Question about [Pure] methods

Is the following method Pure? I'd say so, as it doesn't change in anyway the current class, thus, everything we can now currenly "see开发者_如何转开发" in the class, before running this method will still be exactly the same after. Am I correct?

class Set {
    ...
    public ISet<T> UnionWith(ISet<T> set) {
       ISet<T> unionSet = ...

        foreach (Element element in this) {
            unionSet.Add(element);
        }

        foreach (Element element in set) {
           unionSet.Add(element);
        }

        return unionSet;
    }
}

If by [Pure] you mean labeled with the Pure attribute from System.Diagnostics.Contracts, the documentation says:

Pure methods do not make any visible state changes.

Since your method appears to not make any visible state changes (i.e. no side effects), it would qualify for the [Pure] attribute.