html tidy SaveString method

Hi,
 i have done the following steps :
 1.download the TidyAtl.dll
2.regsvr32 /c tidyAtl.dll
3.TlbImp c:\TidyATL.dll /out:c:\Tidy.dll
 i got a dll that i could use from .NET env.
 i have wrote the following code in C# :
   
private int status=0; 

private Document tdoc = new Document(); 

 string optFile = System.Web.HttpContext.Current.Server.MapPath("config.tidy"); 


status = tdoc.LoadConfig(optFile);

status = tdoc.ParseString(input); 
 
status = tdoc.CleanAndRepair();

status = tdoc.RunDiagnostics();

tdoc.SetOptBool(tidy.TidyOptionId.TidyForceOutput, 1 );

status = tdoc.SaveFile(System.Web.HttpContext.Current.Server.MapPath("
ttttee.xml"));

//this is the line with my problem

string tmp = tdoc.SaveString();

 when i have checked the tmp string in dubug time i saw it is NULL.

 something is wrong with the function SaveString? or am i missing something 
here?

 also i have searched the WEB and found documentation about a tidy dll that 
has more functions, like SaveBuffer...

 HELP !!!!!!!!!!!!!!!!!!!!!!!!!!

 thanks,

 danielb