VS 2010 compiler error: Interop type XXX cannot be embedded. Use the applicable interface instead.

Some users have reported that when using Visual Studio 2010 and referencing an Interop Assembly with "Embed Interop Types" set to TRUE they would get the error literally saying that the type cannot be embedded. Then the error would advise to use an interface - but w/o many additional details on where to get this interface from. This post is intended to explain the error and give an easy guidance on what the interface is.

One of the limitations for 'Embed Interop Types' feature has been that classes cannot be used when embedding an assembly. This limitation has to do with servicing - it is safe to embed metadata but not anything that can potentially contain executable code (class types contain metadata AND code while interfaces only contain metadata). When you try to references a class type from an Interop Assembly for which the "Embed Interop Types" is set to TRUE compiler do issue a warning. But in most cases you do not need to use classes from interop assemblies and minor code tweaking is needed to successfully compile with EIT=true.

Let's address the most common case of this error first.I will also address the cases when the user code uses constants from VSIP assemblies such as EnvDTE.Constants / VSLangProj.PrjKind below...

In most cases (such as error for the usage of UPnPNATClass as noted in one of the comments) this error is the result of code which tries to instantiate a COM object e.g. here piece of code starting up Excel:

Excel.

ApplicationClass xlapp = new Excel.ApplicationClass();

Here it is enough to say that Excel.ApplicationClass derives from Excel.Application interface and one can even instantiate Excel using Excel.Application interface. Rewriting this code as below produces exact same results:

Excel.

Application xlapp = new Excel.Application();

Typically, to change your code to use an interface type instead of a class type you just need to remove the 'Class' suffix and compile the code! Another way to find what the applicable interface is - look at the definition of the class type. The class usually derives from one or more interfaces. Look at the definition of each interface - one of them will have CoClass attribute and this is the interface that you are looking for.

The case with EnvDTE/VSLangProj is a little bit different. I believe there should be a recommendation coming from the VSIP team encouraging to use Type Embedding. Unfortunately, some constants in the EnvDTE/VSLangProj  assemblies cannot be embedded. For these you will need manually embed the values of the constant into your project.Since those are constant fields in the abstract classes and should never change - this is a safe thing to do.

In the IDE you can right click on the value you need to embed, then copy&paste the values locally.

For example, suppose my C# code has code like this:

System.Windows.Forms.

MessageBox.Show(EnvDTE.Constants.vsDocumentKindText);

When this code is compiled I will get compilation errors:
<errors>
'EnvDTE.Constants' does not contain a definition for 'vsDocumentKindText'
Interop type 'EnvDTE.Constants' cannot be embedded. Use the applicable interface instead.
</errors>

In the IDE I will right click on vsDocumentKindText and choose "Go To Definition". This will take me what appears to be the definition of the constants class. I will copy the following lines and paste it in my code:

public abstract class Constants
{
     public const string vsDocumentKindText = "{8E7B96A8-E33D-11D0-A6D5-00C04FB67F6A}";
}

Then I will make the Constants class 'internal', rename the class to EnvDTEConstants.

internal abstract class EnvDTEConstants
{
    public const string vsDocumentKindText = "{8E7B96A8-E33D-11D0-A6D5-00C04FB67F6A}";
}

Next, I will go through my code and change all the occurrences of EnvDTE.Constants to EnvDTEConstants. And now my code does compile!

System.Windows.Forms.MessageBox.Show(EnvDTEConstants.vsDocumentKindText);