實體框架中的Linq查詢錯誤

Question

我想通過EF linq查詢從域對象收集一個值。但我得到一個錯誤，請問可以幫我糾正一下。

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

的ElementAt方法不被支持。在你調用Count()和ElementAt()之前，messageObject查詢不會被執行，並且這些查詢將被視爲LINQ to Entity查詢。

在for循環所採取的動作可以通過添加Select語句中軋製成的LINQ查詢：

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

    return query.ToArray(); 
}