Find index of a value in an array

int keyIndex = Array.FindIndex(words, w => w.IsKey);

That actually gets you the integer index and not the object, regardless of what custom class you have created

For arrays you can use: Array.FindIndex<T>:

int keyIndex = Array.FindIndex(words, w => w.IsKey);

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For lists you can use List<T>.FindIndex:

int keyIndex = words.FindIndex(w => w.IsKey);

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You can also write a generic extension method that works for any Enumerable<T>:

///<summary>Finds the index of the first item matching an expression in an enumerable.</summary>
///<param name="items">The enumerable to search.</param>
///<param name="predicate">The expression to test the items against.</param>
///<returns>The index of the first matching item, or -1 if no items match.</returns>
public static int FindIndex<T>(this IEnumerable<T> items, Func<T, bool> predicate) {
    if (items == null) throw new ArgumentNullException("items");
    if (predicate == null) throw new ArgumentNullException("predicate");

    int retVal = 0;
    foreach (var item in items) {
        if (predicate(item)) return retVal;
        retVal++;
    }
    return -1;
}

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And you can use LINQ as well:

int keyIndex = words
    .Select((v, i) => new {Word = v, Index = i})
    .FirstOrDefault(x => x.Word.IsKey)?.Index ?? -1;

int keyIndex = words.TakeWhile(w => !w.IsKey).Count();