asp.net mvc - ModelState.AddModelError dosen't show error -

my modelstate.addmodelerror dosen't show error. checked username , show error when been duplicated. when use breakpoint see modelstate.addmodelerror filled, doesn't show error msg.

my code:

controller:

    [httppost]     public actionresult register(tbl_user model)     {         if (modelstate.isvalid)         {             if (myclass.isusernameduplicate(model.username) == true)             {  this.modelstate.addmodelerror("username", "the username duplicate");                  return view(model);             }             else             {                 myclass.creatuser(model.username, model.password_user, model.nam);             }         }          return view(model);      } 

view :

         @html.validationsummary(true)          @html.validationmessagefor(model => model.username)          @html.textboxfor(m => m.username, new {@class = "input", @placeholder = "*enter user name" }) 

you need call addmodelerror before checking modelstate.isvalid. many times custom validation in controllers

[httppost] public actionresult register(tbl_user model) {     if (myclass.isusernameduplicate(model.username) == true)     {         this.modelstate.addmodelerror("username", "the username duplicate");     }      if (!modelstate.isvalid)     {         return view(model);     }     else     {         myclass.creatuser(model.username, model.password_user, model.nam);     }      return view(model); }