Linq OrderBy against specific values

Question

Is there a way in Linq to do an OrderBy against a set of values (strings in this case) without knowing the order of the values?

Consider this data:

A         


        

Answer 1

Combined all answers (and more) into a generic LINQ extension supporting caching which handles any data type, can be case-insensitive and allows to be chained with pre- and post-ordering:

public static class SortBySample
{
    public static BySampleSorter Create(IEnumerable fixedOrder, IEqualityComparer comparer = null)
    {
        return new BySampleSorter(fixedOrder, comparer);
    }

    public static BySampleSorter Create(IEqualityComparer comparer, params TKey[] fixedOrder)
    {
        return new BySampleSorter(fixedOrder, comparer);
    }

    public static IOrderedEnumerable OrderBySample(this IEnumerable source, Func keySelector, BySampleSorter sample)
    {
        return sample.OrderBySample(source, keySelector);
    }

    public static IOrderedEnumerable ThenBySample(this IOrderedEnumerable source, Func keySelector, BySampleSorter sample)
    {
        return sample.ThenBySample(source, keySelector);
    }
}

public class BySampleSorter
{
    private readonly Dictionary dict;

    public BySampleSorter(IEnumerable fixedOrder, IEqualityComparer comparer = null)
    {
        this.dict = fixedOrder
            .Select((key, index) => new KeyValuePair(key, index))
            .ToDictionary(kv => kv.Key, kv => kv.Value, comparer ?? EqualityComparer.Default);
    }

    public BySampleSorter(IEqualityComparer comparer, params TKey[] fixedOrder)
        : this(fixedOrder, comparer)
    {
    }

    public IOrderedEnumerable OrderBySample(IEnumerable source, Func keySelector)
    {
        return source.OrderBy(item => this.GetOrderKey(keySelector(item)));
    }

    public IOrderedEnumerable ThenBySample(IOrderedEnumerable source, Func keySelector)
    {
        return source.CreateOrderedEnumerable(item => this.GetOrderKey(keySelector(item)), Comparer.Default, false);
    }

    private int GetOrderKey(TKey key)
    {
        int index;
        return dict.TryGetValue(key, out index) ? index : int.MaxValue;
    }
}

Sample usage using LINQPad-Dump():

var sample = SortBySample.Create(StringComparer.OrdinalIgnoreCase, "one", "two", "three", "four");
var unsorted = new[] {"seven", "six", "five", "four", "THREE", "tWo", "One", "zero"};
unsorted
    .OrderBySample(x => x, sample)
    .ThenBy(x => x)
    .Dump("sorted by sample then by content");
unsorted
    .OrderBy(x => x.Length)
    .ThenBySample(x => x, sample)
    .Dump("sorted by length then by sample");