c# - 为什么这个 LINQ 抛出 "FormatException was unhandled"？

Question

我正在尝试使用 LINQ 从通用列表中获取一个 val，如下所示:

private List<PriceVarianceDataAmalgamated> CombineSubsets(string unit)
{
    List<PriceVarianceDataAmalgamated> combinedSubsets = new List<PriceVarianceDataAmalgamated>();
    if (unit.Equals(CRAFTWORKS_SC))
    {
        foreach (PriceVarianceSubsetData pvsd in craftworksWeek1PVDSubsetList)
        {
            PriceVarianceDataAmalgamated pvda = new PriceVarianceDataAmalgamated
            {
                ShortName = pvsd.ShortName,
                ItemCode = pvsd.ItemCode,
                Description = pvsd.Description,
                Price1 = pvsd.Price,
                Price2 = GetPrice2(CRAFTWORKS_SC, pvsd.ShortName, pvsd.ItemCode)
            };
            pvda.Variance = "0.00";
            decimal price1 = Convert.ToDecimal(pvda.Price1);
            decimal price2 = Convert.ToDecimal(pvda.Price2);
            . . .

关键部分(未能按我的意愿行事)是对 GetPrice2() 的调用。该方法开始于:

private string GetPrice2(string _unit, string _shortname, string _itemcode)
{
    string price2 = "0.00";
    if (_unit.Equals(CRAFTWORKS_SC))
    {
        price2 = craftworksWeek2PVDSubsetList
            .Where(x => x.ShortName.Equals(_shortname))
            .Where(x => x.ItemCode.Equals(_itemcode))
            .Select(x => x.Price).ToString();
    }
    . . .

...当我进入对 price2 的赋值时，赋值的值是:

System.Linq.Enumerable+WhereSelectListIterator`2[Pivotal.PriceVarianceSubsetData,System.String]

?!?

给出的错误信息是:

System.FormatException was unhandled
  HResult=-2146233033
  Message=Input string was not in a correct format.
  Source=mscorlib
  StackTrace:
       at System.Number.StringToNumber(String str, NumberStyles options, NumberBuffer& number, NumberFormatInfo info, Boolean parseDecimal)
       at System.Number.ParseDecimal(String value, NumberStyles options, NumberFormatInfo numfmt)
       at System.Convert.ToDecimal(String value)
       at Pivotal.FormMain.CombineSubsets(String unit) in c:\Projects\PriceVariance\Pivotal\Form1.cs:line 290
       at Pivotal.FormMain.GenerateAndSaveSpreadsheetFile() in c:\Projects\PriceVariance\Pivotal\Form1.cs:line 154
       at Pivotal.FormMain.buttonRun_Click(Object sender, EventArgs e) in c:\Projects\PriceVariance\Pivotal\Form1.cs:line 137
. . .

所以这一行 (290) 失败了:

List<PriceVarianceDataAmalgamated> pvdaCraftworks = CombineSubsets(CRAFTWORKS_SC);

...更具体地说是这个(第 154 行):

decimal price2 = Convert.ToDecimal(pvda.Price2);

显然错误的代码是这样的:

price2 = craftworksWeek2PVDSubsetList
    .Where(x => x.ShortName.Equals(_shortname))
    .Where(x => x.ItemCode.Equals(_itemcode))
    .Select(x => x.Price).ToString();

在它爆炸的地方单步执行时，“_shortname”和“_itemcode”都有一个有效值；完全有可能没有可供选择的值，但我似乎通过为 price2 分配默认值来防御这种情况:

string price2 = "0.00";

price2 在方法结束时返回 - 也许我应该使用 catch block 并在那里分配回退值而不是从 git-go 分配回退值？

Answer 1

使用

.Select(x => x.Price).FirstOrDefault().ToString();

它不会抛出异常，而是为引用类型返回空值，为值类型返回默认值。可能会让你更容易找到问题所在