EDIT: better yet, do it like that:
var filteredProjects =
projects.Where(p => filteredTags.All(tag => p.Tags.Contains(tag)));
EDIT2: Honestly, I don't know which one is better, so if performance is not critical, choose the one you think is more readable. If it is, you'll have to benchmark it somehow.
Probably Intersect is the way to go:
void Main()
{
var projects = new List<Project>();
projects.Add(new Project { Name = "Project1", Tags = new int[] { 2, 5, 3, 1 } });
projects.Add(new Project { Name = "Project2", Tags = new int[] { 1, 4, 7 } });
projects.Add(new Project { Name = "Project3", Tags = new int[] { 1, 7, 12, 3 } });
var filteredTags = new int []{ 1, 3 };
var filteredProjects = projects.Where(p => p.Tags.Intersect(filteredTags).Count() == filteredTags.Length);
}
class Project {
public string Name;
public int[] Tags;
}
Although that seems a little ugly at first. You may first apply Distinct to filteredTags if you aren't sure whether they are all unique in the list, otherwise the counts comparison won't work as expected.