Smart Cards in .NET, Part 2 - Two Versions
(Page 3 of 6 )
The Microsoft Windows application programming interface (Win32 API) usually contains two versions of every function that handles characters and strings: a 1-byte character ANSI version and a 2-byte character Unicode version. When unspecified, the character set, represented by the CharSet field, defaults to ANSI. Some functions can have more than two versions. You may specify the exact version by providing additional attributes, as in the example below for the MoveFile API’s Unicode version of the function. You may lookup for more explanation on these attributes in the .NET online help.
[C#]
[DllImport("KERNEL32.DLL", EntryPoint="MoveFileW", SetLastError=true,
CharSet=CharSet.Unicode, ExactSpelling=true,
CallingConvention=CallingConvention.StdCall)]
public static extern bool MoveFile(Stringstr src, String strdst);
A good practice is to group all the related functions in a class and let the class contain all these function prototypes.
Apart from these function prototypes you’ll also need to provide any Structures’, Unions’, and constants’ definitions because in the .NET FCL these are not available. (For example the POINT structure or the NMHDR structure, if you’re trying to process a Common Control notification in your .NET application.) In the case of our Managed Smart Card Library, we will not be using any callbacks, but if needed you may refer to implementing callbacks in the online help.
Next comes the point of marshalling. It’s quite important that the .NET runtime understands what we’re trying to pass to the unmanaged function, and what we expect back after its execution. We need to use the Reflection namespace for this purpose, so we may include in our code the following lines:
[Visual Basic]
Imports System.Reflection
[C#]
using System.Reflection
Using this attribute we may specify how the data we pass or expect in return should be treated. You’ll need a bit of time to get used to the types you encounter while making calls to the native API. You may also use the table in the MSDN online help for the table that gives a mapping of the .NET types and the unmanaged types. If you use a third party DLL you must be cautious about these MarshalAs attributes, and use them where required. In most cases you’ll need them.
Next: The Managed Smart Card Sample >>
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