問題：通過foreach循環(huán)遍歷IEnumerable是，修改其item值失敗，如下圖

原因探究：

1.IEnumerable接口內(nèi)部實現(xiàn)如下：

  public interface IEnumerable<out T> : IEnumerable
  {
    /// <summary><para>Returns an enumerator that iterates through the collection.</para></summary>
    IEnumerator<T> GetEnumerator();
  }

2.IEnumerator內(nèi)部實現(xiàn)如下

  public interface IEnumerator
  {
    /// <summary><para>Advances the enumerator to the next element of the collection.</para></summary>
    bool MoveNext();

    /// <summary><para>Gets the current element in the collection.</para></summary>
    object Current { get; }

    /// <summary><para>Sets the enumerator to its initial position, which is before the first element in the collection.</para></summary>
    void Reset();
  }

3.foreach遍歷機制相當于通過Enumerator的MoveNext方法和Current屬性進行遍歷，上圖代碼等價于下圖：

解決方案:將IEnumerable轉(zhuǎn)成List，在List中Current會被重寫并允許修改，如下圖：