Hello User.')</script>");

    }