Delphi2005学习笔记2——Using Platform Invoke with Delphi 2005

Marshal class contains a method to retrieve the offset of a given field, it is extremely slow and should be avoided in most situations. Informal performance tests show that for a structure with eight or nine numeric fields, it is much faster to allocate a block of unmanaged memory, copy the byte array to the unmanaged memory and call PtrToStructure than finding the position of just one field using Marshal. OffsetOf and converting the data using the BitConverter class. Borland.Vcl.WinUtils contains helper functions to perform conversions between byte arrays and structures (see StructureToBytes and BytesToStructure ). Special cases There are cases where custom processing is required, such as sending a message with a pointer to an array of integers. For situations like this, the Marshal class provides methods to copy data directly to the unmanaged buffer, at specified offsets (so you can construct an array of a custom data type after allocating a buffer). The following example shows how to send a message where the LParam is a pointer to an array of Integer:

function SendArrayMessage(Handle: HWND; Msg: UINT; WParam: WPARAM;
  LParam: TIntegerDynArray): LRESULT; 
var
  Buffer: IntPtr;
begin
  Buffer := Marshal.AllocHGlobal(Length(LParam) * SizeOf(Integer));
  try
    Marshal.Copy(LParam, 0, Buffer, Length(LParam));
    Result := SendMessage(Handle, Msg, WParam, Buffer);
  finally
    Marshal.FreeHGlobal(Buffer);
  end;
end;

Callback Functions When passing a function pointer for a managed function to an unmanaged API, a reference must be maintained to the delegate or it will be garbage collected. If you pass a pointer to your managed function directly, a temporary delegate will be created, and as soon as it goes out of scope (at the end of MyFunction in the example below), it is subject to garbage collection. Consider the following Delphi 7 code:

function MyFunction: Integer;
begin
  ...
  RegisterCallback(@MyCallback);
  ...
end;

In order for this to work in a managed environment, the code needs to be changed to the following:

const
  MyCallbackDelegate: TFNMyCallback = @MyCallback;

function MyFunction: Integer;
begin
  ...
  RegisterCallback(MyCallbackDelegate);
  ...
end;

This will ensure that the callback can be called as long as MyCallbackDelegate is in scope. Data types The same rules apply for callbacks as any other unmanaged API function. Special cases Any parameters used in an asynchronous process must be declared as IntPtr . The marshaler will free any memory it has allocated for unmanaged types when it returns from the function call. When using an IntPtr , it is your responsibility to free any memory that has been allocated. Passing Object References

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