Combine Multiple Predicates

Question

Is there any way in c# .NET 2.0! to combine multiple Predicates?

Let's say I have the following code.

List<string> names = new List<string>(); names.Add("Jacob"); names.Add("Emma"); names.Add("Michael"); names.Add("Isabella"); names.Add("Ethan"); names.Add("Emily");  List<string> filteredNames = names.FindAll(StartsWithE);  static bool StartsWithE(string s) {     if (s.StartsWith("E"))     {         return true;     }     else     {         return false;     } } 

This gives me:

Emma Ethan Emily 

So this is pretty cool stuff, but I know want to be able to filter using multiple predicates.

So I want to be able to say something like this:

List<string> filteredNames = names.FindAll(StartsWithE OR StartsWithI); 

In order to get:

Emma Isabella Ethan Emily 

How can I achieve this? Currently I am just filtering the complete list twice and combining the results afterwards. But unfortunately this is quite inefficent and even more importantly I lose the original sort order, which is not acceptable in my situation.

I also need to be able to iterate over any number of filters/predicates as there can be quite a lot.

Again it needs to be a .NET 2.0 solution unfortunately I can't use a newer version of the framework

Thanks a lot.

Answer 1

How about:

public static Predicate<T> Or<T>(params Predicate<T>[] predicates) {     return delegate (T item)     {         foreach (Predicate<T> predicate in predicates)         {             if (predicate(item))             {                 return true;             }         }         return false;     }; } 

And for completeness:

public static Predicate<T> And<T>(params Predicate<T>[] predicates) {     return delegate (T item)     {         foreach (Predicate<T> predicate in predicates)         {             if (!predicate(item))             {                 return false;             }         }         return true;     }; } 

Then call it with:

List<string> filteredNames = names.FindAll(Helpers.Or(StartsWithE, StartsWithI)); 

Another alternative would be to use multicast delegates and then split them using GetInvocationList(), then do the same thing. Then you could do:

List<string> filteredNames = names.FindAll(Helpers.Or(StartsWithE+StartsWithI)); 

I'm not a huge fan of the latter approach though - it feels like a bit of an abuse of multicasting.