.NET Questions and Solutions

Symptoms
If your network is set up as a workgroup, you have a drive mapped to a Microsoft Windows NT-based server, and you run a Distributed Component Object Model (DCOM) program that attempts to start a remote server component on the Windows NT-based server, you may receive one of the following error messages on a blue screen.
Windows 95, Windows 98, Windows 98 Second Edition:

Windows
A fatal exception 0E has occurred at 0028:???????? in VXD VREDIR(??)+ ????????. The current application will be terminated.
* Press any key to terminate the current application.
* Press CTRL+ALT+DEL again to restart your computer. You will lose any unsaved information on all applications.
Press any key to continue.Windows Millennium Edition (Me):

Windows
An error has occurred. To continue:
Press Enter to return to Windows, or Press CTRL+ALT+DEL to restart your computer. If you do this, you will lose any unsaved information in all open applications.
File Name: VREDIR(06) + 00006C54 Error: 0E : 0028 : C02D2770
Press any key to continue.If you press ENTER, CoCreateInstanceEx returns RPC_S_CALL_FAILED (0×800706BE). The program may display the following error message:

The remote procedure call failed.See the “More Information” section of this article for the error message values for various versions of the Vredir.vxd file.
Resolution
The redirector (Vredir.vxd) incorrectly references internal resource data for the mapped drive connection while setting up the connection that the DCOM Remote Procedure Call (RPC) traffic will use.

Symptoms
In a Microsoft Visual Basic 2005 or Microsoft Visual Basic .NET application, when you pass late-bound parameters to methods of Microsoft Office objects, you may receive COMException exceptions. For example, when you pass the Name property of a late-bound UserProperty object to the UserProperties.Items method, you may receive the following error message:

An unhandled exception of type ‘System.Runtime.InteropServices.COMException’ occurred in microsoft.visualbasic.dll
Additional information: Property is read-only.Note Similar symptoms may also occur when you call methods of other Microsoft Office objects.
Resolution
If you pass a property of an object to a method by reference and the property has a set accessor, Visual Basic calls the set accessor to set the property to the value that the method returns. At compile time, if you pass a property of a late-bound object, the Visual Basic compiler cannot determine whether the property is passed by reference. Also, the Visual Basic compiler cannot determine whether the property has a set accessor. Therefore, the Visual Basic compiler permits the late binder to determine these details at run time.
However, if the relevant objects are Component Object Model (COM) objects at run time, the late binder cannot obtain sufficient information about these details. The late binder uses managed reflection to try to determine these details. The late binder assumes that such method calls involve a ByRef parameter and that the property that you pass has a set accessor. If the property that you pass does not have a set accessor, the Microsoft .NET Framework generates a MissingMethodException exception. The late binder handles the MissingMethodException exception.
The .NET Framework generates a MissingMethodException exception when the call to a method of a Microsoft Office object returns a HRESULT value of COR_E_MISSINGMETHOD. The Microsoft Office object is a COM object. Visual Basic .NET incorrectly assumes that all the methods of Microsoft Office objects return COR_E_MISSINGMETHOD if the property that you pass does not have a set accessor. However, not all the methods of Microsoft Office objects return COR_E_MISSINGMETHOD if the property that you pass does not have a set accessor. Therefore, the behavior that is mentioned in the “Symptoms” section of this article occurs if you pass a property that does not have a set accessor, such as the UserProperty.Name property, to a method.
Note This behavior does not occur if you use early-bound Microsoft Office objects.

Symptoms
When you assign a value to a property of a Component Object Model (COM) object in .NET, you may receive the following error message when you run your application:

An unhandled exception of type ‘System.Runtime.InteropServices.COMException’ occurred in InterOpDemo.exe
Additional information: Type mismatch
Resolution
The problem occurs if all of the following conditions are true: You are using the COM object in early bound mode.The property in the COM object has both Set and Let methods.You are trying to pass a value type variable to that property. In early bound mode, Microsoft Visual Basic .NET and Visual Basic 2005 always call the Set method of the property if it is available. If you want to call the Let method, you must explicitly specify it.

Symptoms
When you set a public object variable of a Microsoft Component Object Model (COM) component in Microsoft Visual Basic .NET or in Microsoft Visual Basic 2005, you may receive the following error message:

An unhandled exception of type ‘System.Runtime.InteropServices.COMException’ occurred in microsoft.visualbasic.dll
Additional information: Object variable or With block variable not set
Resolution
This behavior can occur if all the following conditions are true: You define a public variable in the COM component.The variable is of the Object type.You reference this COM component by using late binding in Visual Basic .NET or in Visual Basic 2005. When you access the public object by using late binding, Visual Basic .NET or Visual Basic 2005 does not correctly set the BindingFlags enumeration.
The BindingFlags enumeration is used to specify the flags that control binding and the way in which the search for members and types is conducted by reflection.

Symptoms
When you try to add a project reference to a Component Object Model (COM) DLL in Microsoft Visual Studio .NET, you may receive an error message that is similar to the following:

A reference to ‘C:\MyCOMDLL\Debug\MyCOMDLL.dll’ could not be added. Converting the type library to a .NET assembly failed. Could not load type MyCOMDLLLib.MyClassClass from assembly Interop.MyCOMDLLLib, Version=1.0.0.0.
Additionally, when you try to convert the type definitions that are found in a COM DLL by using Microsoft Type Library Importer (Tlbimp.exe) from a command prompt, you may receive an error message that is similar to the following:

TlbImp error: System.TypeLoadException – Could not load type MyCOMDLLLib.MyClassClass from assembly MyCOMDLLLib, Version=1.0.0.0.
Resolution
You may notice the behavior that is mentioned in the “Symptoms” section when the following conditions are true: Your DLL contains a class (such as MyClass) that implements an interface (such as IMyClass) that in turn derives from a base interface (such asIBaseClass).The IMyClass interface contains a method (such as Test) that has the same name as a property that the IBaseClass interface has.You notice this behavior because of the mechanism that Tlbimp.exe uses to disambiguate member names. Under the previous conditions, when the Microsoft .NET Framework tries to create a method implementation to associate the Test method of the MyClass class with the corresponding interface method, the .NET Framework does not know the name of the corresponding interface method. Therefore, Tlbimp.exe generates a System.TypeLoadException error, and then you receive the error message that is mentioned in the “Symptoms” section.
When you try to add a project reference to a COM DLL, Visual Studio .NET internally runs Tlbimp.exe and then handles any generated exceptions. Therefore, under the previous conditions, Visual Studio .NET handles the generated System.TypeLoadException, and then you receive the error message that is mentioned in the “Symptoms” section.

Symptoms
If you impersonate a particular account in your ASP.NET Web Service (.asmx) or in your Web Form (.aspx), and then you call to an Apartment/Single-threaded Component Object Model (COM) component, the process identity (by default, ASPNET account) is used instead of the impersonated account. Therefore, logic errors may occur when you try to access resources from the COM component, or you may receive the following error message:

Access Denied
Resolution
The thread that executes your ASP.NET request is one of the I/O threads or one of the workerThreads in the ASP.NET worker process (aspnet_wp.exe). These threads are all Multi-Threaded Apartment (MTA) threads. If you programmatically impersonate an account in your .aspx or .asmx code, or if you impersonate by using <identity impersonate=”true”> in Web.config or Machine.config, then the impersonation token is held on this MTA thread. If you then make a call into a single-threaded or an apartment-threaded COM component, that component is accessed by an entirely different thread, which is the single thread in its Single-Threaded Apartment (STA). Because this STA thread does not have an impersonation token of its own, and the impersonation token of the MTA thread is not passed to the STA thread, the STA thread then executes under the process identity.