TopologicalSorter.cs

public static class TopologicalSorter
{
    public static IEnumerable<T> Sort<T>(this IEnumerable<T> source, Func<T, IEnumerable<T>> dependsOn)
    {
        var sorted = new List<T>();
        var visited = new HashSet<T>();
        foreach (var item in source) Visit(item, visited, sorted, dependsOn);
        return sorted;
    }

    private static void Visit<T>(T item, ISet<T> visited, ICollection<T> sorted, Func<T, IEnumerable<T>> dependsOn)
    {
        if (!visited.Contains(item))
        {
            visited.Add(item);
            foreach (var dep in dependsOn(item)) Visit(dep, visited, sorted, dependsOn);
            sorted.Add(item);
        }
    }
}