Allowing Optional parameters in your COM objects

Article
05/16/2007

It’s simple to create a VFP object that can be used within other applications. I show how useful it is in Blogs get 300 hits per hour: Visual FoxPro can count.

That sample builds the T1.C1 object that uses methods with multiple parameters. T1.C1 is a general purpose server which in that example is shown to serves up web pages on a web server. It can be used for running any VFP code. Because the code to run is not built into the object itself, modifying the web server functionality doesn’t require shutting down and restarting the web site or web application as most other applications do.

When calling Foxpro methods and functions, all parameters are optional by default. In other words, if the function expects 3 parameters, you can still pass 0 arguments, and the unpassed parameters will default to False(.f.)

When building a COM object, Foxpro generates a TypeLibrary describing all the methods and their parameters. When calling that object via late binding, the additional required parameters default to .f.

It’s simple to modify the generated TypeLib to indicate some parameters are optional: we just add a little code to the project hook posted in Strongly typed methods and properties

The sample below adds to the same project hook to massage the generated TypeLibrary to indicate optional parameters.

If the helpstring has a “|” character, the following text is interpreted as a TypeLibrary directive.

If it starts with “%”, then the value is interpreted as an integer (like 2), indicating that parameters starting with the 2^nd are optional.
Else it is interpreted as a strongly typed object to return (see Strongly typed methods and properties)

Run the code below, then start Excel or VB.Net

Excel

In Excel, choose Tools->Macros->VB Editor. For the menu option to open VB Editor in Excel 2007, use the Developer Tab on your ribbon. If it’s not there, choose Office Button, Excel Options, Show Developer Tab in Ribbon. Alt-F11 works in both versions of Excel. To use early binding, add a reference to T1.DLL by choosing Tools->References and add a checkmark to “T1 Type Library”

Then Insert->Module and paste this code

Sub foo()

Dim xEarly As New t1.c1 ' for early binding

MsgBox (xEarly.MyEval("iif(sys(2334)='1','Early bound call ','Late bound call ') + version(1)"))

Set xLate = CreateObject("t1.c1") ' for late binding

MsgBox (xLate.MyEval("iif(sys(2334)='1','Early bound call ','Late bound call ') + version(1)"))

End Sub

Now hit F5 to run the code

The code uses SYS(2334) to indicate whether or not it was an early or late bound call.

Because the Optional parameters are specified, the line is simpler than

MsgBox (x.MyEval("version(1)", 0, 0, 0, 0))

Also, with early binding Excel’s intellisense indicates optional parameters with square brackets.

To use late binding, don’t add the reference, replace the Dim line with this: Set x = CreateObject(“t1.c1”)

VB.Net

In Visual Studio, choose File->New->Project->VB->Windows Form Application. Again, add a reference to get early binding (Late binding works as in Excel.) Project->Add Reference->Browse and navigate to the T1.DLL you just built.

Dbl click the form to add this code to the Load method:

Dim xEarly As New t1.c1 ' for early binding

Dim cCmd As String = "iif(sys(2334)='1','Early bound call ','Late bound call ') + version(1)"

MsgBox(xEarly.MyEval(cCmd))

Dim xLate = CreateObject("t1.c1") ' for late binding

MsgBox(xLate.MyEval(cCmd))

'This one uses reflection to make the early one invoke latebound

MsgBox(xEarly.GetType().InvokeMember("MyEval", Reflection.BindingFlags.InvokeMethod, _

Nothing, xEarly, New Object() {cCmd}))

Me.Close() ' close the form

In C#:

t1.c1 xEarly = new t1.c1Class();

string cCmd = "iif(sys(2334)='1','Early bound call ','Late bound call ') + version(1)";

System.Windows.Forms.MessageBox.Show(xEarly.MyEval(cCmd, 0, 0, 0, 0).ToString());

string cLate = xEarly.GetType().InvokeMember("MyEval",System.Reflection.BindingFlags.InvokeMethod,

null,xEarly,new object[] {cCmd}).ToString();

System.Windows.Forms.MessageBox.Show(cLate);

this.Close(); // close the form

Does C# support late binding? No, except through reflection.see Programming Office Applications Using Visual C#

Does C# support optional params? (ILDASM of Inerop.T1.Dll looks the same as for VB.) No: see Understanding Optional Parameters in COM Interop

Allowing Optional parameters in your COM objects

Additional resources