SAMPLE: OPTARG: Optional Parameters with Automation

OPTARG is an OLE Automation server application that demonstrates automation methods with optional parameters and variable number of parameters. It implements an Add method that takes optional parameters and a Multiply method that takes a variable number of parameters.

See the README.TXT included with the sample for details to compile and run.

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Optional parameters are implemented by marking the parameter with the optional attribute and by using VARIANT as the type of the parameter. If the controller omits a parameter, the method will be passed a VARIANT with VARIANT.vt = VT_ERROR and VARIANT.scode = DISP_E_PARAMNOTFOUND.

Variable parameters are implemented by marking the method with the vararg attribute and by providing a parameter of type SAFEARRAY(variant). This array will contain the variable number of parameters passed by the controller.

NOTE: vararg methods do not have the upper limit of parameters that methods with optional parameters do.

Because of a problem in the OLE dlls, invoking the Multiply method will cause an exception in the 32-bit OLE that shipped with Windows NT 3.5 and a GP fault in 16-bit OLE 2.02 and 2.03. This problem has been fixed in NT 3.51 and Windows 95.

The problem occurs if a vararg method has a retval parameter. For example, the following will fail: The problem does not occur if the vararg method doesn't have a retval parameter. For example, the following will currently work: