static void Main(string[] args)
{
try
{
MyContext db = new MyContext();

var a = db.a.First();
B b4 = new B
{
name = "B4"
};
b4.a = a;

db.b.Add(b4);
db.SaveChanges();

Console.Write("success");

}
catch (Exception ex)
{
Console.WriteLine(ex.Message);
}
}

public class A
{
[Key]
public int Aid { get; set; }
public string name { get; set; }

public virtual List<B> bList { get; set; }
}

public class B
{
[Key]
public int Bid { get; set; }
[ForeignKey("a")]
public int Aid { get; set; }
public string name { get; set; }

public virtual A a { get; set; }
}

 这样是可以实现你的需求的