ctrl+ shift+ B to build project
Delete entity framework and add new orther
-- Hoang ConnectionString :
<connectionStrings>
<add name="ProjectSem3Entities" connectionString="metadata=res://*/EF.DataContext.csdl|res://*/EF.DataContext.ssdl|res://*/EF.DataContext.msl;provider=System.Data.SqlClient;provider connection string="data source=DESKTOP-61G807S\SQLEXPRESS;initial catalog=ProjectSem3;persist security info=True;user id=sa;password=123456;MultipleActiveResultSets=True;App=EntityFramework"" providerName="System.Data.EntityClient" />
</connectionStrings>
-- Thanh ConnectionString
<connectionStrings>
<add name="ProjectSem3Entities" connectionString="metadata=res://*/EF.DataContext.csdl|res://*/EF.DataContext.ssdl|res://*/EF.DataContext.msl;provider=System.Data.SqlClient;provider connection string="data source=.;initial catalog=ProjectSem3;persist security info=True;user id=sa;password=123456;MultipleActiveResultSets=True;App=EntityFramework"" providerName="System.Data.EntityClient" />
</connectionStrings>