Автор: http://www.swissdelphicenter.ch

{
Converts Unicode string to Ansi string using specified code page.
  @param   ws       Unicode string.
  @param   codePage Code page to be used in conversion.
  @returns Converted ansi string.
}


function WideStringToString(const ws: WideString; codePage: Word): AnsiString;
var
  l: integer;
begin
  if ws = '' then
    Result := ''
else
  begin
    l := WideCharToMultiByte(codePage,
      WC_COMPOSITECHECK or WC_DISCARDNS or WC_SEPCHARS or WC_DEFAULTCHAR,
      @ws[1], -1, nil, 0, nil, nil);
    SetLength(Result, l - 1);
    if l > 1 then
      WideCharToMultiByte(codePage,
        WC_COMPOSITECHECK or WC_DISCARDNS or WC_SEPCHARS or WC_DEFAULTCHAR,
        @ws[1], -1, @Result[1], l - 1, nil, nil);
  end;
end; { WideStringToString }

{:Converts Ansi string to Unicode string using specified code page.
  @param   s        Ansi string.
  @param   codePage Code page to be used in conversion.
  @returns Converted wide string.
 }

function StringToWideString(const s: AnsiString; codePage: Word): WideString;
var
  l: integer;
begin
  if s = '' then
    Result := ''
else
  begin
    l := MultiByteToWideChar(codePage, MB_PRECOMPOSED, PChar(@s[1]), -1, nil,
      0);
    SetLength(Result, l - 1);
    if l > 1 then
      MultiByteToWideChar(CodePage, MB_PRECOMPOSED, PChar(@s[1]),
        -1, PWideChar(@Result[1]), l - 1);
  end;
end; { StringToWideString }