Delphi2005学习笔记2——Using Platform Invoke with Delphi 2005
来源：互联网
作者：西部数码
时间：2008-04-09
西部数码-全国
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When working with for example the Windows API, object references are sometimes passed to the API where they are stored and later passed back to the application for processing usually associated with a given event. This can still be accomplished in .NET, but special care needs to be taken to ensure a reference is kept to all objects (otherwise they can and will be garbage collected). Data types The following table shows
| Unmanaged Data Types |
Managed Data Type |
| Supply Data |
Receive Data |
Pointer (Object reference, user data)
GCHandle
GCHandle The GCHandle provides the primary means of passing an object references to unmanaged code, and ensuring garbage collection does not happen. A GCHandle needs to be allocated, and later freed when no longer needed. There are several types of GCHandle , GCHandleType.Normal being the most useful when an unmanaged client holds the only reference. In order pass a GCHandle to an API function once it is allocated, type cast it to IntPtr (and optionally onwards to LongInt, depending on the unmanaged declaration). The IntPtr can later be cast back to a GCHandle . Note that IsAllocated must be called before accessing the Target property, as shown below: procedure MyProcedure;
var
Ptr: IntPtr;
Handle: GCHandle;
begin
...
if Ptr <> nil then
begin
Handle := GCHandle(Ptr);
if Handle.IsAllocated then
DoSomething(Handle.Target);
end;
...
end;
Advanced techniques The use of a GCHandle , although relatively easy, is fairly expensive in terms of performance. It also has the possibility of resource leaks if handles aren’t freed correctly. If object references are maintained in the managed code, it is possible to pass a unique index, for example the hash code returned by the
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