I am creating application by C# and sql server, I am using entity frameWork library

Question

, but I have a problem: I have a table its name is Tests contain two columns as a foreign key, when I write this code: DataGridView1.DataSource = TestOpperation.Read tests(); "I created this method, it return a list from Tests" But in DataGridView1 the foreign key columns are empty. And this exception : system.NullReferenceException