VBA中的VSTO：AddIn.Object有时会返回Nothing（null）(VSTO in VBA: AddIn.Object returns Nothing (null) sometimes)

鉴于：

VSTO加载项 override object RequestComAddInAutomationService() ，它返回一个类的实例，在我的场景中称为Facade 。 Excel 2007中的VBA宏，它访问AddIn.Object以获取Facade并使用它。很多时候这种方式非常好。 有几次突然出现，这似乎不起作用。

更新：原来这是一个有问题的特定用户。 她总是拥有它，其他人从来没有它（？永远不会说“从不”）

在这“偶尔”中，我得到了

错误：未设置对象变量或With块变量

在尝试访问Facade属性的代码行。简而言之，我可以告诉你， RequestComAddInAutomationService()中的代码中没有任何容易出错的魔术，并且访问加载项的VBA代码也是从Web获取的，看起来也很好。对于那些花时间阅读它的人来说，更长的版本尚未到来:-)

问题：有没有人知道为什么会发生这种情况？ 这是一个Excel问题吗？

承诺的细节：

MyAddIn.cs：

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; } }

Facade.cs：

[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; } }

Facade有一些方法，但没有一个字段。

Controller.cs：

public class Controller { private static Controller instance = null; public static Controller Instance { get { if (instance == null) instance = new Controller(); return instance; } } private Controller() { } }

Controller有一些私人领域。由于字段分配是在创建时执行的，因此我对它们进行了审核。它们中的大多数根本没有初始化，或者它们被设置为null ，因此构造函数几乎不做任何事情。

VBA代码：

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()

最后一行是错误发生的地方。虽然调用的方法返回一个对象，但我很确定它不能是错误的来源：如果返回的引用为null ，那么该语句将简单地计算为Set oResult = Nothing仍然有效。每当在Nothing的引用上执行一个方法时，VBA就会抛出这种类型的错误，在我的情况下就是automationObject 。

另一方面，如果加载项根本不存在， Application.COMAddIns(...)会引发索引超出范围错误，我以前见过。

Given:

A VSTO Add-In An override object RequestComAddInAutomationService() which returns an instance of a class which is called Facade in my scenario. A VBA macro in Excel 2007 which accesses the AddIn.Object to get the Facade and uses it. A plenty of times where this works perfectly fine. A couple of times where out of the blue, this doesn't seem to work.

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")

In this "couple of times" I get

Error: Object variable or With block variable not set

at the line of code which tries to access a property of Facade. In short I can tell you that the code in RequestComAddInAutomationService() 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 :-)

Question: Does anyone have a clue why this can happen? Is it an Excel issue?

Details as promised:

MyAddIn.cs:

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; } }

Facade.cs:

[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; } }

Facade has some methods but not a single field.

Controller.cs:

public class Controller { private static Controller instance = null; public static Controller Instance { get { if (instance == null) instance = new Controller(); return instance; } } private Controller() { } }

Controller 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 null, so the constructor does virtually nothing.

The VBA 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()

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 null, then the statement would simply evaluate to Set oResult = Nothing which is still valid. VBA rather throws this type of error whenever a method is executed on an reference that is Nothing, which is automationObject in my case.

On the other hand, if the add-in wasn't there at all, the Application.COMAddIns(...) would raise an index out of bounds error, I've seen that before.