public class TESTMAIN
 {
        public TESTMAIN()
        {
            this.TESTREFs = new List<TESTREF>();
        }

        public int ID { get; set; }
        public string A { get; set; }
        public string B { get; set; }
        public string C { get; set; }
        public virtual ICollection<TESTREF> TESTREFs { get; set; }
}
 public class TESTREF 
 {
        public int ID { get; set; }
        public string A { get; set; }
        public Nullable<decimal> TEST { get; set; }
        public Nullable<int> REFID { get; set; }
        public virtual TESTMAIN TESTMAIN { get; set; }
}
//map中的配置
 this.HasOptional(t => t.TESTMAIN)
                .WithMany(t => t.TESTREFs)
                .HasForeignKey(d => d.REFID);

/////////////////////////////////////////////////
TESTREF ref1 = new TESTREF() { A = "test", TEST = 1 };
TESTREF ref2 = new TESTREF() { A = "test2", TEST = 2 };

TESTMAIN main1 = new TESTMAIN() { A = "main" };