public key error

Question

i created one  class project i buld it one dll is generated. i am using now in another project but the following error is shown  

 

Could not load type 'Premium_Auto_Merchant.PurchaseInvoice' from assembly 'Premium auto Merchant, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null'.

 

 

 

why this error occour ,how to remove this error.