ImmutableListMicrosoft.Bcl.Immutable中的Remove方法

从NuGet包Microsoft. bcl . immutable version 1.0.34和1.1.22-beta中体验Microsoft ImmutableList的一些意外性能

当从不可变列表中删除项时，性能非常慢。对于包含20000个整数值(1…20000)的ImmutableList，如果从20000到1开始删除，大约需要52秒才能从列表中删除所有项。如果我对一个通用的List<T>做同样的事情，我在每次删除操作后创建一个列表的副本，它需要大约500 ms.

我对这些结果感到有点惊讶，因为我认为ImmutableList将比复制通用List<T>更快，但也许这是意料之中的?

示例代码

// Generic List Test
var genericList = new List<int>();
var sw = Stopwatch.StartNew();
for (int i = 0; i < 20000; i++)
{
    genericList.Add(i);
    genericList = new List<int>(genericList);
}
sw.Stop();
Console.WriteLine("Add duration for List<T>: " + sw.ElapsedMilliseconds);
IList<int> completeList = new List<int>(genericList);
sw.Restart();
// Remove from 20000 -> 0.
for (int i = completeList.Count - 1; i >= 0; i--)
{
    genericList.Remove(completeList[i]);
    genericList = new List<int>(genericList);
}
sw.Stop();
Console.WriteLine("Remove duration for List<T>: " + sw.ElapsedMilliseconds);
Console.WriteLine("Items after remove for List<T>: " + genericList.Count);

// ImmutableList Test
var immutableList = ImmutableList<int>.Empty;
sw.Restart();
for (int i = 0; i < 20000; i++)
{
    immutableList = immutableList.Add(i);
}
sw.Stop();
Console.WriteLine("Add duration for ImmutableList<T>: " + sw.ElapsedMilliseconds);
sw.Restart();
// Remove from 20000 -> 0.
for (int i = completeList.Count - 1; i >= 0; i--)
{
    immutableList = immutableList.Remove(completeList[i]);
}
sw.Stop();
Console.WriteLine("Remove duration for ImmutableList<T>: " + sw.ElapsedMilliseconds);
Console.WriteLine("Items after remove for ImmutableList<T>: " + immutableList.Count);

更新

如果从ImmutableList的开头删除项，就像普通的foreach循环一样，那么性能要比好得多。然后，删除所有项所需的时间少于100毫秒。这不是在所有情况下都可以做的事情，但知道这一点是很好的。