C# Definition:

// Command handler interface
[ComImport]
[InterfaceType(ComInterfaceType.InterfaceIsIUnknown)]
[Guid("75ae0a2d-dc03-4c9f-8883-069660d0beb6")]
public interface IUICommandHandler
{
     // User action callback, with transient execution parameters
     [PreserveSig]
     HRESULT Execute(UInt32 commandId,              // the command that has been executed
             UI_ExecutionVerb verb,         // the mode of execution
             [In] ref PropertyKey key,          // the property that has changed
             [In] ref PropVariant currentValue,     // the new value of the property that has changed
             [MarshalAs(UnmanagedType.Interface)] IUISimplePropertySet commandExecutionProperties); // additional data for this execution

     // Informs of the current value of a property, and queries for the new one
     [PreserveSig]
     HRESULT UpdateProperty(UInt32 commandId,
               [In] ref PropertyKey key,
               [In] ref PropVariant currentValue,
               [In, Out] ref PropVariant newValue);
}

Notes:

None.