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

For a recent project we had to use the Excel Interop Assembly. But after referencing the Interop Assembly in Visual Studio 2010, we get an error when we try to compile.  Setting the "Embed Interop Types" to FALSE  solves the problem but of course than we loose a nice new feature in Visual Studio 2010.

So where does this error comes from? 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.

In most cases this error is the result of code which tries to instantiate a COM object e.g. here piece of code starting up Excel:

   1:  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:

   1:  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.

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