SqlDataReader的連接屬性爲空

Question

我遇到了一個奇怪的問題，即我能夠將調用的結果返回到存儲過程，但代碼追溯失敗。

public IEnumerable<T> ExecuteStoredProcedure<T>(string storedProcedureName, IDataMapper<T> mapper, IDictionary<string, object> parameters) 
{ 
    using (var connection = new SqlConnection(connectionString)) 
    { 
     using (var cmd = new SqlCommand(storedProcedureName, connection)) 
     { 
      cmd.CommandType = CommandType.StoredProcedure; 
      foreach (var key in parameters.Keys) 
      { 
       cmd.Parameters.AddWithValue(key, parameters[key]); 
      } 
      connection.Open(); 
      SqlDataReader reader = cmd.ExecuteReader(); 
      //return MapRecordsToDTOs(reader, mapper); 

      //let's test: 
      IEnumerable<T> result = MapRecordsToDTOs(reader, mapper); 
      var x = (new List<T>(result)).Count; 
      System.Diagnostics.Debug.WriteLine(x); 
      return result; 
     } 
    } 
} 


private static IEnumerable<T> MapRecordsToDTOs<T>(SqlDataReader reader, IDataMapper<T> mapper) 
{ 
    if (reader.HasRows) 
    { 
     while (reader.Read()) 
     { 
      System.Diagnostics.Debug.WriteLine(reader["Id"]); //what's going on... 
      yield return mapper.MapToDto((IDataRecord)reader); 
     } 
    } 
} 

調用此代碼顯示了可變x始終表示，我期望從我的存儲過程的調用看到的行數。

另外我的調試輸出顯示了我期望看到的ID值。

但是，在返回這些結果之後，我會從行if (reader.HasRows)（即已執行的行）收到錯誤An exception of type 'System.InvalidOperationException' occurred in System.Data.dll but was not handled in user code。我從中調用此請求的瀏覽器顯示HTTP Error 502.3 - Bad Gateway。

我懷疑的原因是系統的計算調試ID和X值分別對它將如何返回真實用戶輸出。因此，它執行一個懶惰的操作來獲取IEnumerable值到它必須返回的位置;只有在這一點上，using語句才導致調用處理方法，因此讀者的連接爲空（這是我在調試時檢查reader變量的屬性時看到的內容）。

有沒有人見過這樣的行爲/是它的錯誤;或者我錯過了一些明顯的東西？

---

附加代碼：

public interface IDataMapper<T> 
{ 
    T MapToDto(IDataRecord record); 
} 

public class CurrencyMapper: IDataMapper<CurrencyDTO> 
{ 
    const string FieldNameCode = "Code"; 
    const string FieldNameId = "Id"; 
    const string FieldNameName = "Name"; 
    const string FieldNameNum = "Num"; 
    const string FieldNameE = "E"; 
    const string FieldNameSymbol = "Symbol"; 

    public CurrencyMapper() { } 

    public CurrencyDTO MapToDto(IDataRecord record) 
    { 
     var code = record[FieldNameCode] as string; 
     var id = record[FieldNameId] as Guid?; 
     var name = record[FieldNameName] as string; 
     var num = record[FieldNameNum] as string; 
     var e = record[FieldNameE] as int?; 
     var symbol = record[FieldNameSymbol] as char?; 
     return new CurrencyDTO(id, code, num, e, name, symbol); 
    } 
} 

public class CurrencyRepository 
{ 

    const string SPReadAll = "usp_CRUD_Currency_ReadAll"; 

    readonly SqlDatabase db; 
    public CurrencyRepository() 
    { 
     db = new SqlDatabase(); //stick to SQL only for the moment for simplicity 
    } 
    public IEnumerable<CurrencyDTO> GetCurrencyCodes() 
    { 
     var mapper = new CurrencyMapper(); 
     return db.ExecuteStoredProcedure(SPReadAll, mapper); 
    } 
} 

public class CurrencyDTO 
{ 

    readonly Guid? id; 
    readonly string code; 
    readonly string num; 
    readonly int? e; 
    readonly string name; 
    readonly char? symbol; 

    public CurrencyDTO(Guid? id,string code,string num,int? e,string name, char? symbol) 
    { 
     this.id = id; 
     this.code = code; 
     this.num = num; 
     this.e = e; 
     this.name = name; 
     this.symbol = symbol; 
    } 

    public Guid? Id { get { return id; } } 
    public string Code { get { return code; } } 
    public string Num { get { return num; } } 
    public int? E { get { return e; } } 
    public string Name { get { return name; } } 
    public char? Symbol { get { return symbol; } } 
} 

Answer 1

我暫時實現從而解決了這個問題的解決方法。

這工作：

private static IEnumerable<T> MapRecordsToDTOs<T>(SqlDataReader reader, IDataMapper<T> mapper) 
{ 
    var list = new List<T>(); //use a list to force eager evaluation 
    if (reader.HasRows) 
    { 
     while (reader.Read()) 
     { 
      list.Add(mapper.MapToDto((IDataRecord)reader)); 
     } 
    } 
    return list.ToArray(); 
} 

而不是在原來的：

private static IEnumerable<T> MapRecordsToDTOs<T>(SqlDataReader reader, IDataMapper<T> mapper) 
{ 
    if (reader.HasRows) 
    { 
     while (reader.Read()) 
     { 
      yield return mapper.MapToDto((IDataRecord)reader); 
     } 
    } 
} 

的區別在於我謹受迭代器的代碼，使得它只能通過在列表中的結果迭代;並且不依賴於編譯器明智地理解與IDisposable對象相關的要求。

這是我的理解，編譯器應該能夠爲我處理這個（在這裏證實：https://stackoverflow.com/a/13504789/361842），所以我懷疑這是編譯器中的一個錯誤。

這裏報告：https://connect.microsoft.com/VisualStudio/feedback/details/3113138

其他演示代碼在這裏： https://gist.github.com/JohnLBevan/a910d886df577e442e2f5a9c2dd41293/