Question

我在测试运行报告时收到以下错误：

LINQ to Entities无法识别方法'System.Decimal PerformCurrencyConversion（System.Decimal，System.Decimal，System.Decimal，System.String，System.DateTime）'方法，并且此方法无法转换为存储表达式

这是我的问题：

List<BusinessPlanningElements> productSales = (from sale in ctn.ProductSales
    where sale.DateSold.Year == reportRequest.Year && sale.ProductID == t.ProductID
    group sale by sale.DateSold.Month into ds
    select new BusinessPlanningElements
    {
      Month = ds.Select(it => it.DateSold.Month).FirstOrDefault(),
      EURNumberOfUnitSold = ds.Where(it => it.EURSales.HasValue).Where(it => it.EURSales != 0).Count(),
      GBPNumberOfUnitSold = ds.Where(it => it.GBPSales.HasValue).Where(it => it.GBPSales != 0).Count(),
      USDNumberOfUnitSold = ds.Where(it => it.USDSales.HasValue).Where(it => it.USDSales != 0).Count(),
      EURCumulativeNumberOfUnitsSold = ctn.ProductSales.Where(it => it.EURSales.HasValue && it.EURSales != 0 && it.ProductID == t.ProductID && it.DateSold.Month <= ds.Key).Count(),
      GBPCumulativeNumberOfUnitsSold = ctn.ProductSales.Where(it => it.GBPSales.HasValue && it.GBPSales != 0 && it.ProductID == t.ProductID && it.DateSold.Month <= ds.Key).Count(),
      USDCumulativeNumberOfUnitsSold = ctn.ProductSales.Where(it => it.USDSales.HasValue && it.USDSales != 0 && it.ProductID == t.ProductID && it.DateSold.Month <= ds.Key).Count(),
      USDTotalSales = PerformCurrencyConversion(ds.Sum(it => it.USDSales.HasValue ? it.USDSales.Value : 0), 0M, 0M, reportRequest.Currency, DateTime.Now.AddMonths(-2)),
      GBPTotalSales = PerformCurrencyConversion(0M, ds.Sum(it => it.GBPSales.HasValue ? it.GBPSales.Value : 0), 0M, reportRequest.Currency, DateTime.Now.AddMonths(-2)),
      EURTotalSales = PerformCurrencyConversion(0M, 0M, ds.Sum(it => it.EURSales.HasValue ? it.EURSales.Value : 0), reportRequest.Currency, DateTime.Now.AddMonths(-2)),
      USDCumulativeTotalSales = PerformCurrencyConversion(ctn.ProductSales.Sum(it => it.USDSales.HasValue && it.USDSales != 0 && it.ProductID == t.ProductID && it.DateSold.Month <= ds.Key ? it.USDSales.Value : 0), 0M, 0M, reportRequest.Currency, DateTime.Now.AddMonths(-2)),
      GBPCumulativeTotalSales = PerformCurrencyConversion(ctn.ProductSales.Sum(it => it.GBPSales.HasValue && it.GBPSales != 0 && it.ProductID == t.ProductID && it.DateSold.Month <= ds.Key ? it.GBPSales.Value : 0), 0M, 0M, reportRequest.Currency, DateTime.Now.AddMonths(-2)),
      EURCumulativeTotalSales = PerformCurrencyConversion(ctn.ProductSales.Sum(it => it.EURSales.HasValue && it.EURSales != 0 && it.ProductID == t.ProductID && it.DateSold.Month <= ds.Key ? it.EURSales.Value : 0), 0M, 0M, reportRequest.Currency, DateTime.Now.AddMonths(-2)),
    }).ToList();

PerformCurrencyConversion方法几乎获得转换率列表，并将值转换为用户指定的货币。它接受如下参数：

public decimal PerformCurrencyConversion(decimal usd, decimal gbp, decimal eur, string Currency, DateTime saleMonth)
{
    //Logic
}

我过去在整个代码库中多次使用过这种方法，没有任何麻烦或错误，例如

List<MonthlyProductSales> monthlyProductSales = (from sale in productSales
     orderby sale.DateSold descending
     group sale by new { sale.DateSold.Month, sale.Product.Name } into ds
     select new MonthlyProductSales
     {
         Name = ds.Select(it => it.Product.Name).FirstOrDefault(),
         Month = ds.Select(it => it.DateSold.Month).FirstOrDefault(),
         Year = ds.Select(it => it.DateSold.Year).FirstOrDefault(),
         USDNumberItemsSold = ds.Where(it => it.USDSales.HasValue).Where(it => it.USDSales != 0).Count(),
         GBPNumberItemsSold = ds.Where(it => it.GBPSales.HasValue).Where(it => it.GBPSales != 0).Count(),
         EURNumberItemsSold = ds.Where(it => it.EURSales.HasValue).Where(it => it.EURSales != 0).Count(),
         USDRevenueTotal = PerformCurrencyConversion(ds.Sum(it => it.USDSales.HasValue ? it.USDSales.Value : 0), 0M, 0M, reportRequest.Currency, DateTime.Now.AddMonths(-2)),
         GBPRevenueTotal = PerformCurrencyConversion(0M, ds.Sum(it => it.GBPSales.HasValue ? it.GBPSales.Value : 0), 0M, reportRequest.Currency, DateTime.Now.AddMonths(-2)),
         EURRevenueTotal = PerformCurrencyConversion(0M, 0M, ds.Sum(it => it.EURSales.HasValue ? it.EURSales.Value : 0), reportRequest.Currency, DateTime.Now.AddMonths(-2)),
     }).ToList();

任何人都可以看到有什么区别，或者我如何解决这个错误？

Answer 1

区别在于LINQ实现。

在第一个代码示例中，您使用的是 LINQ to Entities （即实体框架的LINQ实现）。此实现将LINQ转换为最终在数据库上执行的SQL。

在第二个代码示例中，您正在使用 LINQ to Objects （即内存中的集合）。此实现不会转换为SQL或其他语言，而只是创建在内存中集合上执行的一系列操作。因此，LINQ的这种实现可以使用.NET函数，例如PerformCurrencyConversion。

所以你可以做的是首先使用LINQ to Entities查询你的数据库，并将结果存储在一个数组或列表中，然后在该数组或列表上执行LINQ to Objects查询 - 然后你可以使用内置的在.NET函数中。

C＃错误：LINQ to Entities无法识别该方法

1 个答案: