c# - SqlDataReader 的 Connection 属性为 Null

Question

我遇到了一个奇怪的问题，我能够从对存储过程的调用中返回结果，但代码会追溯失败。

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);
        }
    }
}

调用此代码表明变量 x 始终代表我希望从调用我的存储过程中看到的行数。

此外，我的调试输出显示了我希望看到的 ID 值。

但是，返回这些结果后，我收到错误 An exception of type 'System.InvalidOperationException' occurred in System.Data.dll but was not handled in user code 从行 if (reader.HasRows)(即已经执行)。我调用此请求的浏览器显示 HTTP Error 502.3 - Bad Gateway。

我怀疑原因是系统正在计算用于调试的 ID 和 X 值，以及它如何返回真实的用户输出。因此，它执行惰性操作以在必须返回它们时获取 IEnumerable 值；只有在这一点上，using 语句才导致调用 dispose 方法，因此读取器的连接为空(这是我在检查 reader 变量的属性时看到的同时调试)。

有没有人见过这样的行为/这是一个错误吗？还是我只是错过了一些明显的东西？

---

附加代码:

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

我已经临时实现了解决此问题的解决方法。

这个有效:

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();
}

相对于原来的:

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

不同之处在于我移动了受迭代器影响的代码，使其只迭代列表中的结果；并且不依赖编译器明智地理解与 IDisposable 对象相关的要求。

据我了解，编译器应该能够为我处理此问题(已在此处确认:https://stackoverflow.com/a/13504789/361842)，因此我怀疑这是编译器中的错误。

此处报告:https://connect.microsoft.com/VisualStudio/feedback/details/3113138

额外的演示代码在这里: https://gist.github.com/JohnLBevan/a910d886df577e442e2f5a9c2dd41293/