var StringStream: TStringStream;

StringStream := TStringStream.Create(myVariable, TEncoding.Unicode);

var MemoryStream: TMemoryStream;
    str1, str2: UnicodeString;
    lng1, lng2: Integer;

str1 := 'Тестовая строка.';
lng1 := Length(str1);
MemoryStream.WriteBuffer(lng1, SizeOf(lng1)); // write length in chars
if lng1 > 0 then MemoryStream.WriteBuffer(pointer(str1)^, ByteLength(str1));

MemoryStream.Position := 0;
MemoryStream.ReadBuffer(lng2, SizeOf(lng2)); // read length in chars
SetLength(str2, lng2);
if lng2 > 0 then MemoryStream.ReadBuffer(Pointer(str2)^, ByteLength(str2));