How to pass COM optional parameter in C#

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How to pass COM optional parameter in C#                                                        By Philippe Leefsma
  Here is a problem you may encounter when invoking the AutoCAD ActiveX API from C#:
  I'm trying to use AcadSelectionSet.SelectOnScreen in C# with similar approaches I've been using in VB. But I can't omit the optional parameters in C# which I can in VB.      
I tried to create some empty arrays and pass them to SelectOnScreen but I get invalid argument errors.      
Could you offer a solution?
  
  Solution
  Note: this workaround only applies to .Net 3.5 and prior versions of the framework, as C# 4.0 provides missing parameters functionality.
  In VB, you can ignore the optional parameters when you call AcadSelectionSet.SelectOnScreen but you can't do that in C#. This is because C# doesn't support optional parameters. Also, if you really want to pass NULL values as an indication that you don't want any values in the object array (no variant in C#), you can't create arrays with values, the zeros will be trades as values in the DXF code not 'nothing'. That's where the problem is.
  Fortunately, there is a solution in .NET which is using object reflection to specific a missing value that is NULL.   
   object o = System.Reflection.Missing.Value;
  This will be the equivalent of NULL values passed into VB functions call. The following code snippet demonstrates how it works.

2. //
   
3. // error checking code omitted for code brevity
   
4. //
   
5. private void TestMissing(AcadApplication oAcad)
   
6. {
   
7.     AcadDocument oDoc = oAcad.ActiveDocument;
   
8.     AcadSelectionSet ss = oDoc.SelectionSets.Add("ss01");
   
9.     object oMissing01 = System.Reflection.Missing.Value;
   
10.     object oMissing02 = System.Reflection.Missing.Value;
    
11.     ss.SelectOnScreen(oMissing01, oMissing02);
    
12.     System.Windows.Forms.MessageBox.Show(
    
13.         "Number of objects in the selection: " + ss.Count + ".");
    
14.     ss.Delete();
    
15. }

//<br /> // error checking code omitted for code brevity<br /> //<br /> private void TestMissing(AcadApplication oAcad)<br /> {<br />     AcadDocument oDoc = oAcad.ActiveDocument;<br />     AcadSelectionSet ss = oDoc.SelectionSets.Add("ss01");<br />     object oMissing01 = System.Reflection.Missing.Value;<br />     object oMissing02 = System.Reflection.Missing.Value;<br />     ss.SelectOnScreen(oMissing01, oMissing02);<br />     System.Windows.Forms.MessageBox.Show(<br />         "Number of objects in the selection: " + ss.Count + ".");<br />     ss.Delete();<br /> }