How to display an alert box from C# in ASP.NET?

After insertion code,

ScriptManager.RegisterClientScriptBlock(this, this.GetType(), "alertMessage", "alert('Record Inserted Successfully')", true);

Response.Write("<script>alert('Data inserted successfully')</script>");

Write this line after your insert code

 ClientScript.RegisterStartupScript(this.GetType(), "alert", "alert('Insert is successfull')", true);