Question

Having some trouble with Predicates in C#.
I have two sets of code (Both of which I believe should accomplish the same result) but one never works. The reason I am asking is I need this predicate to appear a few times with different elements and so I’d like to keep it as minimal as possible (the non working one is very simplistic while the other contains many lines).

1 Not working:

ItemViewModel item = (App.ViewModel.All_Items.Where(x => (x as ItemViewModel).Name == my_list_of_strings.ElementAt(2)) as ItemViewModel);

Also using "Select" instead of Where isn’t working.

2 Working:

foreach (ItemViewModel it in App.ViewModel.All_Items)
{
  if (item.Name == my_list_of_strings.ElementAt(2))
  {
    MessageBox.Show("Success!!");
    item = it;
    continue; // Leave loop
  }
}

It’s probably something stupid that I’ve overlooked but if anyone knows the solution, that would be great!

Thanks.

Answer 1

IEnumerable<T>.Where(Func<T, bool>) returns a collection however it looks like what you want is a single element. There are a couple of options:

IEnumerable<T>.FirstOrDefault(Func<T, bool>) // returns null if no element found
IEnumerable<T>.First(Func<T, bool>) // throws if no element is found

// These throw an error if more than one element exists that matches the query
IEnumerable<T>.SingleOrDefault(Func<T, bool>) // returns null if no element found
IEnumerable<T>.Single(Func<T, bool>) // throws if no element is found

In your example it would be:

// Just replace "Where" with "FirstOrDefault"
ItemViewModel item = (App.ViewModel.All_Items.FirstOrDefault(x => (x as ItemViewModel).Name == my_list_of_strings.ElementAt(2)) as ItemViewModel);