ASP.Net: Display JavaScript Alert MessageBox using C# and VB.Net

Hi optimistms,

Please refer below sample.

HTML

<asp:Button ID="btnSubmit" runat="server" Text="Submit" OnClick="btnSubmit_Click" />

Code

C#

protected void btnSubmit_Click(object sender, EventArgs e)
{
    try
    {
        string message = "Data is incorrect.";
        System.Text.StringBuilder sb = new System.Text.StringBuilder();
        sb.Append("<script type = 'text/javascript'>");
        sb.Append("window.onload=function(){");
        sb.Append("alert('");
        sb.Append(message);
        sb.Append("')};");
        sb.Append("</script>");
        ClientScript.RegisterClientScriptBlock(this.GetType(), "alert", sb.ToString());
    }
    catch (Exception ex)
    {
    }
}

VB.Net

Protected Sub btnSubmit_Click(ByVal sender As Object, ByVal e As EventArgs) Handles btnSubmit.Click
    Try
        Dim message As String = "Data is incorrect.."
        Dim sb As System.Text.StringBuilder = New System.Text.StringBuilder()
        sb.Append("<script type = 'text/javascript'>")
        sb.Append("window.onload=function(){")
        sb.Append("alert('")
        sb.Append(message)
        sb.Append("')};")
        sb.Append("</script>")
        ClientScript.RegisterClientScriptBlock(Me.GetType(), "alert", sb.ToString())
    Catch ex As Exception
    End Try
End Sub

Screenshot