要确定特定文件夹(在您的情况下为所选目录)的驱动器上的可用空间，可以调用 GetSpaceOnDisk64 功能.它们之间的区别在于，第一个能够返回字节和兆字节的空间信息.后者仅以字节为单位返回此信息.在下面的示例中，我选择了第一个提到的函数，因此您可以通过修改单个布尔参数来决定要在哪个单元中进行操作:

To determine a free space on a drive of a specific folder (in your case the selected directory), you can call the GetSpaceOnDisk or GetSpaceOnDisk64 function. The difference between them is that the first one is able to return space info in bytes as well as in megabytes. The latter returns this info just in bytes. For the following example I chose the first mentioned function, so you can decide in which units you want to operate by modifying a single boolean parameter:

[Setup]
AppName=My Program
AppVersion=1.5
DefaultDirName={pf}\My Program

[Code]
procedure ExitProcess(uExitCode: UINT);
  external 'ExitProcess@kernel32.dll stdcall';

function IsEnoughFreeSpace(const Path: string; MinSpace: Cardinal): Boolean;
var
  FreeSpace, TotalSpace: Cardinal;
begin
  // the second parameter set to True means that the function operates with
  // megabyte units; if you set it to False, it will operate with bytes; by
  // the chosen units you must reflect the value of the MinSpace paremeter
  if GetSpaceOnDisk(Path, True, FreeSpace, TotalSpace) then
    Result := FreeSpace >= MinSpace
  else
    RaiseException('Failed to check free space.');
end;

function NextButtonClick(CurPageID: Integer): Boolean;
begin
  Result := True;

  if CurPageID = wpSelectDir then
  begin
    // the second parameter in this function call is the expected min. space in
    // units specified by the commented parameter above; in this example we are
    // checking if there's at least 1 MB of free space on drive of the selected
    // directory; we need to extract a drive portion of the selected directory,
    // because it's probable that the directory won't exist yet when we check
    if not IsEnoughFreeSpace(ExtractFileDrive(WizardDirValue), 1) then
    begin
      MsgBox('There is not enough space on drive of the selected directory. ' +
        'Setup will now exit.', mbCriticalError, MB_OK);
      // in this input parameter you can pass your own exit code which can have
      // some meaningful value indicating that the setup process exited because
      // of the not enough space reason
      ExitProcess(666);
    end;
  end;
end;