Question

我遇到一个奇怪的问题，我可以通过调用存储过程返回结果，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);
        }
    }
}

调用此代码显示变量x始终表示我希望从对存储过程的调用中看到的行数。

此外，我的调试输出显示了我希望看到的ID值。

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

我怀疑原因是系统分别计算调试的ID和X值以及它将如何返回实际用户输出。因此，它执行一个惰性操作来获取IEnumerable值，它必须返回它们;只有这一点，using语句才会调用dispose方法，因此读者的连接为null（这是我在调试时检查reader变量的属性时所看到的）。 p>

之前有没有人看到过这样的行为/它是一个错误;或者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

我暂时实施了解决此问题的解决方法。

这有效：

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();
}

与原作相反：

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，使得它只遍历列表中的结果;并且不依赖编译器明智地理解与IDisposable对象相关的要求。

我的理解是编译器应该能够为我处理这个问题（在此确认：https://stackoverflow.com/a/13504789/361842），所以我怀疑它是编译器中的一个错误。

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

此处的其他演示代码： https://gist.github.com/JohnLBevan/a910d886df577e442e2f5a9c2dd41293/

SqlDataReader的Connection属性为Null

1 个答案: