C#错误：LINQ to实体无法识别该方法

在测试运行报告时，我收到以下错误：

LINQ to Entities无法识别方法“System.Decimal PerformCurrencyConversion（System.Decimal，System.Decimal，System.Decimal，System.String，System.DateTime）”方法，并且无法将此方法转换为存储表达式

我的问题是：

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();

在整个代码库中，我已经多次使用这种方法，没有遇到任何问题或错误，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();

List monthlyProductSales=（来自productSales中的sale
orderby sale.datesalled
按新{sale.datesalled.Month，sale.Product.Name}将销售分组到ds中
选择新的MonthlyProductSales
{
Name=ds.Select（it=>it.Product.Name）.FirstOrDefault（），
Month=ds.Select（it=>it.datesell.Month）.FirstOrDefault（），
Year=ds.Select（it=>it.datesell.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）、0、0、0、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（）；

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有人能看出区别是什么吗？或者我该如何避免这个错误

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SQL，最终在数据库上执行

在第二个代码示例中，您使用的是LINQ to对象（即内存中的集合）。此实现不会转换为SQL或其他语言，而是简单地创建在内存集合上执行的操作链。因此，LINQ的这个实现可以使用.NET函数，例如

PerformCurrencyConversion

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因此，您可以首先使用LINQ to Entities查询数据库，并将结果存储在数组或列表中，然后在该数组或列表上执行LINQ to Objects查询，然后您可以使用内置的.NET函数。

使用LINQ to Entities或LINQ to Objects的工作代码是吗？@AmiramKorach，它在标记中。

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();