What's the deal with [ComVisible] default and public classes COM exposure?

c# com comvisible
user2166888 picture user2166888 · Mar 28, 2013 · Viewed 28.7k times · Source

MSDN has this article about [ComVisible] attribute. I don't quite get what happens when one sets [ComVisible(true)].

MSDN says

The default is true, which indicates that the managed type is visible to COM. This attribute is not needed to make public managed assemblies and types visible; they are visible to COM by default. Only public types can be made visible.

So they say public types are visible to COM by default. But they also say only public types can be made visible by setting [ComVisible(true)]. It does not makes sense: if public types are visible by default, then how does setting [ComVisible(true)] make public types visible? If they're already visible how will they get more visible?

Perhaps my understanding is not correct. I shall appreciate if anyone can put some light on the above statements.

Answer

sharptooth picture sharptooth · Mar 29, 2013

The trick is you can also add this attribute at assembly level (in AssemblyInfo.cs). If you specify [assembly: ComVisible(true)] (or don't specify that at assembly level and so have the same effect by default) then all the public classes and interfaces and public methods thereof become COM-visible by default.

You could just as well set [assembly: ComVisible(false)] at assembly level and then all the public entities would by default have the same effect as if they had [ComVisible(false)] on them and so you could only mark those classes/interfaces/methods COM-visible ([ComVisible(true)]) which you really need.

This helps you to not expose too much when you have lots of public entities as here. Without this mechanism you would have to set [ComVisible(false)] to each class/interface/method that you don't want exposed. Using [assembly: ComVisible(false)] lets you only expose the stuff you need.

And you only can expose public entities to COM (be default or explicitly) - entities with stricter visibility can't be exposed to COM.

Related questions

Is there any point in specifying a Guid when using ComVisible(false)?

When you create a new C# project in Visual Studio, the generated AssemblyInfo.cs file includes an attribute specifying an assembly GUID. The comment above the attribute states that it is used "if this project is exposed to COM". None …

Read More
Microsoft.Office.Core Reference Missing

Using the example provided in codeproject I am struggling to work out where I can find the reference to the library Microsoft.Office.Core. I am getting the error "The referenced component 'Microsoft.Office.Core' could not be found." I …

Read More
How can I generate UUID in C#

I am creating an .idl file programmatically. How do I create UUIDs for the interfaces and Methods Programmatically. Can I generate the UUID programmatically?

Read More