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PORestricting class method access to one other class
 

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    <p>Greetings.</p> <p>I have two classes, 'Database' and 'Group.' I want to be able to create instances of 'Group' and call methods on those instances from within 'Database' and to be able to distribute 'Group' instance references publicly. However, I do not want to provide public access to the constructor or other methods in 'Group.'</p> <p>I originally thought that I could achieve this access restriction by making 'Group' a private inner class of 'Database,' but I discovered that I couldn't publicly distribute references to 'Group' if it was private. Also, my attempts at making 'Group' a public inner class failed, because if its methods were all private, 'Database' couldn't access them, and if they were public, access was possible beyond 'Database.'</p> <p><strong>I am looking for a best-practice technique for solving or circumventing this problem.</strong> Maybe I missed a necessary keyword somewhere? Nothing I have found thus far in my research has indicated that C# permits this granularity of control. I found a messy way to get around the problem, as I've provided below in code. Its essence is this: before every call in 'Database' to a method in 'Group,' set a field in 'Database,' publicly readable but only privately settable, that 'Group''s methods all check in their creating instances of 'Database' before performing their intended operations. When reading the field (via a public method in 'Database,') the field is reset, preventing any further method calls on 'Group' until 'Database' sets the field again.</p> <pre><code>public class Database { // Field; true if Database has just authorized a method call to a %Group. private bool group_isNextCallAuthorized = false; // Invoked before every method call to a %Group. private void Group_AuthorizeNextCall() { group_isNextCallAuthorized = true; } // Method, ordinarily called from %Group, that checks its creating %Database // that the %Database itself authorized the method call on the %Group. It // automatically resets the authorization to false to prepare for the next, // perhaps unauthorized, method call. public bool Group_IsNextCallAuthorized() { bool previousValue = group_isNextCallAuthorized; group_isNextCallAuthorized = false; return previousValue; } // Constructor; creates a demo %Group. public Database() { // Create a %Group, first authorizing the action. Group_AuthorizeNextCall(); Group group = Group.Create(this); // Call some method on the group Group_AuthorizeNextCall(); group.SomeGroupMethod(); } } public class Group { // Field; refers to the %Database that created this %Group instance. private Database db; // Generates an instance of %Group, requiring the creating %Database as an // argument. After checking that the %Database %db isn't null, it verifies // that %db actually requests and authorized this %Group's creation via the // %Group_IsNextCallAuthorized(.) method provided by %Database. public static Group Create(Database db) { // It will not create a dud instance of %Group; it will throw an exception // instead. if ((db == null) || !db.Group_IsNextCallAuthorized()) throw new System.Exception("Unauthorized access."); return new Group(db); } // This constructor is referenced internally by static %Create(.) as above. private Group(Database db) { this.db = db; } // This is an arbitrary method showing that it must check that the %Database // that created this %Group authorized this method call before it will // perform its intended function. public void SomeGroupMethod() { if (!db.Group_IsNextCallAuthorized()) throw new System.Exception("Unauthorized access."); // intended functionality... } } </code></pre>
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