How to assert all items in a collection using fluent-assertions?

The recommended way is to use OnlyContain:

items.Should().OnlyContain(x => x.IsActive, "because I said so!");

These will also work:

items.All(x => x.IsActive).Should().BeTrue("because I said so!");

items.Select(x => x.IsActive.Should().BeTrue("because I said so!"))
     .All(x => true); 

Note that the last line (.All(x => true)) forces the previous Select to execute for each item.

Something like replacing your foreach loop with a foreach method should do the trick (at least a little bit).

var items = CreateABunchOfActiveItems();
items.ForEach(item => item.IsActive.Should().BeTrue("because I said so, too!"));

I find this syntax a little bit more fluent than traditional foreach loop :)

ForEach method is not defined if your method CreateABunchOfActiveItems returns an IEnumerable. But it can be easily implemented as an extension method:

public static IEnumerable<T> ForEach<T>(this IEnumerable<T> enumeration, 
    Action<T> action)
{
    // I use ToList() to force a copy, otherwise your action 
    // coud affect your original collection of items!. If you are confortable 
    // with that, you can ommit it
    foreach (T item in enumeration.ToList())
    {
        action(item);
        yield return item;
    }
}