C# 4 and .NET 4

698 ❘ ChaPTer 26 interOp
can see in Figure 26-3, a dual interface is derived from IDispatch but provides the additional methods of
the interface directly in the vtable. Scripting clients can use the IDispatch interface to invoke the Add and
Sub methods, whereas clients aware of the vtable can call the Add and Sub methods directly.
QueryInterface
AddRef
Release
GetTypeInfoCount
"Add"
"Sub"
47
48
GetIDsOfNames
Invoke
Add
47 pAdd
48 pSub
figure 26-3
Sub
Casting and Queryinterface
If a .NET class implements multiple interfaces, casts can be done to get one interface or another. With
COM, the interface IUnknown offers a similar mechanism with the method QueryInterface(). As
discussed in the previous section, the interface IUnknown is the base interface of every interface, so
QueryInterface() is available anyway.
method binding
How a client maps to a method is defined with the terms early and late binding. Late binding means that the
method to invoke is looked for during runtime. .NET uses the System.Reflection namespace to make this
possible (see Chapter 14).
COM uses the IDispatch interface discussed earlier for late binding. Late binding is possible with dispatch
and dual interfaces.
With COM, early binding has two different options. One way of early binding, also known as vtable
binding, is to use the vtable directly — this is possible with custom and dual interfaces. The second option
for early binding is also known as ID binding. Here the dispatch ID is stored inside the client code, so
during runtime only a call to Invoke() is necessary. GetIdsOfNames() is called during design time. With
such clients, it is important to remember that the dispatch ID must not be changed.
data Types
For dual and dispatch interfaces, the data types that can be used with COM are restricted to a list of
automation-compatible data types. The Invoke() method of the IDispatch interface accepts an array
of VARIANT data types. The VARIANT is a union of many different data types, such as BYTE, SHORT, LONG,
FLOAT, DOUBLE, BSTR, IUnknown*, IDispatch*, and so on. VARIANTs have been easy to use from Visual
Basic, but it was complex to use them from C++. .NET has the Object class instead of VARIANTs.
With custom interfaces, all data types available with C++ can be used with COM. However, this also
restricts the clients that can use this component to certain programming languages.
registration
.NET distinguishes between private and shared assemblies, as discussed in Chapter 18. With COM, all
components are globally available through a registry configuration.
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