delphi调用图形识别dll出错!!!!!!!

delphi 调用图形识别dll总是报地址之类的错误，请大家帮忙看看是什么问题,代码如下:
implementation
function OCR(imgname:string;i:integer):PChar;stdcall;external 'AspriseOCR.dll';
{$R *.dfm}

procedure TForm1.Button1Click(Sender: TObject);
var
vcode:PChar;
filename:string;
begin
filename:='test1.bmp';
vcode:=OCR('e:\poll\demo\test1.bmp',-1);
edit1.Text:=strpas(vcode);
end;

end.

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谁给你做的dll, 第一个参数是 string ?

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dll用delphi写的吗?

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不是的，是在国外的一个网站下载的DLL，说明文档说是可以支持delphi但没有例程，这是我仿VB的例程写出的。
VB的例程是：
private declare function ocr lib "aspriseocl.dll"(byval file as string,byval imagetype as long )as string
private declare function ocrbarcodes lib "asperiseocr.dll"(byval file as string,byval imagetype as long ) as string
private declare function ocrpart lib "aspriseocr.dll" (byval file as string,byval imagetype as long,byval startx as long,byval starty as long,byval width as long,byval height as long) as string
private sub command2_click()
dim ret as string
ret=ocr("c:\test.bmp",-1)
end sub

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回楼上，改了还是报一样的错
access violation at 0x00000000:read of address ox00000000'.

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那就应该查一下文件路径是否正确、该Dll是否需要调用初始化过程。
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