使用 using 返回 IEnumerable

发布于 2024-09-28 19:33:58 字数 819 浏览 10 评论 0原文

我遇到了一个有趣的问题，这是完全有道理的。我有一个像这样的通用方法：

public TResult Run<TResult>(Func<SqlDataReader, TResult> resultDelegate)
{
   TResult result;

   using (SqlDataReader reader = command.ExecuteReader()) // command is SqlCommand with attached SqlConnection
   {
      result = resultsDelegate(reader);
   }

   // Some other unrelated code (but that's why result has a variable)

   return result;
}

在一种情况下，resultDelegate 的返回类型 (TResult) 是 IEnumerable

我很难找到解决这个问题的最佳方法，我知道我可以返回一个具体的列表，但如果可能的话，我想避免这种情况，但我还是可以在委托中移动 using 语句。，如果我可以避免为每个代表这样做，那就太好了。

原文

Interesting problem I ran across which makes total sense. I have a generic method like so:

public TResult Run<TResult>(Func<SqlDataReader, TResult> resultDelegate)
{
   TResult result;

   using (SqlDataReader reader = command.ExecuteReader()) // command is SqlCommand with attached SqlConnection
   {
      result = resultsDelegate(reader);
   }

   // Some other unrelated code (but that's why result has a variable)

   return result;
}

In one case, the resultDelegate's return type (TResult) is IEnumerable<object>. The problem is that the Run function returns immediately due to deferred execution, disposing the SqlDataReader. Later in the code when I try to read through the results (which the delegate does reader.Read(), I get an InvalidOperationException: Invalid attempt to call Read when reader is closed.

I'm having a hard time figuring out the best way around this. I know I can return a concrete list, but I would like to avoid that if possible. I can also move the using statement inside the delegate, but once again, if I can avoid doing that for every delegate it would be nice. Any ideas?

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欢你一世 2024-10-05 19:33:58

也许：

public TResult Run<TResult>(Func<SqlDataReader, TResult> resultDelegate)
{
   TResult result;

   using (SqlDataReader reader = command.ExecuteReader()) // command is SqlCommand with attached SqlConnection
   {
      result = resultsDelegate(reader);
      if (typeof(TResult) == typeof(IEnumerable<object>)) 
      {
         var enumerable = result as IEnumerable<object>;
         if (enumerable != null) 
         {
            result = enumerable.ToList();  
         }
      }
   }

   // Some other unrelated code (but that's why result has a variable)

   return result;

}

Perhaps:

public TResult Run<TResult>(Func<SqlDataReader, TResult> resultDelegate)
{
   TResult result;

   using (SqlDataReader reader = command.ExecuteReader()) // command is SqlCommand with attached SqlConnection
   {
      result = resultsDelegate(reader);
      if (typeof(TResult) == typeof(IEnumerable<object>)) 
      {
         var enumerable = result as IEnumerable<object>;
         if (enumerable != null) 
         {
            result = enumerable.ToList();  
         }
      }
   }

   // Some other unrelated code (but that's why result has a variable)

   return result;

}

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