When you interoperate between COM and the managed code of the .NET Framework, you may encounter one or more of the following common issues.
At times, you may have to use data types that are not part of the .NET Framework. Interop assemblies handle most of the work for COM objects, but you may have to control the data types that are used when managed objects are exposed to COM. For example, structures in class libraries must specify the BStr unmanaged type on strings sent to COM objects created by Visual Basic 6.0 and earlier versions. In such cases, you can use the MarshalAsAttribute attribute to cause managed types to be exposed as unmanaged types.
In Visual Basic 6.0 and earlier versions, strings are exported to COM objects as sequences of bytes without a null termination character. For compatibility with other languages, Visual Basic 2005 includes a termination character when exporting strings. The best way to address this incompatibility is to export strings that lack the termination character as arrays of Byte or Char.
Managed class hierarchies flatten out when exposed as COM objects. For example, if you define a base class with a member, and then inherit the base class in a derived class that is exposed as a COM object, clients that use the derived class in the COM object will not be able to use the inherited members. Base class members can be accessed from COM objects only as instances of a base class, and then only if the base class is also created as a COM object.
Although you can create overloaded methods with Visual Basic, they are not supported by COM. When a class that contains overloaded methods is exposed as a COM object, new method names are generated for the overloaded methods.
For example, consider a class that has two overloads of the Synch method. When the class is exposed as a COM object, the new generated method names could be Synch and Synch_2.
The renaming can cause two problems for consumers of the COM object.
Clients might not expect the generated method names.
The generated method names in the class exposed as a COM object can change when new overloads are added to the class or its base class. This can cause versioning problems.
To solve both problems, give each method a unique name, instead of using overloading, when you develop objects that will be exposed as COM objects.
You use interop assemblies almost as if they are managed code replacements for the COM objects they represent. However, because they are wrappers and not actual COM objects, there are some differences between using interop assemblies and standard assemblies. These areas of difference include the exposure of classes, and data types for parameters and return values.
Unlike classes in standard assemblies, COM classes are exposed in interop assemblies as both an interface and a class that represents the COM class. The interface's name is identical to that of the COM class. The name of the interop class is the same as that of the original COM class, but with the word "Class" appended. For example, suppose you have a project with a reference to an interop assembly for a COM object. If the COM class is named MyComClass, IntelliSense and the Object Browser show an interface named MyComClass and a class named MyComClassClass.
Generally, you create an instance of a .NET Framework class using the New statement with a class name. Having a COM class represented by an interop assembly is the one case in which you can use the New statement with an interface. Unless you are using the COM class with an Inherits statement, you can use the interface just as you would a class. The following code demonstrates how to create a Command object in a project that has a reference to the Microsoft ActiveX Data Objects 2.8 Library COM object:
However, if you are using the COM class as the base for a derived class, you must use the interop class that represents the COM class, as in the following code:
Interop assemblies implicitly implement interfaces that represent COM classes. You should not try to use the Implements statement to implement these interfaces or an error will result.
Unlike members of standard assemblies, interop assembly members may have data types that differ from those used in the original object declaration. Although interop assemblies implicitly convert COM types to compatible common language runtime types, you should pay attention to the data types that are used by both sides to prevent runtime errors. For example, in COM objects created in Visual Basic 6.0 and earlier versions, values of type Integer assume the .NET Framework equivalent type, Short. It is recommended that you use the Object Browser to examine the characteristics of imported members before you use them.
Most COM objects are used by creating an instance of a COM class using the New keyword and then calling methods of the object. One exception to this rule involves COM objects that contain AppObj or GlobalMultiUse COM classes. Such classes resemble module level methods in Visual Basic 2005 classes. Visual Basic 6.0 and earlier versions implicitly create instances of such objects for you the first time that you call one of their methods. For example, in Visual Basic 6.0 you can add a reference to the Microsoft DAO 3.6 Object Library and call the DBEngine method without first creating an instance:
Dim db As DAO.Database ' Open the database. Set db = DBEngine.OpenDatabase("C:\nwind.mdb") ' Use the database object.
Visual Basic 2005 requires that you always create instances of COM objects before you can use their methods. To use these methods in Visual Basic 2005, declare a variable of the desired class and use the new keyword to assign the object to the object variable. The Shared keyword can be used when you want to make sure that only one instance of the class is created.
' Class level variable. Shared DBEngine As New DAO.DBEngine Sub DAOOpenRecordset() Dim db As DAO.Database Dim rst As DAO.Recordset Dim fld As DAO.Field ' Open the database. db = DBEngine.OpenDatabase("C:\nwind.mdb") ' Open the Recordset. rst = db.OpenRecordset( _ "SELECT * FROM Customers WHERE Region = 'WA'", _ DAO.RecordsetTypeEnum.dbOpenForwardOnly, _ DAO.RecordsetOptionEnum.dbReadOnly) ' Print the values for the fields in the debug window. For Each fld In rst.Fields Debug.WriteLine(fld.Value.ToString & ";") Next Debug.WriteLine("") ' Close the Recordset. rst.Close() End Sub
One common interop problem involves errors in event handlers that handle events raised by COM objects. Such errors are ignored unless you specifically check for errors using On Error or Try...Catch...Finally statements. For example, the following example is from a Visual Basic 2005 project that has a reference to the Microsoft ActiveX Data Objects 2.8 Library COM object.
' To use this example, add a reference to the ' Microsoft ActiveX Data Objects 2.8 Library ' from the COM tab of the project references page. Dim WithEvents cn As New ADODB.Connection Sub ADODBConnect() cn.ConnectionString = _ "Provider=Microsoft.Jet.OLEDB.4.0;" & _ "Data Source=C:\NWIND.MDB" cn.Open() MsgBox(cn.ConnectionString) End Sub Private Sub Form1_Load(ByVal sender As System.Object, _ ByVal e As System.EventArgs) Handles MyBase.Load ADODBConnect() End Sub Private Sub cn_ConnectComplete( _ ByVal pError As ADODB.Error, _ ByRef adStatus As ADODB.EventStatusEnum, _ ByVal pConnection As ADODB.Connection) _ Handles cn.ConnectComplete ' This is the event handler for the cn_ConnectComplete event raised ' by the ADODB.Connection object when a database is opened. Dim x As Integer = 6 Dim y As Integer = 0 Try x = CInt(x / y) ' Attempt to divide by zero. ' This procedure would fail silently without exception handling. Catch ex As Exception MsgBox("There was an error: " & ex.Message) End Try End Sub
This example raises an error as expected. However, if you try the same example without the Try...Catch...Finally block, the error is ignored as if you used the OnError Resume Next statement. Without error handling, the division by zero silently fails. Because such errors never raise unhandled exception errors, it is important that you use some form of exception handling in event handlers that handle events from COM objects.
Understanding COM interop errors
Without error handling, interop calls often generate errors that provide little information. Whenever possible, use structured error handling to provide more information about problems when they occur. This can be especially helpful when you debug applications. For example:
You can find information such as the error description, HRESULT, and the source of COM errors by examining the contents of the exception object.
Most ActiveX controls that work with Visual Basic 6.0 work with Visual Basic 2005 without trouble. The main exceptions are container controls, or controls that visually contain other controls. Some examples of older controls that do not work correctly with Visual Studio are as follows:
Microsoft Forms 2.0 Frame control
Up-Down control, also known as the spin control
Sheridan Tab Control
There are only a few workarounds for unsupported ActiveX control problems. You can migrate existing controls to Visual Studio if you own the original source code. Otherwise, you can check with software vendors for updated .NET-compatible versions of controls to replace unsupported ActiveX controls.
Visual Basic 2005 sometimes raises COM errors such as "Error 0x800A017F CTL_E_SETNOTSUPPORTED" when you pass ReadOnly properties of some older ActiveX controls as ByRef parameters to other procedures. Similar procedure calls from Visual Basic 6.0 do not raise an error, and the parameters are treated as if you passed them by value. The error message you see in Visual Basic 2005 is the COM object reporting that you are trying to change a property that does not have a property Set procedure.
If you have access to the procedure being called, you can prevent this error by using the ByVal keyword to declare parameters that accept ReadOnly properties. For example:
If you do not have access to the source code for the procedure being called, you can force the property to be passed by value by adding an extra set of brackets around the calling procedure. For example, in a project that has a reference to the Microsoft ActiveX Data Objects 2.8 Library COM object, you can use:
Deploying assemblies that expose COM interfaces presents some unique challenges. For example, a potential problem occurs when separate applications reference the same COM assembly. This situation is common when a new version of an assembly is installed and another application is still using the old version of the assembly. If you uninstall an assembly that shares a DLL, you can unintentionally make it unavailable to the other assemblies.
To avoid this problem, you should install shared assemblies to the Global Assembly Cache (GAC) and use a MergeModule for the component. If you cannot install the application in the GAC, it should be installed to CommonFilesFolder in a version-specific subdirectory.
Assemblies that are not shared should be located side by side in the directory with the calling application.