private void PurchaseProductComboBox_SelectedIndexChanged(object sender, EventArgs e)
{
ModelPurchase _modelPurchase = new ModelPurchase();
_modelPurchase.purchaseProduct = purchaseProductComboBox.Text;
PurchaseManager _purchaseManager = new PurchaseManager();
availableQtyTextBox.Text = _purchaseManager.AvaiablePurchaseQuantity(_modelPurchase);
totalMrpTextBox.Text = _purchaseManager.TotalMrp(_modelPurchase);
}