This attribute (and, in C#, the keyword “extern”) specifies that a method definition is implemented externally (usually in a DLL). Apply this attribute to a method that has been declared (but not defined) to specify the DLL name and entry point in which the method can be found.
The attribute can be customized in a number of different ways to help
control the binding against the external method. The
value dictates how the parameters to the call (and return value coming back)
should be sent to the function.
CallingConvention.StdCall (used for calling
_stdcall-declared functions, which is most of the Win32 API set) and
CallingConvention.Cdecl (used for calling functions declared directly from
C or C++) are the two most common values. The
value indicates which character set parameters to the call is
expected to be, either two-byte Unicode or one-byte ANSI.
EntryPoint indicates the name of the exported function
from the DLL to bind to (normally this is guessed from the name of the
.NET-declared method), and
whether the .NET compiler should attempt to “best match” a declared
DllImport method against a possible set of exported functions. The
PreserveSig value indicates how .NET should treat
[retval]-declared parameters. By default, the .NET
compilers ignore the HRESULT return value on IDL-declared methods
and use the
[retval]-declared parameter as the return value; setting