Dynamic Keyword, C# and Interop?

Question

Ok, I am calling an interop dll which I have no access to. Here is the pseudo code:

dynamic myVariable = null;

firstInteropMethod(ref myVariable);
secondInteropMethod(myVariable); //Not by ref

The method signatures for the two methods are

firstInteropMethod(ref object someObject);
secondInteropMethod(object someObject);

The expected value is a double array of the definition

double[,]

Now the fun part. My original code gets the wrong results but no error. However, this code:

firstInteropMethod(ref myVariable);
secondInteropMethod((double[,]) myVariable);

Gives the expected results.

Using watches and type of statements I have determined that nothing changes between the two calls so what gives? Why would there be a difference and what would that difference be?

Answer 1

This MSDN article on dynamic explains why casting is needed for COM Interop when operations declare the parameter type as object and indicates that using the /link:filelist compiler option will allow you to define the COM method signatures as dynamic as well.