C# 遍历Dictionary并修改其中的Value

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：

             // defined the Dictionary variable
             Dictionary<int, string> td = new Dictionary<int, string>();
             td.Add(, "str1");
             td.Add(, "str2");
             td.Add(, "str3");
             td.Add(, "str4");
             // test for
             TestForDictionary(td);
             // test for each
             TestForEachDictionary(td);

TestForDictionary Code

         static void TestForDictionary(Dictionary<int, string> paramTd)
         {

             for (int i = ;i<= paramTd.Keys.Count;i++)
             {
                 paramTd[i] = "string" + i;
                 Console.WriteLine(paramTd[i]);
             }
         }

TestForDictionary的执行结果

string1
string2
string3
string4

TestForEachDictionary Code

         static void TestForEachDictionary(Dictionary<int, string> paramTd)
         {
             int forEachCnt = ;
             foreach (KeyValuePair<int,string> item in paramTd)//System.InvalidOperationException: 'Collection was modified; enumeration operation may not execute.'
             {
                 paramTd[item.Key] = "forEach" + forEachCnt;
                 Console.WriteLine(paramTd[item.Key]);
                 forEachCnt += ;
             }
         }

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。

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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：

     [ComVisible(false)]
     [DebuggerDisplay("Count = {Count}")]
     [DebuggerTypeProxy(typeof(Generic.Mscorlib_DictionaryDebugView<,>))]
     [DefaultMember("Item")]
     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
     {
     ...
     }

Dictionary也有无参公共方法GetEnumerator

 //
         // 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();

我们再看看Dictionary<TKey, TValue>.Enumerator的代码：

         // Summary:
         //     Enumerates the elements of a System.Collections.Generic.Dictionary<TKey,TValue>.
         [Serializable]
         public struct Enumerator : IEnumerator<KeyValuePair<TKey, TValue>>, IDisposable, IDictionaryEnumerator, IEnumerator
         {

             // Summary:
             //     Gets the element at the current position of the enumerator.
             //
             // Returns:
             //     The element in the System.Collections.Generic.Dictionary<TKey,TValue> at
             //     the current position of the enumerator.
             public KeyValuePair<TKey, TValue> Current { get; }

             // Summary:
             //     Releases all resources used by the System.Collections.Generic.Dictionary<TKey,TValue>.Enumerator.
             public void Dispose();
             //
             // Summary:
             //     Advances the enumerator to the next element of the System.Collections.Generic.Dictionary<TKey,TValue>.
             //
             // Returns:
             //     true if the enumerator was successfully advanced to the next element; false
             //     if the enumerator has passed the end of the collection.
             //
             // Exceptions:
             //   System.InvalidOperationException:
             //     The collection was modified after the enumerator was created.
             public bool MoveNext();
         }

让我们看看，我们能否修改KeyValuePair的值

     public struct KeyValuePair<TKey, TValue>
     {
         //
         // Summary:
         //     Initializes a new instance of the System.Collections.Generic.KeyValuePair<TKey,TValue>
         //     structure with the specified key and value.
         //
         // Parameters:
         //   key:
         //     The object defined in each key/value pair.
         //
         //   value:
         //     The definition associated with key.
         public KeyValuePair(TKey key, TValue value);

         // Summary:
         //     Gets the key in the key/value pair.
         //
         // Returns:
         //     A TKey that is the key of the System.Collections.Generic.KeyValuePair<TKey,TValue>.
         public TKey Key { get; }
         //
         // Summary:
         //     Gets the value in the key/value pair.
         //
         // Returns:
         //     A TValue that is the value of the System.Collections.Generic.KeyValuePair<TKey,TValue>.
         public TValue Value { get; }

         // Summary:
         //     Returns a string representation of the System.Collections.Generic.KeyValuePair<TKey,TValue>,
         //     using the string representations of the key and value.
         //
         // Returns:
         //     A string representation of the System.Collections.Generic.KeyValuePair<TKey,TValue>,
         //     which includes the string representations of the key and value.
         public override string ToString();
     }

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：

 static void ModifyDictionaryValueForEach()
  {
      Span<int> storage = stackalloc int[];
      int num = ;
      foreach (ref int item in storage)
      {
          item = num++;
      }

      foreach (ref readonly var item in storage)
      {
          Console.Write($"{item} ");
      }
      // Output:
      // 0 1 2 3 4 5 6 7 8 9
  }

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();