C#的Dictionary类型的值，知道key后，value可以修改吗？答案是肯定能修改的。我在遍历的过程中可以修改Value吗？答案是也是肯定能修改的，但是不能用For each循环。否则会报以下的Exception.
System.InvalidOperationException: 'Collection was modified; enumeration operation may not execute.'
之所以会报Exception是For each本身的问题，和Dictionary没关系。For each循环不能改变集合中各项的值，如果需要迭代并改变集合项中的值，请用For循环。
大家来看下例子：
1 // defined the Dictionary variable 2 Dictionary<int, string> td = new Dictionary<int, string>(); 3 td.Add(1, "str1"); 4 td.Add(2, "str2"); 5 td.Add(3, "str3"); 6 td.Add(4, "str4"); 7 // test for 8 TestForDictionary(td); 9 // test for each 10 TestForEachDictionary(td);
TestForDictionary Code
1 static void TestForDictionary(Dictionary<int, string> paramTd) 2 { 3 4 for (int i = 1;i<= paramTd.Keys.Count;i++) 5 { 6 paramTd[i] = "string" + i; 7 Console.WriteLine(paramTd[i]); 8 } 9 }
TestForDictionary的执行结果
string1
string2
string3
string4
TestForEachDictionary Code
1 static void TestForEachDictionary(Dictionary<int, string> paramTd) 2 { 3 int forEachCnt = 1; 4 foreach (KeyValuePair<int,string> item in paramTd)//System.InvalidOperationException: 'Collection was modified; enumeration operation may not execute.' 5 { 6 paramTd[item.Key] = "forEach" + forEachCnt; 7 Console.WriteLine(paramTd[item.Key]); 8 forEachCnt += 1; 9 } 10 }
TestForEachDictionary里的For each会在循环第二次的时候报错，也就是说它会在窗口中打印出“forEach1”后断掉。
---------------------------------------------------------------------------------------------------------------------------------------------------
为什么foreach循环不能改变Dictionary中Key对应的Value?
首先，我们知道要使用Foreach in语句需要满足下列条件：
1.迭代集合实现了System.Collections.IEnumerable或者System.Collections.Generic.IEnumerable<T>接口；
2.有public Enumerator GetEnumerator();方法
3.GetEnumerator的返回类型是Enumerator,那就必须有Current属性和MoveNext方法
其实1,2,3是一个东西，关联性很强。
我迭代的集合是Dictionary，我把Dictionary的代码片段贴出来：
1 [ComVisible(false)] 2 [DebuggerDisplay("Count = {Count}")] 3 [DebuggerTypeProxy(typeof(Generic.Mscorlib_DictionaryDebugView<,>))] 4 [DefaultMember("Item")] 5 public class Dictionary<TKey, TValue> : IDictionary<TKey, TValue>, ICollection<KeyValuePair<TKey, TValue>>, IEnumerable<KeyValuePair<TKey, TValue>>, IEnumerable, IDictionary, ICollection, IReadOnlyDictionary<TKey, TValue>, IReadOnlyCollection<KeyValuePair<TKey, TValue>>, ISerializable, IDeserializationCallback 6 { 7 ... 8 }
Dictionary也有无参公共方法GetEnumerator
1 // 2 // Summary: 3 // Returns an enumerator that iterates through the System.Collections.Generic.Dictionary<TKey,TValue>. 4 // 5 // Returns: 6 // A System.Collections.Generic.Dictionary<TKey,TValue>.Enumerator structure 7 // for the System.Collections.Generic.Dictionary<TKey,TValue>. 8 public Dictionary<TKey, TValue>.Enumerator GetEnumerator();
我们再看看Dictionary<TKey, TValue>.Enumerator的代码：
1 // Summary: 2 // Enumerates the elements of a System.Collections.Generic.Dictionary<TKey,TValue>. 3 [Serializable] 4 public struct Enumerator : IEnumerator<KeyValuePair<TKey, TValue>>, IDisposable, IDictionaryEnumerator, IEnumerator 5 { 6 7 // Summary: 8 // Gets the element at the current position of the enumerator. 9 // 10 // Returns: 11 // The element in the System.Collections.Generic.Dictionary<TKey,TValue> at 12 // the current position of the enumerator. 13 public KeyValuePair<TKey, TValue> Current { get; } 14 15 // Summary: 16 // Releases all resources used by the System.Collections.Generic.Dictionary<TKey,TValue>.Enumerator. 17 public void Dispose(); 18 // 19 // Summary: 20 // Advances the enumerator to the next element of the System.Collections.Generic.Dictionary<TKey,TValue>. 21 // 22 // Returns: 23 // true if the enumerator was successfully advanced to the next element; false 24 // if the enumerator has passed the end of the collection. 25 // 26 // Exceptions: 27 // System.InvalidOperationException: 28 // The collection was modified after the enumerator was created. 29 public bool MoveNext(); 30 }
让我们看看，我们能否修改KeyValuePair的值
1 public struct KeyValuePair<TKey, TValue> 2 { 3 // 4 // Summary: 5 // Initializes a new instance of the System.Collections.Generic.KeyValuePair<TKey,TValue> 6 // structure with the specified key and value. 7 // 8 // Parameters: 9 // key: 10 // The object defined in each key/value pair. 11 // 12 // value: 13 // The definition associated with key. 14 public KeyValuePair(TKey key, TValue value); 15 16 // Summary: 17 // Gets the key in the key/value pair. 18 // 19 // Returns: 20 // A TKey that is the key of the System.Collections.Generic.KeyValuePair<TKey,TValue>. 21 public TKey Key { get; } 22 // 23 // Summary: 24 // Gets the value in the key/value pair. 25 // 26 // Returns: 27 // A TValue that is the value of the System.Collections.Generic.KeyValuePair<TKey,TValue>. 28 public TValue Value { get; } 29 30 // Summary: 31 // Returns a string representation of the System.Collections.Generic.KeyValuePair<TKey,TValue>, 32 // using the string representations of the key and value. 33 // 34 // Returns: 35 // A string representation of the System.Collections.Generic.KeyValuePair<TKey,TValue>, 36 // which includes the string representations of the key and value. 37 public override string ToString(); 38 }
大家看到了，TValue Value { get; }是我们需要修改的项，只有get方法，所以它是只读的。
上面是从代码方面解释foreach迭代时不可改变集合项，还有一个原因是是你改变值类型的值时，它对应的栈地址也就相应的改变了，这个大家都知道，我就不多说了。那么问题来了，如果我不改变变量的地址，是不是我就可以用Foreach改变集合项呢？答案是肯定的。可以看下边MSDN的代码：
1 static void ModifyDictionaryValueForEach() 2 { 3 Span<int> storage = stackalloc int[10]; 4 int num = 0; 5 foreach (ref int item in storage) 6 { 7 item = num++; 8 } 9 10 foreach (ref readonly var item in storage) 11 { 12 Console.Write($"{item} "); 13 } 14 // Output: 15 // 0 1 2 3 4 5 6 7 8 9 16 }
上边的代码必须是在C#版本是7.3下运行.Span<T>是.net Core2.1里的类型。然后使用ref就可以改变了。大家可以试试。
// // Summary: // Returns an enumerator that iterates through the System.Collections.Generic.Dictionary<TKey,TValue>. // // Returns: // A System.Collections.Generic.Dictionary<TKey,TValue>.Enumerator structure // for the System.Collections.Generic.Dictionary<TKey,TValue>. public Dictionary<TKey, TValue>.Enumerator GetEnumerator();