Windows 应用商店应用 - C# 格式十进制到货币/货币

我想将我用math.round（variable， 2）四舍五入的小数格式化为货币格式，因此它将始终将4转换为4.00。我尝试这样做：

    public ProductItem(String itemNo, String description, String unitOfMeasure, decimal unitPriceExclVAT, decimal purchasePrice, decimal margin, int actualStock, String imagePath)
    {
        this.ItemNo = itemNo;
        this.Description = description;
        this.UnitOfMeasure = unitOfMeasure;
        this.UnitPriceExclVAT = Math.Round(unitPriceExclVAT, 2);
        this.PurchasePrice = Math.Round(purchasePrice, 2);
        this.Margin = Math.Round(margin, 2);
        this.ActualStock = actualStock;
        this.ImagePath = imagePath;
    }
    public string ItemNo { get; private set; }
    public string Description { get; private set; }
    public string UnitOfMeasure { get; private set; }
    public decimal UnitPriceExclVAT { get; set; }
    public decimal PurchasePrice { get; set; }
    public decimal Margin { get; set; }
    public int ActualStock { get; private set; }
    public string ImagePath { get; private set; }

                foreach (JsonValue itemValue in groupObject["Items"].GetArray())
                {
                    if (uniqueGroupItemsCount != 36)
                    {
                        JsonObject itemObject = itemValue.GetObject();
                        ProductItem product = new ProductItem(itemObject["ItemNo"].GetString(),
                                                        itemObject["Description"].GetString(),
                                                        itemObject["UnitOfMeasure"].GetString(),
                                                        Convert.ToDecimal(itemObject["UnitPriceExclVAT"].GetString().Replace(',', '.')),
                                                        Convert.ToDecimal(itemObject["PurchasePrice"].GetString().Replace(',', '.')),
                                                        Convert.ToDecimal(itemObject["Margin"].GetString().Replace(',', '.')),
                                                        Convert.ToInt32(itemObject["ActualStock"].GetString().Replace(',', '.')),
                                                        itemObject["ImagePath"].GetString());

                        if (product.Description.ToString().ToLower().Trim().Contains(productItems) || product.ItemNo.ToString().ToLower().Trim().Contains(productItems))
                        {
                            var money = Convert.ToDecimal(string.Format("{0:C}", product.Margin));//here is where it goes wrong, i know i can format it like this, but its not working.
                            product.Margin = money;
                            searchedGroup.Items.Add(product);
                            uniqueGroupItemsCount++;
                        }

上面的代码会给我一个错误。错误是：mscorlib 中发生了类型为"System.FormatException"的异常.dll但未在用户代码中处理

希望你能帮我:)

编辑：它不需要是像 €20.00 这样的货币值，只有 20.00 对我来说就足够了，因为我可以在 XAML 中制作欧元符号。