Improper threading can cause serious problems when using a COM object in ColdFusion. Make sure that the object is thread-safe. An object is thread-safe if it can be called from many programming threads simultaneously, without causing errors.

Visual Basic ActiveX DLLs are typically not thread-safe. If you use such a DLL in ColdFusion, you can make it thread-safe by using the OLE/COM Object Viewer to change the threading model of the object to the Apartment model.

If you are planning to store a reference to the COM object in the Application, Session, or Server scope, do not use the Apartment threading model. This threading model is intended to service only a single request. If your application requires you to store the object in any of these scopes, keep the object in the Both threading model, and lock all code that accesses the object, as described in Locking code with cflock.

1. Open the OLE/COM Object Viewer.

2. Select All Objects under Object Classes in the left pane.

3. Locate your COM object. The left pane lists the objects by name.

4. Select your object.

5. Select the Implementation tab in the right pane.

6. Select the Inproc Server tab, below the App ID field.

7. Select the Threading Model drop-down list and select Apartment or Both, as appropriate.

COM object methods in arguments are passed by value. The COM object gets a copy of the variable value, so you can specify a ColdFusion variable without surrounding it with quotation marks.

COM object out method arguments are passed by reference. The COM object modifies the contents of the variable on the calling page, so the calling page can use the resulting value. To pass a variable by reference, surround the name of an existing ColdFusion variable with quotation marks. If the argument is a numeric type, assign the variable a valid number before you make the call. For example:

<cfset inStringArg="Hello Object"> 
<cfset outNumericArg=0> 
<cfset result=myCOMObject.calculate(inStringArg, "outNumericArg")>

The string "Hello Object" is passed to the object's calculate method as an input argument. The method sets the value of outNumericArg to a numeric value.

The following table described some error messages you could encounter when using COM objects: