问题：通过foreach循环遍历IEnumerable是，修改其item值失败，如下图

原因探究：

1.IEnumerable接口内部实现如下：

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

2.IEnumerator内部实现如下

  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遍历机制相当于通过Enumerator的MoveNext方法和Current属性进行遍历，上图代码等价于下图：

解决方案:将IEnumerable转成List，在List中Current会被重写并允许修改，如下图：