{"id":3581,"date":"2005-03-09T20:40:00","date_gmt":"2005-03-09T20:40:00","guid":{"rendered":"https:\/\/blogs.msdn.microsoft.com\/heaths\/2005\/03\/09\/guidelines-for-com-interoperability-from-net-2\/"},"modified":"2005-03-09T20:40:00","modified_gmt":"2005-03-09T20:40:00","slug":"guidelines-for-com-interoperability-from-net-2","status":"publish","type":"post","link":"https:\/\/devblogs.microsoft.com\/setup\/guidelines-for-com-interoperability-from-net-2\/","title":{"rendered":"Guidelines for COM Interoperability from .NET"},"content":{"rendered":"

In developer forums in which I participate I often read and respond to questions asking about COM interoperability (interop) and my reply is almost always the same. There are guidelines – if not rules – for exposing .NET Framework components to COM<\/a> and they are all based on guidelines for COM development<\/a>. If you have experience writing COM components these guidelines should sound familiar; if not, I hope you find this information useful.\nIn COM, you interact with components through interfaces, which all derive from IUnknown<\/font>. Each interface exposed to COM clients has a unique interface ID (IID) and the classes that implement those interfaces have unique class IDs (CLSIDs). In .NET you should attribute all COM-visible types with a unique value in the GuidAttribute<\/font> attribute. You can create these using uuidgen.exe<\/em> or guidgen.exe<\/em>.<\/p>\n

[Guid(“231d685b-6e22-49ef-a046-74e187ed0d21”)]<\/p>\n

<\/span><\/p>\n

public<\/span> interface<\/span> IFoo<\/p>\n

<\/span><\/p>\n

{<\/p>\n

<\/span><\/p>\n

}<\/p>\n

<\/span><\/p>\n

 <\/p>\n

<\/span>\n<\/p>\n

[Guid(“54c0c685-4ef2-4a1a-8693-b5773dbed295”)]<\/p>\n

<\/span><\/p>\n

[ClassInterface(ClassInterfaceType.None)]<\/p>\n

<\/span><\/p>\n

public<\/span> class<\/span> Foo : IFoo<\/span><\/p>\n

\n

<\/span>{\n<\/span><\/p>\n

}<\/p>\n

<\/span>\nNow the interfaces and classes you register will always have the same IDs from which they’re identified. To make sure the type library ID doesn’t change, you should also attribute your assembly with a unique GuidAttribute<\/font> value as well.<\/p>\n

[<\/span>assembly<\/span>: Guid(“1D2E5B02-438F-49d3-8D33-BC3451418551”)]<\/span>\nAlso, you should never use automatically-generated class interfaces using the enumerations ClassInterfaceType.AutoDispatch<\/font> or ClassInterfaceType.AutoDual<\/font>. You should define your class interface as you see above for IFoo<\/font> <\/font>explicitly and implement them as the first interface of your class, as well as use ClassInterfaceType.None<\/font> as the value for the ClassInterfaceAttribute<\/font> <\/font>on your class. If you use auto-generated class interfaces, your IID will most likely change (a problem for VTBL or “early” binding) and any changes to the order of your methods will change the order of functions in the VTBL, which is also a problem. The VTBL is a virtual table with function address used for COM clients to query using IUnknown::QueryInterface<\/font> and to call those functions. If the functions change order, your compiled client could be calling the wrong function with the wrong parameters. This order problem may also affect automation clients that cache dispatch IDs, though you can resolve that issue by attributing each property or method with the DispIdAttribute<\/font>.\nFor the same reason, you should never change a published interface; that is, you should never change the methods or properties of an interface once you’ve released it. You should create a new interface that derives from the older interface and implement that interface as your new class interface instead:<\/p>\n

<\/p>\n

[Guid(“231d685b-6e22-49ef-a046-74e187ed0d21”)]<\/p>\n

<\/span><\/p>\n

public<\/span> interface<\/span> IFoo<\/p>\n

<\/span><\/p>\n

{<\/p>\n

<\/span><\/p>\n

      <\/span>int<\/span> Add(int<\/span> a, int<\/span> b);<\/p>\n

<\/span><\/p>\n

      <\/span>int<\/span> Subtract(int<\/span> a, int<\/span> b);<\/p>\n

<\/span><\/p>\n

}<\/p>\n

<\/span><\/p>\n

 <\/p>\n

<\/span><\/p>\n

[Guid(“bbf3e617-ee6a-4e5a-a6a5-1623edd3e6c2”)]<\/p>\n

<\/span><\/p>\n

public<\/span> interface<\/span> IFoo2 : IFoo<\/p>\n

<\/span><\/p>\n

{<\/p>\n

<\/span><\/p>\n

      <\/span>int<\/span> Multiply(int<\/span> a, int<\/span> b);<\/p>\n

<\/span><\/p>\n

      <\/span>float<\/span> Divide(int<\/span> a, int<\/span> b);<\/p>\n

<\/span><\/p>\n

}<\/p>\n

<\/span><\/p>\n

 <\/p>\n

<\/span>\n<\/p>\n

[Guid(“54c0c685-4ef2-4a1a-8693-b5773dbed295”)]<\/p>\n

<\/span><\/p>\n

[ClassInterface(ClassInterfaceType.None)]<\/p>\n

<\/span>\n<\/p>\n

public<\/span> <\/font>class<\/span> Foo : IFoo2<\/p>\n

<\/font><\/span><\/p>\n

{<\/p>\n

<\/font><\/span><\/p>\n

      <\/span>#region<\/span> IFoo2 Members<\/p>\n

<\/font><\/span><\/p>\n

 <\/font><\/p>\n

<\/span><\/p>\n

      <\/font><\/span>public<\/span> <\/font>int<\/span> Multiply(<\/font>int<\/span> a, <\/font>int<\/span> b)<\/p>\n

<\/font><\/span><\/p>\n

      <\/span>{<\/p>\n

<\/font><\/span><\/p>\n

      <\/span>      <\/span><\/font>return<\/span> a * b;<\/p>\n

<\/font><\/span><\/p>\n

      <\/span>}<\/p>\n

<\/font><\/span><\/p>\n

 <\/font><\/p>\n

<\/span><\/p>\n

      <\/font><\/span>public<\/span> <\/font>float<\/span> Divide(<\/font>int<\/span> a, <\/font>int<\/span> b)<\/p>\n

<\/font><\/span><\/p>\n

      <\/span>{<\/p>\n

<\/font><\/span><\/p>\n

            <\/font><\/span>return<\/span> (<\/font>float<\/span>)a \/ (<\/font>float<\/span>)b;<\/p>\n

<\/font><\/span><\/p>\n

      <\/span>}<\/p>\n

<\/font><\/span><\/p>\n

 <\/font><\/p>\n

<\/span><\/p>\n

      <\/span>#endregion<\/p>\n

<\/span><\/p>\n

 <\/p>\n

<\/span><\/p>\n

      <\/span>#region<\/span> IFoo Members<\/p>\n

<\/font><\/span><\/p>\n

 <\/font><\/p>\n

<\/span><\/p>\n

      <\/font><\/span>public<\/span> <\/font>int<\/span> Add(<\/font>int<\/span> a, <\/font>int<\/span> b)<\/p>\n

<\/font><\/span><\/p>\n

      <\/span>{<\/p>\n

<\/font><\/span><\/p>\n

            <\/font><\/span>return<\/span> a + b;<\/p>\n

<\/font><\/span><\/p>\n

      <\/span>}<\/p>\n

<\/font><\/span><\/p>\n

 <\/font><\/p>\n

<\/span><\/p>\n

      <\/font><\/span>public<\/span> <\/font>int<\/span> Subtract(<\/font>int<\/span> a, <\/font>int<\/span> b)<\/p>\n

<\/font><\/span><\/p>\n

      <\/span>{<\/p>\n

<\/font><\/span><\/p>\n

            <\/font><\/span>return<\/span> a – b;<\/p>\n

<\/font><\/span><\/p>\n

      <\/span>}<\/p>\n

<\/font><\/span><\/p>\n

 <\/font><\/p>\n

<\/span><\/p>\n

      <\/span>#endregion<\/p>\n

<\/span><\/p>\n

}<\/font><\/span><\/p>\n

\n

<\/span><\/span><\/span>\nWhen registering your managed assembly for exposure to COM, you should use regasm.exe<\/em> that ships with the .NET Framework redistributable. If you’re installing the assembly into the Global Assembly Cache (GAC) – which means it must be strong named – you can run the following:\nregasm.exe AssemblyName.dll<\/em><\/font>\nIf you’re not installing your assembly into the GAC, you should pass the \/codebase switch to register the full path:\nregasm.exe \/codebase AssemblyName.dll<\/em><\/font>\nIn both cases, you can generate a type library (typelib) using the \/tlb option. This is useful for automation clients.<\/p>\n

To summarize:<\/p>\n