How to call a COM Object by ProgID with Reflection in C#

Question

class Device{
    private object device;
    public Device(string ProgID)
    {
        if (ProgID == "") ProgID = "ScopeSim.Telescope";
        device = Activator.CreateInstance(Type.GetTypeFromProgID(ProgID));
        Console.WriteLine("Connected");
    }
    public object Invoke(string Name, object[] args)
    {
        var v1 = device.GetType(); //this is a com object in debug
        var v2 = v1.GetMethod(Name);
        var v3 = v2.Invoke(device,args); //throws exception, v2 is null
        return v3;
    }
}
//somwhere else in another method in another class that has this in a field...
Console.WriteLine(new Device("").Invoke("A Method Name that is a string but is not known and could be anything, for testing, the name is 'Unpark'", object[] args));

This throws a NullReferenceException. The Unpark method does exist but it does not have a return type, but it does exist. Also, when it stopped to debug (on the exception) the ProgID field in the constructor was null. I would assume that this is normal though, right? It would have already run. Does anyone know why it throws it? If I declare device as dynamic, says it can't bind at runtime to a null object (basically the same thing).

Response to First Answer: I think reflection requires the variables as an array of objects. Yes, Unpark is written with a capital U. The ProgID thing apparently seems to be irrelevant.

you mean like device.InvokeMethod(Name);? Oh hum, I did it again. The method name is not known at compile time. — Arlen Beiler
– Arlen Beiler, Commented Oct 23, 2012 at 16:03
What I mean is that I don't even know what methods will be called at compile time. The only reason I know it now is for testing. I have an interface which the device is required to implement, but that is only relevant for testing, the program couldn't care less as long as it exists. — Arlen Beiler
– Arlen Beiler, Commented Oct 23, 2012 at 16:14

Peter Stock · Accepted Answer · 2012-10-23 16:03:39Z

0

Well, if ProgID is null, it won't be set, as you only check if the string is empty. I always use string.IsNullOrEmpty(s) instead of s == "".

Check this:

Is the Unpark Method really written with a capital U?
Does it require the object[] argument?

answered Oct 23, 2012 at 16:03

Peter Stock

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1 Comment

Arlen Beiler Over a year ago

I think it is reflection that requires those objects.

Community · Accepted Answer · 2017-05-23 12:31:08Z

0

Well, I think I finally figured it out, thanks to this answer: https://stackoverflow.com/a/3199919/258482. The problem is that you have to use InvokeMember to do anything to a COM object.

edited May 23, 2017 at 12:31

CommunityBot

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answered Nov 13, 2012 at 16:29

Arlen Beiler

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1 Comment

user585968 Over a year ago

Though a fine answer at the time, c# 4 makes it much easier to perform COM-interop via the dynamic keyword so there is no need for InvokeMember. See my answer on that link

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