.Net5 下Dictionary 为什么可以在foreach中Remove |
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.Net5 下Dictionary 为什么可以在foreach中Remove
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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(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();
19 return false;
20 }
View Code
.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(entry.key, entry.value);
17 return true;
18 }
19 }
20
21 _index = _dictionary._count + 1;
22 _current = default;
23 retur
View Code
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.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()) 40 { 41 entry.key = default!; 42 } 43 44 if (RuntimeHelpers.IsReferenceOrContainsReferences()) 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。看到这可能会直呼内行啊，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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