C# What should be key to cache results for different Hashsets

Question

I am coding an ASP.NET MVC 3 application. The Navigation menu is created dynamically from DB based on permission set for user, using a child action. Users are allocated to Usergroups/Roles, but then they can also have some certain specific additional permissions. I am having a Hashset being returned by Authorization Services for all the privileges for current user. Most of users would have same privileges as the usergroup (no additional privileges). So, i want to cache the navigation menu.

Questions, 1. I need to use VaryByCustom option for OutputCache attribute? 2. How should i create unique keys for hashtables having same set of permissions (may be added to hashset in different order)?

Answer 1

1. I need to use VaryByCustom option for OutputCache attribute?

I suppose that's the best version. See here on SO or in this blog.

2. How should i create unique keys for hashtables having same set of permissions (may be added to hashset in different order)?

To build the cache key, I have used something similar to this in the past (uses a GetHashCode implementation):

public class Permission
{
    public Guid Id { get; set; }
    public string Name { get; set; }
    public override int GetHashCode() { return this.Id.GetHashCode(); }
}

public static string BuildCacheKey(HashSet<Permission> permissions)
{
    var hashCode = GetHashCode(permissions);
    return string.Format(CultureInfo.InvariantCulture, "permission_{0}", hashCode);
}

private static int GetHashCode(IEnumerable<Permission> permissions)
{
    unchecked
    {
        var hash = 17;
        foreach (var permission in permissions.OrderBy(p => p.Id))
        {
            hash = hash * 23 + permission.GetHashCode();
        }

        return hash;
    }
}

This should work with several permissions IMHO. If you only have a few permission combinations and not too complicated permission names, you could also just concat the names.

public static string BuildCacheKey(HashSet<Permission> permissions)
{
    var builder = new StringBuilder("permission_");
    foreach (var permission in permissions.OrderBy(p => p.Name))
    {
        builder.Append(permission.Name);
    }

    return builder.ToString();
}