Passing a COM method default parameter

Question

I have a ComVisible COM class written in C#. I want to call it from another C# bit of code using COM and pass the default value for the parameter. I can call plenty of other methods without default arguments.

This is the best I can come up with. The first two lines work for all my other methods.

Type mytype = Type.GetTypeFromProgID("MyType");
dynamic myinstance = Activator.CreateInstance(mytype);
object missingValue = System.Reflection.Missing.Value;
myinstance.generatecsvdocument("mystring", ref missingValue);

My method looks like this:

public void generatecsvdocument(string mystring, string rowseperator = "\n")

When I run it I get the error:

The best overloaded method match for 'generatecsvdocument(string, string)' has some invalid arguments

Answer 1

  object missingValue = System.Reflection.Missing.Value;

That cannot work here. It is only valid for a COM method that takes a VARIANT as an argument. Looks like object or dynamic in C#. A very different kind of default argument mechanism than C# supports, it is the callee that determines the default value. In C# it is the caller that determines it, the C# compiler uses metadata to know that default.

Missing.Value turns in a variant of type vtError with the value DISP_E_PARAMNOTFOUND at runtime. Signalling the COM method to use the default value. Not actually that commonly used, usually only implemented in COM servers that support scripting languages. Office Automation is the most common example, probably what inspired you to try this.

But no, your argument is string , not a variant. There is no way to discover the default either when you use late binding, implicit is that you don't know anything about the default value stored in metadata. Otherwise the reason that the vtError mechanism exists, scripting languages have the same problem. The only real way to get ahead is to rewrite the method and test for a null argument, substituting "\\n" if that's the case.