Insert and update records by submit button

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C#

ASP.NET

Hi,

I am trying to insert new record and update by single button while i am updating record that is working but when try to insert record on that time error is
input string was not in correct format.

actually i am not insert id for this record because it is auto generated for insert record.
for update i am using id for reference.

following is my coding when i debug first check if condition for update is right but for insert it is not going to else block it is again if block what is the reason?

protected void btnSave_Click(object sender, EventArgs e)
   {
       connection = new SqlConnection(ConfigurationManager.AppSettings["connectionstring"].ToString());
      if(lblProduct_Id.Text==@lblProduct_Id.Text)

          {
              connection.Open();
              string strUpdate = "Update product_detail set product_name=@product_name,product_quantity=@product_quantity,product_rate=@product_rate where Product_Id=@Product_Id";
              SqlCommand cmd = new SqlCommand(strUpdate, connection);
              cmd.CommandType = CommandType.Text;
              cmd.Parameters.AddWithValue("@product_name", SqlDbType.NVarChar).Value = txtProduct_Name.Text;
              cmd.Parameters.AddWithValue("@product_quantity", SqlDbType.Int).Value = Convert.ToInt32(txtProduct_Quantity.Text);
              cmd.Parameters.AddWithValue("@product_rate", SqlDbType.Int).Value = Convert.ToInt32(txtProduct_Rate.Text);
              cmd.Parameters.AddWithValue("@product_id", SqlDbType.Int).Value = Convert.ToInt32(lblProduct_Id.Text);
              cmd.ExecuteNonQuery();
              connection.Close();
              bindgrid();
      }
        else
       {

           connection.Open();
           string strSave = "insert into product_detail (product_name,product_quantity,product_rate) values(@product_name,@product_quantity,@product_rate)";
           SqlCommand cmd = new SqlCommand(strSave, connection);
           cmd.CommandType = CommandType.Text;
           cmd.Parameters.Add("@product_name", SqlDbType.NVarChar).Value = txtProduct_Name.Text;
           cmd.Parameters.Add("@product_quantity", SqlDbType.Int).Value = txtProduct_Quantity.Text.ToString();
           cmd.Parameters.Add("@product_rate", SqlDbType.Int).Value = txtProduct_Rate.Text.ToString();
           cmd.Parameters.Add("@product_id", SqlDbType.Int).Value = lblProduct_Id.Text.ToString();
           cmd.ExecuteNonQuery();
           radGrid1.Visible = true;
           connection.Close();
           bindgrid();

       }

   }