c# - 为什么我读取成功的Npgsql数据消失了？

Question

我有以下代码形状。看来我误解了 C# 方法的返回值。 “完整”枚举数怎么可能返回为空枚举数？

class ThingDoer
{
    public NpgsqlDataReader DoQuery()
    {
        NpgsqlCommand c = new NpgsqlCommand(...);
        NpgsqlDataReader dataread = c.ExecuteReader();
        return dataread;  // Debugger confirms that six data are enumerable here.
    }
}

...

class OtherThing
{
    public void higherLevelFunction()
    {
        NpgsqlDataReader result = myThingDoer.DoQuery();
        result.Read();  // No data! result's enumerable returns nothing!
    }
}

Answer 1

您没有详细说明您的连接来自何处。假设它是这样的:

public NpgsqlDataReader DoQuery()
{
    using(NpgsqlConnection = GetConnectionCode())
    {
        NpgsqlCommand c = new NpgsqlCommand(...);
        NpgsqlDataReader dataread = c.ExecuteReader();
        return dataread;
    }//Connection closes at this using-scope being left because that triggers Dispose()
}

然后将其更改为:

public NpgsqlDataReader DoQuery()
{
    bool ownershipPassed = false;
    NpgsqlConnection conn = GetConnectionCode();
    try
    {
        NpgsqlCommand c = new NpgsqlCommand(...);
        NpgsqlDataReader dataread = c.ExecuteReader(CommandBehavior.CloseConnection);
        ownershipPassed = true;
        return dataread;
    }
    finally
    {
        if(!ownershipPassed)//only if we didn't create the reader than takes charge of the connection
          conn.Dispose();
    }
}

然后在使用读取器的地方，必须处置它以依次处置连接的底层连接到数据库:

public void higherLevelFunction()
{
    using(NpgsqlDataReader result = myThingDoer.DoQuery())
      result.Read();
}