C#. Do if( a == (b or c or d)). Is it possible?

Question

Is there another way to write something like this:

if (a == x || a == y || a == z)

One way that I found is doing it like this:

Answer 1

I often use an extension method that mimics SQLs IN:

public static bool IsIn(this T obj, params T[] collection) {
   return collection.Contains(obj);
}

That way I can do

if(a.IsIn(b, c, d)) { ... }