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Header Files Created by Import
#import creates two header files that reconstruct the type library contents in C++ source code. The primary header file is similar to that produced by the Microsoft Interface Definition Language (MIDL) compiler, but with additional compiler-generated code and data. The primary header file has the same base name as the type library, plus a .TLH extension. The secondary header file has the same base name as the type library, with a .TLI extension. It contains the implementations for compiler-generated member functions, and is included (#include) in the primary header file.

If importing a dispinterface property that uses byref parameters, #import will not generate __declspec(property) statement for the function.

Both header files are placed in the output directory specified by the /Fo (name object file) option. They are then read and compiled by the compiler as if the primary header file was named by a #include directive.

The following compiler optimizations come with the #import directive:

The header file, when created, is given the same timestamp as the type library.
When #import is processed, the compiler first checks if the header exists and is up to date. If yes, then it does not need to be re-created.
The #import directive also participates in minimal rebuild and can be placed in a precompiled header file. See Creating Precompiled Header Files for more information.