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    <p><code>leftClass.IsInstanceOfType(right)</code> means that "the right is a left". <code>rightClass.IsInstanceOfType(left)</code> means that "the left is a right".</p> <p>If the right is a left, but the left is not a right, then the left must be the super class, and the right must be the subclass. This is equivalent to the first case, where you have the comment "rightClass is a subclass of leftClass". So, it seems like the comments' intentions are actually accurate.</p> <p>However, I see a few problems with the method. The comments are the least of your worries.</p> <ol> <li><p>It will throw <code>NullReferenceException</code> if either parameter is null.</p></li> <li><p>It makes unnecessary calls to <code>GetType()</code>, because the actual source code for <code>IsInstanceOfType</code> looks like this:</p> <pre><code>public virtual bool IsInstanceOfType(object o) { if (o == null) { return false; } return this.IsAssignableFrom(o.GetType()); } </code></pre> <p>You should take @p.s.w.g.'s advice and use <code>IsAssignableFrom</code>, and possibly consider refactoring the signature compare two Types instead of two Objects.</p></li> <li><p>Any two concrete types will always have at least a common base type of <code>System.Object</code>. Returning null is not an acceptable result here.</p></li> <li><p>It doesn't handle cases where one type is not linearly derived from the other type, but both still have a common base class that is more derived than <code>System.Object</code>. For example,</p> <pre><code>public class Base { } public class A : Base { } public class B : Base { } </code></pre> <p>Your method would say that <code>A</code> and <code>B</code> are unrelated and return null, where the correct "common base" would be <code>Base</code>.</p></li> </ol> <p>I would look at the implementation provided at <a href="https://stackoverflow.com/questions/353430/easiest-way-to-get-a-common-base-class-from-a-collection-of-types">Easiest way to get a common base class from a collection of types</a>, which still is not perfect, but is better than the one you posted.</p> <h2>Update : Working code</h2> <p>I decided not to be stingy and I've posted the method that I use for this purpose below. Good luck.</p> <pre><code> /// &lt;summary&gt; Finds the most derived common base class of all the provided types, or System.Object if there is no common base class &lt;/summary&gt; public static Type CommonBaseClass(params Type[] types) { if(ReferenceEquals(types,null)) return typeof(object); types = types.Where(x =&gt; !ReferenceEquals(x,null)).Distinct().ToArray(); switch (types.Length) { case 0: return typeof(object); case 1: return types[0].IsInterface ? typeof(object): types[0]; default: IEnumerable&lt;IEnumerable&lt;Type&gt;&gt; hierarchies = types.Select(ClassHierarchy).OrderBy(x =&gt; x.Count()); Queue&lt;Type&gt; smallest = new Queue&lt;Type&gt;(hierarchies.First().Reverse()); hierarchies = hierarchies.Skip(1); do { int maxPossible = smallest.Count; hierarchies = hierarchies.Select(each =&gt; each.Take(maxPossible)); Type candidate = smallest.Dequeue(); if (hierarchies.All(each =&gt; each.Last() == candidate)) return candidate; } while (smallest.Count &gt; 1); return typeof(object); } } ///&lt;summary&gt;Gets the class hierarchy of the provided type, in order of derivation, e.g. : (System.Object,CustomBaseType,CustomConcreteType,...), or the singleton of System.Object type if the provided type is an interface or null &lt;/summary&gt; public static IEnumerable&lt;Type&gt; ClassHierarchy(this Type type) { if (type == null || type.IsInterface) type = typeof(object); var stack = new Stack&lt;Type&gt;(); do { stack.Push(type); type = type.BaseType; } while (type != null); return stack; } </code></pre>
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    1. COI have the feeling your example code is a little bloated for the purpuse... In what is the result different from my example code?
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    2. CO@MartinMulder, (1) Our methods handle interfaces differently. (2) Your code doesn't handle the cases I described in point #4 in my answer
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    3. CO@MartinMulder, (3) Your operation is binary. Mine is n-ary. (which may very well be an example of "bloat for the purpose", but it does produce a more generic, reusable method)
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