当通过接口访问时，DbContext会在查询时抛出exception

我创建了一个我的DbContext类实现的接口，这使我能够为unit testing创​​建一个假的db上下文。 到目前为止，这适用于我的所有LINQ查询，但是我得到以下exception：

Unable to create a constant value of type 'DemoApp.Member'. Only primitive types ('such as Int32, String, and Guid') are supported in this context. 

通过接口执行LINQ查询会引发上述exception，但是当直接在我的DBContext上执行完全相同的查询时，查询将100％正常工作。 这是接口和相关的演示代码定义：

  interface IDemoContext : IDisposable { IDbSet Members { get; set; } IDbSet Teams { get; set; } } public partial class DemoContext : DbContext, IDemoContext { public DemoContext() : base("name=DemoContext"){} public IDbSet Members { get; set; } public IDbSet Teams { get; set; } } public partial class Member { public Member() { this.SecondaryTeams = new HashSet(); } public int ID { get; set; } public string Name { get; set; } public int? PrimaryTeamID { get; set; } public virtual Team PrimaryTeam { get; set; } public virtual ICollection SecondaryTeams { get; set; } } public partial class Team { public Team() { this.PrimaryMembers = new HashSet(); this.SecondaryMembers = new HashSet(); } public int ID { get; set; } public string Name { get; set; } public virtual ICollection PrimaryMembers { get; set; } public virtual ICollection SecondaryMembers { get; set; } } 

每个成员可能属于一个主要团队，也可能属于许多二级团队。 以下演示代码抛出exception：

 using (IDemoContext dbi = new DemoContext()) { var members = (from member in dbi.Members select new { Name = member.Name, Team = member.PrimaryTeam.Name, SecondaryTeams = from secondaryTeam in member.SecondaryTeams join primaryMember in dbi.Members on secondaryTeam.ID equals primaryMember.PrimaryTeamID into secondaryTeamMembers select new { Name = secondaryTeam.Name, Count = secondaryTeamMembers.Count() } }).ToList(); } 

如果我将第一行更改为：

 using (DemoContext dbi = new DemoContext()) 

然后查询执行完美。

所以我的问题是：

1. 为什么它通过DemoContext而不是IDemoContext工作？
2. 如何更改IDemoContext以使此查询通过界面正常工作？

Answer 1

尝试用dbi.Members中的局部变量替换dbi.Members 。

  using (IDemoContext dbi = new DemoContext()) { IQueryable memberSet = dbi.Members; var members = (from member in memberSet select new { Name = member.Name, Team = member.PrimaryTeam.Name, SecondaryTeams = from secondaryTeam in member.SecondaryTeams join primaryMember in memberSet on secondaryTeam.ID equals primaryMember.PrimaryTeamID into secondaryTeamMembers select new { Name = secondaryTeam.Name, Count = secondaryTeamMembers.Count() } }).ToList(); } 

Answer 2

找到解决方案，诀窍是在查询中包含SecondaryTeams：

  using (IDemoContext dbi = new DemoContext()) { var memberset = dbi.Members.Include(i => i.SecondaryTeams); var members = (from member in memberset select new { Name = member.Name, Team = member.PrimaryTeam.Name, SecondaryTeams = from secondaryTeam in member.SecondaryTeams join primaryMember in memberset on secondaryTeam.ID equals primaryMember.PrimaryTeamID into secondaryTeamMembers select new { Name = secondaryTeam.Name, Count = secondaryTeamMembers.Count() } }).ToList(); }