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COM Callable Wrapper

When a COM client calls a .NET object, the common language runtime creates the managed object and a COM callable wrapper (CCW) for the object. Unable to reference a .NET object directly, COM clients use the CCW as a proxy for the managed object.

The runtime creates exactly one CCW for a managed object, regardless of the number of COM clients requesting its services. As the following illustration shows, multiple COM clients can hold a reference to the CCW that exposes the INew interface. The CCW, in turn, holds a single reference to the managed object that implements the interface and is garbage collected. Both COM and .NET clients can make requests on the same managed object simultaneously.

Accessing .NET objects through COM callable wrapper

COM callable wrapper

COM callable wrappers are invisible to other classes running within the .NET Framework. Their primary purpose is to marshal calls between managed and unmanaged code; however, CCWs also manage the object identity and object lifetime of the managed objects they wrap.

The runtime allocates memory for the .NET object from its garbage-collected heap, which enables the runtime to move the object around in memory as necessary. In contrast, the runtime allocates memory for the CCW from a noncollected heap, making it possible for COM clients to reference the wrapper directly.

Unlike the .NET client it wraps, the CCW is reference-counted in traditional COM fashion. When the reference count on the CCW reaches zero, the wrapper releases its reference on the managed object. A managed object with no remaining references is collected during the next garbage-collection cycle.

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