IndexOf with Linq, accepting lambda expression

Question

Is there a way to find the index from list of partial prefixes with Linq, something like:

List<string> PartialValues = getContentsOfPartialList();
string wholeValue  = "-moz-linear-gradient(top,  #1e5799 0%, #7db9e8 100%)";
int indexOfPartial = PartialValues
                      .IndexOf(partialPrefix=>wholeValue.StartsWith(partialPrefix));

Unfortunately, IndexOf() doesn't accept lambda expression. Is there a similar Linq method for this?

Answer 1

You don't need LINQ at all, List<T> has a method FindIndex.

int indexOfPartial = PartialValues
    .FindIndex(partialPrefix => wholeValue.StartsWith(partialPrefix));

For the sake of completeness, you can use LINQ, but it's not necessary:

int indexOfPartial = PartialValues
  .Select((partialPrefix , index) => (partialPrefix , index))
  .Where(x => wholeValue.StartsWith(x.partialPrefix))
  .Select(x => x.index)
  .DefaultIfEmpty(-1)
  .First();

Answer 2

If you have List<T>, see accepted answer.

In case you have IEnumerable (or other collection that implements it) instead of List, you can use following LINQ code:

int index = PartialValues.TakeWhile(partialPrefix=> ! wholeValue.StartsWith(partialPrefix)).Count();

Pay attention at the negation operator (!) in lambda expression: lambda expression should return true for elements that do NOT match your condition.

Code will return number of elements (e.g. index will point at position immediately after the last element) if no elements match the condition.