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POVSTO in VBA: AddIn.Object returns Nothing (null) sometimes
 

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    <p>Given:</p> <ul> <li>A VSTO Add-In</li> <li>An <code>override object RequestComAddInAutomationService()</code> which returns an instance of a class which is called <code>Facade</code> in my scenario.</li> <li>A VBA macro in Excel 2007 which accesses the <code>AddIn.Object</code> to get the Facade and uses it.</li> <li>A plenty of times where this works perfectly fine.</li> <li><strong>A couple of times where out of the blue, this doesn't seem to work.</strong></li> </ul> <p><strong>Update: Turns out that it's a particular user that has the problem. She has it all the time, others never have it (? never say "never")</strong></p> <p>In this "couple of times" I get</p> <blockquote> <p>Error: Object variable or With block variable not set</p> </blockquote> <p>at the line of code which tries to access a property of <code>Facade</code>. In short I can tell you that the code in <code>RequestComAddInAutomationService()</code> doesn't have any error-prone magic in it, and the VBA code to access the add-in has been taken from the web and looks fine, too. The longer version is yet to come, for those who'll take the time to read it :-)</p> <p><strong>Question: Does anyone have a clue why this can happen? Is it an Excel issue?</strong></p> <hr> <p>Details as promised:</p> <p>MyAddIn.cs:</p> <pre><code>public partial class MyAddIn { public Facade Facade { get; private set; } protected override object RequestComAddInAutomationService() { if (this.Facade == null) this.Facade = new Facade(Controller.Instance); return this.Facade; } } </code></pre> <p>Facade.cs:</p> <pre><code>[ComVisible(true)] [Guid("1972781C-A71A-48cd-9675-AE47EACE95E8")] [InterfaceType(ComInterfaceType.InterfaceIsDual)] public interface IFacade { // some methods } [ComVisible(true)] [ClassInterface(ClassInterfaceType.None)] public class Facade : IFacade { private Controller Controller { get; set; } public Facade(Controller controller) { this.Controller = controller; } } </code></pre> <p><code>Facade</code> has some methods but not a single field.</p> <p>Controller.cs:</p> <pre><code>public class Controller { private static Controller instance = null; public static Controller Instance { get { if (instance == null) instance = new Controller(); return instance; } } private Controller() { } } </code></pre> <p><code>Controller</code> has some private fields. Since the fields assignments are executed on creation, I reviewed them. Most of them are not initialized at all, or they are set to <code>null</code>, so the constructor does virtually nothing.</p> <p>The VBA code:</p> <pre><code>Dim addin As Office.COMAddIn Dim automationObject As Object Set addin = Application.COMAddIns("My AddIn") Set automationObject = addin.Object Dim oResult As Object Set oResult = automationObject.SomeMethodThatReturnsAnObject() </code></pre> <p>The last line is where the error happens. Although the method called returns an object, I am pretty sure that it cannot be the source of the error: If the reference returned was <code>null</code>, then the statement would simply evaluate to <code>Set oResult = Nothing</code> which is still valid. VBA rather throws this type of error whenever a method is executed on an reference that is <code>Nothing</code>, which is <code>automationObject</code> in my case.</p> <p>On the other hand, if the add-in wasn't there at all, the <code>Application.COMAddIns(...)</code> would raise an <em>index out of bounds</em> error, I've seen that before.</p>
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