C# 4 and .NET 4

710 ❘ ChaPTer 26 interOp
Threading issues
As discussed earlier in this chapter, a COM component marks the apartment (STA or MTA) it wants to live
in, based on whether or not it is implemented as thread - safe. However, the thread has to join an apartment.
What apartment the thread should join can be defi ned with the [STAThread] and [MTAThread] attributes,
which can be applied to the Main() method of an application. The attribute [STAThread] means that the
thread joins an STA, whereas the attribute [MTAThread] means that the thread joins an MTA. Joining an
MTA is the default if no attribute is applied.
It is also possible to set the apartment state programmatically with the ApartmentState property of
the Thread class. The ApartmentState property allows you to set a value from the ApartmentState
enumeration. ApartmentState has the possible values STA and MTA (and Unknown if it wasn ’ t set). Be
aware that the apartment state of a thread can be set only once. If it is set a second time, the second setting
is ignored.
What happens if the thread chooses a different apartment from the apartments
supported by the component The correct apartment for the COM component is
created automatically by the COM runtime. Yet, the performance decreases if the
apartment boundaries are crossed while calling the methods of a component.
adding Connection Points
To see how COM events can be handled in a .NET application, the COM component must be extended.
First, you have to add another interface to the interface defi nition fi le COMDemo.idl . The interface
_ICompletedEvents is implemented by the client, which is the .NET application, and called by the
component. In this example, the method Completed() is called by the component when the calculation
is ready. Such an interface is also known as an outgoing interface. An outgoing interface must be either a
dispatch or a custom interface. Dispatch interfaces are supported by all clients. The custom attribute with
the ID 0F21F359 - AB84 – 41e8 – 9A78 – 36D110E6D2F9 defi nes the name of the interface that will be created
in the RCW. The outgoing interface must also be written to the interfaces supported by the component
inside the coclass section, and marked as a source interface:
library COMServerLib
{
importlib(“stdole2.tlb”);
[
uuid(5CFF102B-0961–4EC6–8BB4–759A3AB6EF48),
helpstring(“_ICompletedEvents Interface”),
custom(0F21F359-AB84–41e8–9A78–36D110E6D2F9,
“Wrox.ProCSharp.COMInterop.Server.ICompletedEvents”),
]
dispinterface _ICompletedEvents
{
properties:
methods:
[id(1)] void Completed(void);
};
[
]
uuid(ACB04E72-EB08-4D4A-91D3-34A5DB55D4B4),
helpstring(“COMDemo Class”)
custom(0F21F359-AB84–41e8–9A78–36D110E6D2F9,
“Wrox.ProCSharp.COMInterop.Server.COMDemo”),
www.it-ebooks.info