MFC ActiveX Controls: Advanced Property Implementation
This article describes topics related to implementing advanced properties in an ActiveX control:
Read-Only and Write-Only Properties
The Add Property Wizard provides a quick and easy method to implement read-only or write-only properties for the control.
To implement a read-only or write-only property
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Load your control's project.
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In Class View, expand the library node of your control.
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Right-click the interface node for your control (the second node of the library node) to open the shortcut menu.
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From the shortcut menu, click Add and then click Add Property.
This opens the Add Property Wizard.
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In the Property Name box, type the name of your property.
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For Implementation Type, click Get/Set Methods.
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In the Property Type box, select the proper type for the property.
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If you want a read-only property, clear the Set function name. If you want a write-only property, clear the Get function name.
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Click Finish.
When you do this, the Add Property Wizard inserts the function SetNotSupported or GetNotSupported in the dispatch map entry in place of a normal Set or Get function.
If you want to change an existing property to be read-only or write-only, you can edit the dispatch map manually and remove the unnecessary Set or Get function from the control class.
If you want a property to be conditionally read-only or write-only (for example, only when your control is operating in a particular mode), you can provide the Set or Get function, as normal, and call the SetNotSupported or GetNotSupported function where appropriate. For example:
void CSampleCtrl::SetMyProperty( short propVal )
{
if ( m_bReadOnlyMode ) // some control-specific state
SetNotSupported( );
else
m_ipropVal = propVal; // set property as normal
}
This code sample calls SetNotSupported if the m_bReadOnlyMode data member is TRUE. If FALSE, then the property is set to the new value.
Returning Error Codes From a Property
To indicate that an error has occurred while attempting to get or set a property, use the COleControl::ThrowError function, which takes an SCODE (status code) as a parameter. You can use a predefined SCODE or define one of your own. For a list of predefined SCODEs and instructions for defining custom SCODEs, see Handling Errors in Your ActiveX Control in the article ActiveX controls: Advanced Topics.
Helper functions exist for the most common predefined SCODEs, such as COleControl::SetNotSupported, COleControl::GetNotSupported, and COleControl::SetNotPermitted.
 Note |
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| ThrowError is meant to be used only as a means of returning an error from within a property's Get or Set function or an automation method. These are the only times that the appropriate exception handler will be present on the stack. |
For more information on reporting exceptions in other areas of the code, see COleControl::FireError and the section Handling Errors in Your ActiveX Control in the article ActiveX Controls: Advanced Topics.
