refer the inline comments, as an advice, Please use the proper naming convention to the controls so that it will be easy to find the correct control on the fly.
string barCode = textBox3.Text.Trim();
string Material = textBox4.Text.Trim();
string Location = textBox5.Text.Trim();
string Quantity = textBox6.Text.Trim();
string date = textBox7.Text.Trim();
SqlCommand cmd = new SqlCommand("INSERT into [Materials Inventory] (Barcode, Material, Location, Quanitiy, [Date Added]) " +
" VALUES ( @Barcode,@Material,@Location,@Quantity,@Date)", sqlcon);
cmd.Parameters.AddWithValue("@Barcode", barCode);
cmd.Parameters.AddWithValue("@Material", Material);
cmd.Parameters.AddWithValue("@Location", Location);
cmd.Parameters.AddWithValue("@Quantity", Quantity);
cmd.Parameters.AddWithValue("@Date", date);
sqlcon.Open();
cmd.ExecuteNonQuery();
sqlcon.Close();