c# - Entity Framework 中的 Linq 查询错误

Question

我正在尝试通过 EF linq 查询从域对象收集值。但我收到错误，请帮我纠正这个错误。

public String[] ReturnPatientIDs(int CounsellingRoomID)
{

    var messageObject = this.context.CounsellingMessages.Where(c => c.CounsellingRoomID == CounsellingRoomID).Distinct();

    String[] PatientIDs = new String[messageObject.Count()];

    for (int k = 0; k < PatientIDs.Length; k++)
    {
        PatientIDs[k] = messageObject.ElementAt(k).Chatname;
    }

    return PatientIDs;
}

LINQ to Entities does not recognize the method 'Me.Domain.General.CounsellingMessage ElementAt[CounsellingMessage](System.Linq.IQueryable`1[Me.Domain.General.CounsellingMessage], Int32)' method, and this method cannot be translated into a store expression.

Description: An unhandled exception occurred during the execution of the current web request. Please review the stack trace for more information about the error and where it originated in the code.

Exception Details: System.NotSupportedException: LINQ to Entities does not recognize the method 'MedicalApp.Domain.General.CounsellingMessage ElementAt[CounsellingMessage](System.Linq.IQueryable`1[MedicalApp.Domain.General.CounsellingMessage], Int32)' method, and this method cannot be translated into a store expression.

Source Error:

Line 43:             for (int k = 0; k < PatientIDs.Length; k++ )
Line 44:             {
Line 45:                 PatientIDs[k] = messageObject.ElementAt(k).Chatname;
Line 46:             }
Line 47: 

Answer 1

不支持 ElementAt 方法。 messageObject 查询只有在调用 Count() 和 ElementAt() 时才会执行，并且这些查询将被视为 LINQ to Entity查询。

通过添加 Select 语句，可以将 for 循环中执行的操作整合到 LINQ 查询中:

public String[] ReturnPatientIDs(int CounsellingRoomID)
{
    var query = this.context.CounsellingMessages
                    .Where(c => c.CounsellingRoomID == CounsellingRoomID)
                    .Select(c => c.Chatname)
                    .Distinct();

    return query.ToArray();
}