如何有效地返回C＃中SortedList <>中的第一个和最后一个项目？

Question

我的问题就像下面的问题，但与SortedList<Tkey, TValue>而不是原来的SortedList。

Return first element in SortedList in C#

似乎没有像GetKey()这样的功能。

Answer 1

你可以简单地使用list.FirstOrDefault()和list.LastOrDefault()。

如果列表为空，这两个方法将返回default(KeyValuePair<TKey,TValue>)。

并且最好使用它们，因为使用list.First()和list.Last()会在列表为空时抛出错误。

Answer 2

根据您的（不太清楚）问题，您想要检索列表中第一个和最后一个项目的键。

SortedList<TKey, TValue>实施IEnumerable<KeyValuePair<TKey,TValue>>所以这样做：

sortedList.FirstOrDefault().Key

和

sortedList.LastOrDefault().Key

将返回这些键

编辑：正如@mjwills所建议的那样，使用Keys属性在性能方面是一个更好的主意，因为它实现了IList<TKey>和LastOrDefault被优化以适用于这种情况（不是获取整个集合来获取最后一项）：

sortedList.Keys.FirstOrDefault()
sortedList.Keys.LastOrDefault()

Answer 3

使用IEnumerable <>。Last或IEnumerable <>。LastOrDefault方法仅对IList <>值有效。让我们看看the source code - 访问非IList <>的最后一项需要迭代所有项目。

为了更高效率，更好地依赖于类型IList <>（SortedList<>.Keys）的SortedList<>.Values或https://dotnetfiddle.net/MLmDIL属性：

        var sl = new SortedList<string, int>();

        for (var i = 0; i < 50000; i++) {
            sl.Add(i.ToString("X8"), i);
        }       

        Console.WriteLine("SortedList<> is IList<>: " + (sl is IList<KeyValuePair<string, int>>));
        Console.WriteLine("Keys-property in SortedList<> is IList<>: " + (sl.Keys is IList<string>));


        Console.WriteLine("\n\nPerformance measurement - get last Key:");

        var firstKey = sl.First().Key;

        watch.Restart();        
        var lastKey = sl.Last().Key;
        watch.Stop();
        Console.WriteLine("   * Non IList<> access takes {0} (first key: {1}, last key: {2})", watch.Elapsed.TotalMilliseconds, firstKey, lastKey);

        firstKey = sl.Keys.First();

        watch.Restart();        
        lastKey = sl.Keys.Last();
        watch.Stop();
        Console.WriteLine("   * IList<> access takes {0} (first key: {1}, last key: {2})", watch.Elapsed.TotalMilliseconds, firstKey, lastKey);


        Console.WriteLine("\n\nPerformance measurement - get last Value:");

        var firstValue = sl.First().Value;

        watch.Restart();        
        var lastValue = sl.Last().Value;
        watch.Stop();
        Console.WriteLine("   * Non IList<> access takes {0} (first value: {1}, last value: {2})", watch.Elapsed.TotalMilliseconds, firstValue, lastValue);

        firstValue = sl.Values.First();

        watch.Restart();        
        lastValue = sl.Values.Last();
        watch.Stop();
        Console.WriteLine("   * IList<> access takes {0} (first value: {1}, last value: {2})", watch.Elapsed.TotalMilliseconds, firstValue, lastValue);     


        Console.WriteLine("\n\nPerformance measurement - get last Value by Key:");

        watch.Restart();        
        lastKey = sl.Keys.Last();
        lastValue = sl[lastKey];
        watch.Stop();
        Console.WriteLine("   * IDictionary<> access takes {0} (last key: {1}, last value: {2})", watch.Elapsed.TotalMilliseconds, lastKey, lastValue); 

    /*
    Execution result:

SortedList<> is IList<>: False
Keys-property in SortedList<> is IList<>: True


Performance measurement - get last Key:
   * Non IList<> access takes 0.7146 (first key: 00000000, last key: 0000C34F)
   * IList<> access takes 0.0032 (first key: 00000000, last key: 0000C34F)


Performance measurement - get last Value:
   * Non IList<> access takes 0.7366 (first value: 0, last value: 49999)
   * IList<> access takes 0.0003 (first value: 0, last value: 49999)


Performance measurement - get last Value by Key:
   * IDictionary<> access takes 0.0036 (last key: 0000C34F, last value: 49999)
    */