C# Cannot convert from 'String' To 'Int' retreiving data from database sql

I'm creating a POS system for grocery store i am fetching data from tables into text boxes but i got error. help will be appreciated@! image for reference Check Image here

Here is my code

void AddItemViaStockCode()
    {
        try
        {
            con = new SqlConnection(constring);
            con.Open();
            mytra = con.BeginTransaction();
            cmd = new SqlCommand();
            cmd.Connection = con;
            cmd.Transaction = mytra;
            cmd.CommandText = "SELECT * FROM sitemcode WHERE stk_code = '" + txtStockCode.Text.Trim() + "'";
            reader = cmd.ExecuteReader();
            while (reader.Read())
            {

               string code = reader.GetString("stk_code");
                string item = reader.GetString("stk_item");
                string cat =  reader.GetString("stk_category");
                string unit = reader.GetString("unit");
                string minip =reader.GetString("min_price");
               string selling=reader.GetString("selling_price");
                string costp =reader.GetString("cost_price"); 
            }

        }catch(Exception ex)
        {
            MessageBox.Show(ex.Message, "Add Error", MessageBoxButtons.OK, MessageBoxIcon.Error);
        }

    }