Custom sorting order in LINQ (ORDER BY WEIGHTING)

I have developed a C# LINQ extension method to allow very flexible ordering by assigning weightings to the sort keys of the elements to be ordered. I was inspired by the functionality offered by SQL Server’s ORDER BY CASE WHEN:

Source code

ORDER BY
CASE SEASON
WHEN 'WINTER' THEN 1
WHEN 'SPRING' THEN 2
WHEN 'SUMMER' THEN 3
WHEN 'AUTUMN' THEN 4
END

The extension method I have created lets you pass a lambda function which allows the use of logic to apply custom weightings to the sort keys.

Source code
   
var data = dt.Select(g => new
{
    Season = g.season,
    AverageTemp = g.temp
}).OrderByWeight(a => a.Season, x =>
{
    if (x == "WINTER") return 1;
    if (x == "SPRING") return 2;
    if (x == "SUMMER") return 3;
    if (x == "AUTUMN") return 4;
    return 99;
});

The above will return an IOrderedEnumerable sorted by Season in the order WINTER, SPRING, SUMMER, AUTUMN. However, the OrderByWeight extension method provides even more powerful functionality as the lambda allows us to perform string comparisons and more:

Source code
   
var data = dt.Select(g => new
{
    Year = g.year,
}).OrderByWeight(a => a.Year, x =>
{
    if (x.Contains("2008")) return 1;
    if (x.Contains("2009")) return 2;
    if (x.Contains("2010") return 3;
    return 99;
});

Using the above, a non-trivial list such as {‘London 2010′, ‘Leeds 2008′, ‘Cardiff 2009′, ‘Glasgow 2011′} can be easily sorted to {‘Leeds 2008′, ‘Cardiff 2009′, ‘London 2010′, ‘Glasgow 2011′}

The extension method used to implement this functionality is:

Source code
   
public static IOrderedEnumerable<TSource> OrderByWeight<TSource, TKey>(this IEnumerable<TSource> source, Func<TSource, TKey> keySelector, Func<TKey, int> weighting) where TKey : IComparable
{
    Dictionary<TSource, int> order = new Dictionary<TSource, int>();
    foreach (TSource item in source)
    {
        if (!order.ContainsKey(item)) order.Add(item, weighting(keySelector(item)));
    }
    return source.OrderBy(s => order[s]);
}

It has not been throughly unit tested yet but it should have the same behaviour as GroupBy. Whilst I have only provided the C# implementation I’m sure it can be converted to other languages using the CLR.