ObservableCollection Contains doesnt work correctly

hi guys! when press button find throwexeption if use after start findbycode find work correctly

 public icommand findbycode          {                           {                  return new relaycommand(() =>                  {                                            libmaterials =                          new observablecollection<libmaterial>(                              _repo._context.libmaterial.where(material => material.code.contains(code)));                  });              }          }            public icommand find          {                           {                  return new relaycommand(() =>                  {                      ////loadbook();                      ////code = "";                        if (findgenre == null)                      {                          libmaterials =                              new observablecollection<libmaterial>(                                  libmaterials                                      .where(material => material.author.contains(autor))                                      .where(material => material.name.contains(bookname)));                      }                      else                      {                          libmaterials =                              new observablecollection<libmaterial>(                                  libmaterials.where(material => material.tree_id == findgenre.id)                                      .where(material => material.author.contains(autor))                                      .where(material => material.name.contains(bookname)));                      }                  });              }          }  

libmateriaks not null if change code 

.where(material => material.name==bookname));

all work good.

but want smart find(if inpu 3 symbol in collection find first 3 symbol)

Visual Studio Languages  ,  .NET Framework  >  Visual C#