问猜测缓冲区大小并回忆api函数，或者每次调用它两次以获得确切的缓冲区大小。

EN

StringBuffer = 10 // (byte) guessing returned string will fit in 10 bytes  

result = MsiGetProductInfoW(
  product,
  INSTALLPROPERTY_VERSIONSTRING,
  VersionString,
  StringBuffer
); //maybe it fits in 10

if result = ERROR_MORE_DATA then //doesnt fit in 10 so recall to get the correct buffer size
begin
  MsiGetProductInfoW(
    product,
    INSTALLPROPERTY_VERSIONSTRING,
    nil,
    StringBuffer
  );

  Inc(StringBuffer); // cause null-terminated string

  // recall it with matching
  MsiGetProductInfoW(
    product,
    INSTALLPROPERTY_VERSIONSTRING,
    VersionString,
    StringBuffer
  );
end;

StringBuffer = 0;

// get buffer size
MsiGetProductInfoW(
  product,
  INSTALLPROPERTY_VERSIONSTRING,
  nil,
  StringBuffer
);

Inc(StringBuffer); // cause null-terminated string

// use it with the correct buffersize
MsiGetProductInfoW(
  product,
  INSTALLPROPERTY_VERSIONSTRING,
  VersionString,
  StringBuffer
);