C# List, Removing all null values *after* last non-null element

Question

I have a list of strings, example(C#):

new List<string> { "string1", null, null "string2", "string3", null, null, null }

I have many of those, all with different amounts of strings and nulls, where every one can be in a different place and the list length is not the same for each list, also not necessarily string list.

How would I remove the remaining null values after the last string, after the last non null value, keeping the null values that are between and in front of?

Thanks!

/Fredman

Answer 1

Check your list from last item to the beginning and remove null values until a non null values is reached:

List<string> list = new List<string> { "string1", null, null "string2", "string3", null, null, null };

for(int i=list.Count - 1; i>=0; i--)
{
     if(list[i]==null) list.RemoveAt(i);
     else break;
}

Answer 2

To avoid iterating the list twice to determine whether all entries are null (as you are currently doing), I'd suggest a slight tweak of Ashkan's solution:

var list = new List<string> { "string1", null, null, "string2", "string3", null, null, null };

int? firstNonNullIndex = null;

for (int i = list.Count - 1; i >= 0; i--)
{
    if (list[i] != null)
    {
        firstNonNullIndex = i;
        break;
    }
}

if (firstNonNullIndex == null) {
    // Do nothing as per your requirements (i.e. this handles your `All` call)
}
else
{
    list.RemoveRange(firstNonNullIndex.Value + 1, list.Count - firstNonNullIndex.Value - 1);
    // Do whatever you need to do with the `List` here
}

This solution has two main benefits:

• A single RemoveRange call is faster than multiple Remove calls
• No need to remove all (or indeed any!) of the elements if they are all null (ie this scenario gets much faster)

Answer 3

这是删除所有尾随null项的一种非常简单的方法：

while (items.Any() && items.Last() == null) items.RemoveAt(items.Count - 1);

Answer 4

Linq way:

int position = list.IndexOf(
          list.Where(x =>x!=null).OrderByDesc.FirstOrDefault());
return position == list.Count -1 ? list: list.RemoveRange(position+1, list.Count - 1);