c# - 在 LINQ 查询中将枚举转换为字符串时出错

Question

我有以下查询，它使用的是 Entity Framework 。

Analytic firstSent = (from a in Repository.Query<Analytic>()
                      where a.EntityType == "Proposal" &&
                      a.EntityId == Program.ActiveProposal.Id &&
                      a.Marker == AnalyticMarker.EmailProposalUrl.ToString()
                      orderby a.TimestampUtc
                      select a).FirstOrDefault();

在运行时，我收到以下错误:

LINQ to Entities does not recognize the method 'System.String ToString()' method, and this method cannot be translated into a store expression.

a.Marker 是一个字符串列，AnalyticMarker.EmailProposalUrl 是一个枚举值，我想将该列与该枚举的名称进行比较。

我知道 SQL 不支持从枚举到字符串的转换，但为什么它不解析这个字符串的值，然后将结果字符串传递给 SQL？那应该工作得很好。

Answer 1

试试这个:

var emailProposalUrl = AnalyticMarker.EmailProposalUrl.ToString();
Analytic firstSent = (from a in Repository.Query<Analytic>()
                      where a.EntityType == "Proposal" &&
                      a.EntityId == Program.ActiveProposal.Id &&
                      a.Marker == emailProposalUrl
                      orderby a.TimestampUtc
                      select a).FirstOrDefault();

This other answer正在解释为什么这也可以工作的原因..

The problem arises because ToString() isn't really executed, it is turned into a MethodGroup and then parsed and translated to SQL. Since there is no ToString() equivalent, the expression fails.