Using GDI in the Managed Environment

This article has been excerpted from book "Graphics Programming with GDI+".

One important feature of the .NET runtimes is COM and Win32 interoperability. With runtime interoperability services, developers can use both COM and Win32 libraries in managed applications. The classes related to these services are defined in the System.Runtime.InteropServices namespace.

We can use COM libraries in managed application by simply adding a reference to the COM library using the Add Reference option of VS.NET or the Type Library Importer (Tlbimp.exe) .NET tool. Both of these options allow developers to convert a COM library of a .NET assembly, which can then be treated as other .NET assemblies. The graphical user interface (GUI) functionality of Windows is defined in a Win32 library called Gdi32.dll. Using Win32 libraries in managed code is a little more difficult than using COM libraries. However, there is nothing to worry about because the System.Runtime.InteropServices.DllImportAttribute class allows developers to use functionality defined in unmanaged libraries such as Gdi32.dll.

The DllImportAttribute Class

The DllImportAttribute class allows developers to import Win32 SDK functionality into managed applications. The DllImportAttribute constructor is used to create a new instance of the DllImportAttribute class with the name of the DLL containing the method to import. For example, the GDI functionality is defined in Gdi32.dll. So if we want to use GDI functions in our application, we need to import them using DllImportAttribute. The following code imports the Gdi32.dll library:

[System.Runtime.InteropServices.DllImportAttribute ("gdi32.dll"]

After adding this code, we're ready to use the functions defined in the Gdi32.dll library in our .NET application.

Now let's take a look at simple program that uses the MoveFile function of Win32 defined in the KERNAL32.dll library. The code in Listing 14.1 first imports the library and then calls the MoveFile function to move a file from one location to another.

LISTING 14.1: Using the Win32 MoveFile function defined in KERNEL32.dll

[System.Runtime.InteropServices.DllImportAttribute ("KERNEL32.dll"]
public static extern bool MoveFile
(String src, String dst);

private void Move_Click (object sender, System.EventArgs e)
{
MoveFile ("C:\\outpit.jpeg",
"f:\\NewOutput.jpeg");
}

As with KERNEL32.dll, we can import other Win32 libraries to use them in .NET applications. The DllImportAttribute class provides six field members, which are described in Table 14.1.

The CallingConvention enumeration specifies the calling convention required to call methods implemented in unmanaged code. Its members are defined in Table 14.2.

The DllImportAttribute class has two properties: TypeId and Value. TypeId gets a unique identifier for an attribute when the attribute is implemented in the derived class, and Value returns the name of the DLL with the entry point.

TABLE 14.1: DllImportAttribute field members

Method	Description
CallingConvention	Required to call methods implemented in unmanaged code; represented by the CallingConvention enumeration.
CharSet	Controls name mangling and indicate how to marshal String arguments to the method.
EntryPoint	Identifies the name or ordinal of the DLL entry point to be called.
ExactSpelling	Indicates whether the name of the entry point in the unmanaged DLL should be modified to correspond to the CharSet value specified in the CharSet field.
PreserveSig	Specifies that the managed method signature should not be transformed into an unmanaged signature that returns an HRESULT structure, and may have an additional argument (out or retval) for the return value.
SetLastError	Specifies that the callee will call the Win32 API SetLastError method before returning from the named method.

TABLE 14.2: CallingConvention members

Method	Description
Cdec1	The caller cleans the stack. The property enables calling functions withvarargs.
FastCall	For future use.
StdCall	The called cleans the stack. This is the default convention for calling unmanaged functions from managed code.
ThisCall	The first parameter is the this pointer and is stored in the ECX register. Other parameters are pushed onto the stack. This calling convention is used to call methods in classes exported from an unmanaged DLL.
Winapi	Uses the default platform-calling convention. For example, on Windows it's StdCall, and on Windows CE it's Cdecl.