C# 4 and .NET 4

Platform invoke ❘ 723
To reuse an unmanaged library that doesn ’ t contain COM objects, just exported functions, platform invoke
(p/invoke) can be used. With p/invoke, the CLR loads the DLL that includes the function that should be
called and marshals the parameters.
To use the unmanaged function, fi rst you have to fi nd out the name of the function as it is exported. You
can do this by using the dumpbin tool with the /exports option.
For example, the command
dumpbin /exports c:\windows\system32\kernel32.dll | more
lists all exported functions from the DLL kernel32.dll . In the example, you use the CreateHardLink()
Windows API function to create a hard link to an existing fi le. With this API call, you can have several
fi le n a m e s t h at refere nc e t he s a m e fi le a s long a s t he fi le n a m e s a re on ju s t one h a rd d i sk . T h i s A PI c a l l i s not
available from .NET Framework 4, so platform invoke must be used.
To call a native function, you have to defi ne a C# external method with the same number of arguments,
and the argument types that are defi ned with the unmanaged method must have mapped types with
managed code.
The Windows API call CreateHardLink() has this defi nition in C++:
BOOL CreateHardLink(
LPCTSTR lpFileName,
LPCTSTR lpExistingFileName,
LPSECURITY_ATTRIBUTES lpSecurityAttributes);
Now, this defi nition must be mapped to .NET data types. The return type is a BOOL with unmanaged code;
this simply maps to the bool data type. LPCTSTR defi nes a long pointer to a const string. The Windows
API uses the Hungarian naming convention for the data type. LP is a long pointer, C a const, and STR is
a null - terminated string. The T marks the type as a generic type, and the type is either resolved to LPCSTR
(an ANSI string) or LPWSTR (a wide Unicode string), depending on the compiler ’ s settings. C strings map to
the .NET type String. LPSECURITY_ATTRIBUTES , which is a long pointer to a struct of type SECURITY_
ATTRIBUTES . Because you can pass NULL to this argument, mapping this type to IntPtr is okay. The C#
declaration of this method must be marked with the extern modifi er, because there ’ s no implementation of
this method within the C# code. Instead, the implementation of this method is found in the DLL kernel32
.dll , which is referenced with the attribute [DllImport] . Because the return type of the .NET declaration
CreateHardLink() is of type bool , and the native method CreateHardLink() returns a BOOL , some
additional clarifi cation is useful. Because there are different Boolean data types with C++ (for example
the native bool and the Windows - defi ned BOOL , which have different values), the attribute [MarshalAs]
specifi es to what native type the .NET type bool should map.
[DllImport("kernel32.dll", SetLastError="true",
EntryPoint="CreateHardLink", CharSet=CharSet.Unicode)]
[return: MarshalAs(UnmanagedType.Bool)]
public static extern bool CreateHardLink(string newFileName,
string existingFilename,
IntPtr securityAttributes);
The web site http://www.pinvoke.net and the tool P/Invoke Interop Assistant,
which can be downloaded from http://www.codeplex.com, are very helpful with the
conversion from native to managed code.
The settings that you can specify with the attribute [DllImport] are listed in the following table.
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