从对象转换到 IList<T> 派生类：foreach 失败

Question

I have a custom class MyList<T> : MyBaseList, IList<T> and some instances of it myList1, myList2... , which i store as object (comes from reflection, i can not change) in a List<object> ListOfLists = new List<object>() , so for example myList1 boxed as an object is stored at ListOfLists[0] and so on.

My class MyList<T> implements:

private List<T> items = new List<T>();
...
IEnumerator IEnumerable.GetEnumerator()
{
   return this.items.GetEnumerator();
}

When trying to iterate through every element of myList1, myList2... like...

bool Iterate<T>(T targetElem)
{
   foreach (var listObj in ListOfLists)
   {
      var myIList= (IList<T>)listObj;
      var myEList = myIList.Cast<MyList<T>>();

      foreach (var myList in myEList)
      {
         foreach (var listElem in myList)
         {
            if (EqualityComparer<T>.Default.Equals(listElem , targetElem)) return true;
         }
      }
   }
   return false;
}

... I receive Error System.InvalidCastException at foreach (var myList in myEList) .

Has anyone an idea on how to solve this issue?
Thanks in advance!

Answer 1

You seem to have written an extra foreach loop. To iterate over a list of lists, you only need 2. You also seem to be trying to cast each T object in each sublist to MyList<T> , which doesn't make much sense.

You probably intended this:

foreach (var listObj in ListOfLists)
{
    var myIList= (IList<T>)listObj;

    foreach (var listElem in myIList)
    {
        if (EqualityComparer<T>.Default.Equals(listElem , targetElem)) return true;
    }
}
return false;

I don't know why you are so sure that every sublist must be a list of T . Maybe one of them isn't, who knows? If this is the case, you can use as to cast and check for null:

foreach (var listObj in ListOfLists)
{
    var myIList= listObj as IList<T>;

    if (myIList == null) continue;

    foreach (var listElem in myIList)
    {
        if (EqualityComparer<T>.Default.Equals(listElem , targetElem)) return true;
    }
}
return false;

Answer 2

The problem lies here:

var myEList = myIList.Cast<MyList<T>>();

Here you are trying to cast every item in myIList to an instance of MyList<T> . Since the execution of Cast is deferred you get the error not before the foreach .

As far as I understand your intention you should do

var myEList = myIList.Cast<T>();