How to compare string to String Array in C#?

So you want the word within the array which occurs earliest in the target string? That sounds like you might want something like:

return array.Select(word => new { word, index = target.IndexOf(word) })
            .Where(pair => pair.index != -1)
            .OrderBy(pair => pair.index)
            .Select(pair => pair.word)
            .FirstOrDefault();

Those steps in detail:

• Project the words into a sequence of word/index pairs, where the index is the index of that word within the target string
• Omit words that didn't occur in the target string by removing pairs where the index is -1 (string.IndexOf returns -1 if it's not found)
• Order by index to make the earliest word occur as the first pair
• Select the word within each pair, as we no longer care about indexes
• Return the first word, or null if the sequence is empty