如何在 C# 中使用修改后的列表进行迭代和重新迭代时删除项目列表

Question

I have a list of strings say List1 and for each item of List1 , another list will be generated dynamically inside foreach loop. If generated list items are existed in List1 for any item , those items should be removed from List1 and re-iterated with rest of List1 items

Example: Lets say for item1 , List2 is generated

ForEach loop started:

1st loop

-- item1 --> Generated List say List2

-- item1 --> remove the List2 items from List1 ( In this case, items(item1,item2,item4) are removed and re-iterate List1 with only 'item3' 

2nd loop

 -- item3 ---> generated List say List3

  -- item3 ---> check and remove the List3 items from List1 if existed

Loop Ends.

What is best way to achieve this?

        private static void IterateListAndRemoveItemsWhileIterating()
        {
            List<string> list1 = new List<string>();
            list1.Add("item1");
            list1.Add("item2");
            list1.Add("item3");
            list1.Add("item4");


            List<string> list2 = new List<string>();
            list2.Add("item1");
            list2.Add("item2");
            list2.Add("item4");

            foreach (var item in list1.ToList())
            {
                Console.WriteLine(item); 
                //Do some logic with each item and get another list of Items and then remove  from List1. (Ex: List2 which is another list of items after processing some logic with item1) 
                list1.RemoveAll(r => list2.Contains(r)); // After removing list2 items in list1, I want to re-iterate with rest of items but its not happening
            }
        }

Actual Output:

item1
item3
item2
item4

Expected Output:

item1
item3

Answer 1

Your variable names are ambiguous item is the name of the variable from the foreach loop and the lambda syntax. so the compiler may be using the foreach variable in your lambda meaning you would get all the items in list1 printed to console. try changing one of the var names like this:

foreach (var item in list1.ToList())
{
  Console.WriteLine(item); 
  list1.RemoveAll(s => list2.Contains(s));
}

I hope that will get you the intended result

Correction

using list1.ToList() makes a clone of the original list so the enumerable wont change on the foreach loop even if you make changes to the original list. you can avoid this by keeping a second list:

List<string> enumerationList = list1.ToList();

foreach (var item in enumerationList)
{
  if(list1.Contains(item))
  {
    Console.WriteLine(item); 
    list1.RemoveAll(s => list2.Contains(s));
  }
}

I am sure there are other ways to do this as well.