c# - 是否可以为 yield-return 方法设置 'finally' 代码块？

Question

背景

大家好。我有一个名为 BaseRecordFetcher<TEntity> 的抽象类它有一个方法从子类中获取读取/排序/翻译/移动下一个方法，并且 yield 将结果作为模型实体返回。

当我读取多个数据行时，它会正确执行 yield return对于每个实体，然后在 do...while 之后到达 Trace 消息没有任何问题。

问题

但是我注意到当我使用 IEnumerable.FirstOrDefault() 时在集合上，系统假定不再需要 yield 返回，并且它不会完成此方法的执行!

除了关于返回了多少记录的跟踪输出之外，我对这种行为没有太多问题，但这让我开始思考:“如果我确实需要一些……我们称之为finally 代码？”

问题

有没有办法始终确保系统在 yield return 之后运行一些后处理代码？ ？

代码

/// <summary>
///     Retrieve translated entities from the database. The methods used to do
///     this are specified from the child class as parameters (i.e. Action or
///     Func delegates).
/// </summary>
/// <param name="loadSubsetFunc">
///     Specify how to load a set of database records. Return boolean
///     confirmation that records were found.
/// </param>
/// <param name="preIterationAction">
///     Specify what should happen to sort the results.
/// </param>
/// <param name="translateRowFunc">
///     Specify how a database record should translate to a model entity.
///     Return the new entity.
/// </param>
/// <param name="moveNextFunc">
///     Specify how the database row pointer should move on. Return FALSE on a
///     call to the final row.
/// </param>
/// <returns>
///     A set of translated entities from the database.
/// </returns>
/// <example><code>
///
/// return base.FetchRecords(
///     _dOOdad.LoadFacilitySites,
///     () => _dOOdad.Sort = _dOOdad.GetAutoKeyColumn(),
///     () => 
///         {
///             var entity = new FacilitySite();
///             return entity.PopulateLookupEntity(
///                 _dOOdad.CurrentRow.ItemArray);
///         },
///     _dOOdad.MoveNext);
/// 
/// </code></example>
protected virtual IEnumerable<TEntity> FetchRecords(
    Func<bool> loadSubsetFunc, Action preIterationAction,
    Func<TEntity> translateRowFunc, Func<bool> moveNextFunc)
{
    // If records are found, sort them and return set of entities
    if (loadSubsetFunc())
    {
        Trace.WriteLine(string.Format(
            "# FOUND one or more records: Returning {0}(s) as a set.",
            typeof(TEntity).Name));

        int recordCount = 0;

        preIterationAction();

        do
        {
            recordCount++;
            var entity = translateRowFunc();
            yield return entity;
        }
        while (moveNextFunc());

        // This code never gets reached if FirstOrDefault() is used on the set,
        // because the system will assume no more enities need to be returned
        Trace.WriteLine(string.Format(
            "# FINISHED returning records: {0} {1}(s) returned as a set.",
            recordCount, typeof(TEntity).Name));
    }
    else
    {
        Trace.WriteLine(string.Format(
            "# ZERO records found: Returning an empty set of {0}.",
            typeof(TEntity).Name));
    }
}

---

编辑(添加解决方案；谢谢@Servy 和@BenRobinson):

try
{
    do
    {
        recordCount++;
        var entity = translateRowFunc();
        yield return entity;
    }
    while (moveNextFunc());
}
finally
{
    // This code always executes, even when you use FirstOrDefault() on the set.
    Trace.WriteLine(string.Format(
        "# FINISHED returning records: {0} {1}(s) returned as a set.",
        recordCount, typeof(TEntity).Name));
}

Answer 1

是的，你可以提供finally迭代器 block 中的 block ，是的，它们旨在处理这种情况。

IEnumerator<T>工具 IDisposable .当编译器将迭代器 block 转换为实现时 Dispose枚举器的方法将执行任何应根据枚举器当前所在位置执行的 finally block 。

这意味着只要谁在迭代IEnumerator确保始终处理他们的枚举器，你可以确定你的 finally block 将运行。

另请注意 using block 将被转换为 try/finally block ，因此基本上可以像在迭代器 block 中希望的那样工作。如果需要，它们将在处理迭代器时清理给定的资源。