C# 无法将“System.Linq.Expressions.MethodCallExpression”类型的对象强制转换为“System.Linq.Expressions.LambdaExpression”类型

我正在使用EntityFramework，遇到上述错误。当我调用

var documents = docRepository.GetAllSavedDocuments();
var withRatings = documentFilter.GetDocumentsWithRating(documents, 1);

我得到了标题中的错误

我的代码分为几个类

public class DocRepository
{
    public IQueryable<Document> GetAllSavedDocuments(User user)
    {
        return (from d in AllUsersDocuments(user)
                select d)
            .Include(d => d.Calculations)
            .Include("Calculations.CalculationResults");
    }
}

public class DocumentFilter
{
    public IQueryable<Tuple<Document, Rating>> GetDocumentsWithRating(IQueryable<Document> documents, int id)
    {
        return documents.Select(x => new
        {
            Document = x,
            Rating = _ratingsProvider.GetRating.Compile()(x, id)
        }).AsEnumerable()
            .Select(x => new Tuple<Document, Rating>(x.Document, x.Rating)).AsQueryable().Where(x.Item2 == Rating.Ok);
    }
}

public class RatingProvider
{
    public Expression<Func<Document, int, Rating>> GetRating
    {
        get
        {
            return (document, id) =>
                document.Calculations.Where(x => x.CId == id).Any() ?
                document.Calculations.Single(x => x.CId == id).Rating.Value :
                Rating.Unknown;
        }
    }
}

---

但我想保持它的可查询性，所以我们在SQL端执行它，而不是枚举代码中的所有对象。请有人建议我如何做到这一点？

您可以先为_ratingsProvider.GetRating分配一个变量，以避免标题中出现错误

之后，您将出现另一个错误，即实体框架不支持您的查询。你需要像LINQKit这样的东西。AsExpandable和Invokex，id来自LINQKit

using LinqKit;

---

GetDocumentsWithRatingIQueryable文档中的id是什么，int id？非常感谢。我已经接受了这个答案，但我的低代表性意味着它还不会出现。谢谢你也指出linqkit。

using LinqKit;

    public IQueryable<Tuple<Document, Rating>> GetDocumentsWithRating(IQueryable<Document> documents, int id)
    {
        var getRatingExpression = _ratingsProvider.GetRating;
        return documents.AsExpandable().Select(x => new
        {
            Document = x,
            Rating = getRatingExpression.Invoke(x, id)
        }).AsEnumerable()
            .Select(x => new Tuple<Document, Rating>(x.Document, x.Rating)).AsQueryable().Where(x.Item2 == Rating.Ok);
    }

    public Expression<Func<Document, int, Rating>> GetRating
    {
        get
        {
            return (document, id) =>
                document.Calculations.Where(x => x.CId == id).Any() ?
                document.Calculations
                        .Where(x => x.CId == id)
                        .Select(x => x.Rating.HasValue ? x.Rating.Value : Rating.None)
                        .Single()
                Rating.Unknown;
        }
    }

public class DocumentRating
{
    public Document { get; set; }
    public Rating { get; set; }
}

    public IQueryable<DocumentRating> GetDocumentsWithRating(IQueryable<Document> documents, int id)
    {
        var getRatingExpression = _ratingsProvider.GetRating;
        return documents.AsExpandable().Select(x => new DocumentRating
        {
            Document = x,
            Rating = getRatingExpression.Invoke(x, id)
        })
        .Where(x.Item2 == Rating.Ok);
    }