Get the index of item in a list given its property

Question

In MyList List<Person> there may be a Person with its Name property set to "ComTruise". I need the index of first occurrence of "ComTruise" in MyList , but not the entire Person element.

What I'm doing now is:

string myName = ComTruise;
int thatIndex = MyList.SkipWhile(p => p.Name != myName).Count();

If the list is very large, is there a more optimal way to get the index?

Answer 1

You could use FindIndex

string myName = "ComTruise";
int myIndex = MyList.FindIndex(p => p.Name == myName);

Note: FindIndex returns -1 if no item matching the conditions defined by the supplied predicate can be found in the list.

Answer 2

As it's an ObservableCollection , you can try this

int index = MyList.IndexOf(MyList.Where(p => p.Name == "ComTruise").FirstOrDefault());

It will return -1 if "ComTruise" doesn't exist in your collection.

As mentioned in the comments, this performs two searches. You can optimize it with a for loop.

int index = -1;
for(int i = 0; i < MyList.Count; i++)
{
    //case insensitive search
    if(String.Equals(MyList[i].Name, "ComTruise", StringComparison.OrdinalIgnoreCase)) 
    {
        index = i;
        break;
    } 
}

Answer 3

It might make sense to write a simple extension method that does this:

public static int FindIndex<T>(
    this IEnumerable<T> collection, Func<T, bool> predicate)
{
    int i = 0;
    foreach (var item in collection)
    {
        if (predicate(item))
            return i;
        i++;
    }
    return -1;
}

Answer 4

var p = MyList.Where(p => p.Name == myName).FirstOrDefault();
int thatIndex = -1;
if (p != null)
{
  thatIndex = MyList.IndexOf(p);
}

if (p != -1) ...