.Net5 下Dictionary 为什么可以在foreach中Remove
通信的搞程序 人气:0在一个讨论群里，看见有人说Dictionary可以在foreach中直接调用Remove了，带着疑问，写了简单代码进行尝试
class Program
{
static void Main(string[] args)
{
var dic = Enumerable.Range(1, 10).ToDictionary(t => t, t => t);
foreach (var i in dic)
{
if (i.Key.GetHashCode() % 2 == 0)
{
dic.Remove(i.Key);
}
else
{
Console.WriteLine($"{i.Key}");
}
}
Console.WriteLine("Hello World!");
}
}
执行果然没有报错，输出正常。
终于不再需要进行单独执行Remove
要想知道为啥在.Net Framework上不行，在.Net5下却可以，就需要知道在.Net5中Dictionary有着什么样的变化
.Net Framework中源码 .Net5中源码
我们看下两者有什么区别：
Framework中是这样的：
1 public bool MoveNext() {
2 if (version != dictionary.version) {
3 ThrowHelper.ThrowInvalidOperationException(ExceptionResource.InvalidOperation_EnumFailedVersion);
4 }
5
6 // Use unsigned comparison since we set index to dictionary.count+1 when the enumeration ends.
7 // dictionary.count+1 could be negative if dictionary.count is Int32.MaxValue
8 while ((uint)index < (uint)dictionary.count) {
9 if (dictionary.entries[index].hashCode >= 0) {
10 current = new KeyValuePair<TKey, TValue>(dictionary.entries[index].key, dictionary.entries[index].value);
11 index++;
12 return true;
13 }
14 index++;
15 }
16
17 index = dictionary.count + 1;
18 current = new KeyValuePair<TKey, TValue>();
19 return false;
20 }
.Net5中是这样的：
1 public bool MoveNext()
2 {
3 if (_version != _dictionary._version)
4 {
5 ThrowHelper.ThrowInvalidOperationException_InvalidOperation_EnumFailedVersion();
6 }
7
8 // Use unsigned comparison since we set index to dictionary.count+1 when the enumeration ends.
9 // dictionary.count+1 could be negative if dictionary.count is int.MaxValue
10 while ((uint)_index < (uint)_dictionary._count)
11 {
12 ref Entry entry = ref _dictionary._entries![_index++];
13
14 if (entry.next >= -1)
15 {
16 _current = new KeyValuePair<TKey, TValue>(entry.key, entry.value);
17 return true;
18 }
19 }
20
21 _index = _dictionary._count + 1;
22 _current = default;
23 retur
细看好像两者并没什么很明显的区别。我们知道，在对Dictionary进行操作的时候，_version会自增改变，从而导致报错。难道.Net5中进行Remove操作_version不会改变。
.Net5中Remove代码：
1 public bool Remove(TKey key)
2 {
3 // The overload Remove(TKey key, out TValue value) is a copy of this method with one additional
4 // statement to copy the value for entry being removed into the output parameter.
5 // Code has been intentionally duplicated for performance reasons.
6
7 if (key == null)
8 {
9 ThrowHelper.ThrowArgumentNullException(ExceptionArgument.key);
10 }
11
12 if (_buckets != null)
13 {
14 Debug.Assert(_entries != null, "entries should be non-null");
15 uint collisionCount = 0;
16 uint hashCode = (uint)(_comparer?.GetHashCode(key) ?? key.GetHashCode());
17 ref int bucket = ref GetBucket(hashCode);
18 Entry[]? entries = _entries;
19 int last = -1;
20 int i = bucket - 1; // Value in buckets is 1-based
21 while (i >= 0)
22 {
23 ref Entry entry = ref entries[i];
24
25 if (entry.hashCode == hashCode && (_comparer?.Equals(entry.key, key) ?? EqualityComparer<TKey>.Default.Equals(entry.key, key)))
26 {
27 if (last < 0)
28 {
29 bucket = entry.next + 1; // Value in buckets is 1-based
30 }
31 else
32 {
33 entries[last].next = entry.next;
34 }
35
36 Debug.Assert((StartOfFreeList - _freeList) < 0, "shouldn't underflow because max hashtable length is MaxPrimeArrayLength = 0x7FEFFFFD(2146435069) _freelist underflow threshold 2147483646");
37 entry.next = StartOfFreeList - _freeList;
38
39 if (RuntimeHelpers.IsReferenceOrContainsReferences<TKey>())
40 {
41 entry.key = default!;
42 }
43
44 if (RuntimeHelpers.IsReferenceOrContainsReferences<TValue>())
45 {
46 entry.value = default!;
47 }
48
49 _freeList = i;
50 _freeCount++;
51 return true;
52 }
53
54 last = i;
55 i = entry.next;
56
57 collisionCount++;
58 if (collisionCount > (uint)entries.Length)
59 {
60 // The chain of entries forms a loop; which means a concurrent update has happened.
61 // Break out of the loop and throw, rather than looping forever.
62 ThrowHelper.ThrowInvalidOperationException_ConcurrentOperationsNotSupported();
63 }
64 }
65 }
66 return false;
67 }
一看果然_version不会变化。看到这可能会有喊知乎内行啊，一行代码就解决问题，那为什么Framework中不这样做呢。相关提交讨论
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