Invoking COM Properties and Methods

Question

I am trying to dynamically create COM object, call COM method and set COM properties. The COM class is a VB6 ActiveX DLL. The implementation is exactly equal to the VB6 code from this page http://msdn.microsoft.com/en-us/library/ms973800.aspx.

In short words, the project is PhysServer and the class name is Temperature which has two properties Celsius and Fahrenheit and two methods GetCelsius() and GetFahrenheit().

I have already run regsvr32 to register the ActiveX DLL to the computer. The ProgID is PhysServer.Temperature.

I have three block of codes

Code Block 1 (works)

Option Explicit Off
Option Strict Off
...
Dim objType = Type.GetTypeFromProgID("PhysServer.Temperature")
Dim comObj = Activator.CreateInstance(objType)
comObj.Celsius = 100
Dim f As Double = comObj.GetFahrenheit()
Console.WriteLine(f) ' shows 212

Code Block 2 (works)

Option Explicit On
Option Strict On
...
Dim objType = Type.GetTypeFromProgID("PhysServer.Temperature")
Dim comObj = Activator.CreateInstance(objType)
Microsoft.VisualBasic.CallByName(comObj, "Celsius", CallType.Let, 100)
Dim f As Double = CDbl(Microsoft.VisualBasic.CallByName(comObj, "GetFahrenheit", CallType.Method, Nothing))
Console.WriteLine(f) ' shows 212

Code Block 3 (doesn't work)

Option Explicit On
Option Strict On
...
Dim objType = Type.GetTypeFromProgID("PhysServer.Temperature")
Dim comObj = Activator.CreateInstance(objType)
Dim f As Double = CDbl(objType.InvokeMember("GetFahrenheit", Reflection.BindingFlags.InvokeMethod, Nothing, comObj, Nothing))
Console.WriteLine(f) ' shows the default value of GetFahrenheit '
objType.InvokeMember("Celsius", Reflection.BindingFlags.SetField Or Reflection.BindingFlags.InvokeMethod, Nothing, comObj, New Object() {100}) ' throws exception: Number of parameters specified does not match the expected number '
f = CDbl(objType.InvokeMember("GetFahrenheit", Reflection.BindingFlags.InvokeMethod, Nothing, comObj, Nothing))
Console.WriteLine(f)

I understand Code Block 1 and Code Block 2. However, how could I use set a COM object using reflection like Code Block 3? By some reasons, I cannot use Code Block 1 and Code Block 2. So the only way is Code Block 3... but it doesn't work.

Does anyone know the solution of Code Block 3? Thanks!

Chris Dickson Chris Dickson · Accepted Answer · 2010-11-29T18:08:42

Try this:

objType.InvokeMember("Celsius", Reflection.BindingFlags.SetProperty Or ...

instead of SetField.

comObj is a Runtime-Callable Wrapper, and Celsius will be a Property thereof, not a field.

Its also possible you may need to specify the BindingFlags.Instance flag as well.

Invoking COM Properties and Methods

1 Answers