why compiled and it return incompatible error ?

Hi , 

why when i compile, Delphi complained that i have incompatible types , got TclientDataset
expected  TWebClientDataset ? (in fact there is no ClientDataset component which i have
used in my project , 

===>   MobileDataPool.SaleOrder.Dataset := Datamodule1.tbl_sales;

which i checked my MobileDataPool is just a normal object , and the code are as follow ,

    TWebMobileDataset = class
            property Dataset : TWebClientDataSet read fDataset write fDataset ;
     end ; 

     TSaleOrder = class(TWebMobileDataset)

     end ;

     TMobileDataPool = class
              SaleOrder : TSaleOrder ;  //(which TSaleOrder is class(TWebMobileDataset)) 
    end ; 
Please Advice . thanks.    

Can you try:

 property Dataset : TClientDataSet read fDataset write fDataset ;

Technically, TWebClientDataSet is declared as:
  TWebClientDataSet = class(TClientDataSet);
so defining the property type as TClientDataSet should work.