P/Invoke
It is possible, though generally undesirable, to invoke unmanaged
code from within C#. The .NET
platform invoke
facility
(P/Invoke
) was originally
intended only to provide access to the Windows API, but you can use
it to expose functions in any DLL.
To see how this works, let’s revisit Example 21-3 from Chapter 21. You will
recall that you used the
Stream
class to rename
files by invoking the MoveTo( )
method:
file.MoveTo(fullName + ".bak");
You can accomplish the same thing by using Windows’
kernel32.dll
and invoking the
MoveFiles
method. To do so, you need to declare
the method as a static
extern
and use the DllImport
attribute:
[DllImport("kernel32.dll", EntryPoint="MoveFile", ExactSpelling=false, CharSet=CharSet.Unicode, SetLastError=true)] static extern bool MoveFile( string sourceFile, string destinationFile);
The
DllImportAttribute
class is used to indicate that an
unmanaged method will be invoked through P/Invoke
.
The parameters are:
-
EntryPoint
Indicates the name of the DLL entry point (the method) to call.
-
ExactSpelling
Setting this to
false
allows matching of the entry point name without case sensitivity.-
CharSet
Indicates how the string arguments to the method should be marshaled.
-
SetLastError
Setting this to
true
allows you to callGetLastError
to check if an error occurred when invoking this method.
The rest of the code is virtually unchanged, except for the
invocation of the MoveFile( )
method itself. Notice that
MoveFile( )
is declared to be a static ...
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