c# 4.0 DLR and COM

Question

a common example to illustrate the benefits of DLR is to use it with legacy components such as COM and having the ability to call methods that are not visible at compile time.

is the point of this to skip the COM Interop step? Because once the component has been Regasm'd the compiler will have the metadata.

and let's say we do skip the Interop step, any changes to COM signature (that is used by .net) will still require re-compilation.

if i had to make another guess, the DLR also provides caching of the calls, so any subsequent calls should be faster than those using regular reflection.. so maybe that's one benefit of the DLR here.. but syntactically i'm just not seeing it..

Answer 1

It is specifically useful to interop with latebound COM. The kind where you don't add a reference to a COM type library or PIA. Very painful to do in C# before the dynamic keyword became available, it requires reflection code. If you have a type library then you almost always want to use it since it is a lot faster, catches mistakes at compile time and supports IntelliSense. The only reason you would not want to use one is when you try to make your code flexible enough to handle different versions of the COM server. It is however quite risky with a runtime exception always around the corner to ruin your day. It is in fact rare to not have a type library, almost all COM component authors provide one since the advantages are so great.

Regasm is the exact opposite, you only use that when you write your own [ComVisible] server in C#. There's no benefit to dynamic then.

Answer 2

is the point of this to skip the COM Interop step?

Not, it just simplifies the calling of methods which could be quite a pain with Reflection. No matter whether you use Reflection or C# 4.0 dynamic, both use Interop to communicate with the unmanaged code.

and let's say we do skip the Interop step, any changes to COM signature (that is used by .net) will still require re-compilation.

It depends. If you rename a COM method you will need to recompile the .NET calling code because it will break at runtime when you attempt to call an non-existing method.