inno setup 怎样检测程序已安装

[Registry]
Root: HKLM; Subkey: Software\The Application; ValueType: string; ValueName: Installed version; ValueData: '2.0'; Flags: uninsdeletekey

[Code]
function GetInstalledVersion(): String;
var
InstalledVersion: String;
begin
InstalledVersion :='';
RegQueryStringValue(HKLM, 'Software\The Application', 'Installed version', InstalledVersion);
Result := InstalledVersion;
end;

function InitializeSetup(): Boolean;
var
PrevVer: String;
begin
PrevVer := GetInstalledVersion();
result := true;
if length(PrevVer) > 0 then begin
//如果发现程序已经安装过了
MsgBox ('本程序的 ' + PrevVer + ' 版本已经安装，请卸载后再安装本程序. 安装程序将关闭.', mbError, MB_OK);
result := false;
end;
end;