Expression中Convert有什么用

    public class RefClas
    {
        public int id;
        public int age;
        public RefClas(int id, int age)
        {
            this.id = id;
            this.age = age;
        }
        public static MyStudent RefClasToMyStudent(RefClas rc)
        {
            MyStudent st = new MyStudent();
            st.id = rc.id;
            return st;
        }
        
        public static explicit operator RefClas(Int32 id)
        {
            RefClas rc = new RefClas(id,0);
            return rc;
        }
        public static implicit operator int(RefClas rc)
        {
            return rc.id;
        }
    }
    public class MyStudent
    {
        public int id1 { get; set; }                   //属性
        public int id2;                                 //字段
        public int id3 { get { return 10; } }    //静态属性
        public int id4 = 20;                     //静态字段
        public int id;
        public int new_id;
        public DateTime time;
        public static implicit operator MyStudent(int id)
        {
            Console.WriteLine("public static implicit operator MyStudent(int id)" + id);
            MyStudent st = new MyStudent();
            st.id = id;
            return st;
        }
        public static implicit operator int(MyStudent st)
        {
            Console.WriteLine("public static implicit operator int(MyStudent st)" + st.id);
            return st.id;
        }
        //public static implicit operator Object(MyStudent st)//不允许进行以基类为转换源或目标的用户自定义转换
        //public static implicit operator MyStudent()
        //{
        //    Console.WriteLine("public static implicit operator Object(MyStudent st)" + st.id);
        //    return st.id;
        //}
    }
    public class StudentHelper
    {
        public int id1 { get; set; }                   //属性
        public int id2;                                 //字段
        public static int id3 { get { return 10; } }    //静态属性
        public static int id4 = 20;                     //静态字段

    }

            StudentHelper h = new StudentHelper();
            h.id1 = 1;
            h.id2 = 2;
            Expression<Func<MyStudent, bool>> la1 = n => n.id1 == h.id1;
            Expression<Func<MyStudent, bool>> la2 = n => n.id2 == h.id2;
            Expression<Func<MyStudent, bool>> la3 = n => n.id3 == StudentHelper.id3;
            Expression<Func<MyStudent, bool>> la4 = n => n.id4 == StudentHelper.id4;
            Expression<Func<MyStudent, bool>> la5 = n => n.id == n.new_id;//变量n未定义异常
            test(la1, "属性");
            test(la2, "字段");
            test(la3, "静态属性");
            test(la4, "静态字段");
            //test(la5, "自身参数");//在Eval中异常 上面表达式中只有n.new_id有不同
            RefClas refClass = new RefClas(20,21);
            Expression<Func<MyStudent, bool>> la6 = n => n.id == refClass;
            test(la6, "implicit test");
        public static void test(Expression<Func<MyStudent, bool>> la, string name)
        {
           Console.WriteLine("\n\n*****************" + name + "*********************");
            Expression B_exp; // = la.Body as BinaryExpression;
            B_exp = la.Body as BinaryExpression;
            Console.WriteLine("Expression类名:" + ((BinaryExpression)B_exp).Right.GetType().Name);
            if ((((BinaryExpression)B_exp).Right as UnaryExpression) != null)
            {
                UnaryExpression cast = ((BinaryExpression)B_exp).Right as UnaryExpression;
                var deg = Expression.Lambda<Func<int>>(cast).Compile();
                object obj = deg();
                Console.WriteLine(B_exp + "值为：" + obj);
            }
            else
            {

            }
            MemberExpression m_exp = ((BinaryExpression)B_exp).Right as MemberExpression; //为什么能将B_exp.Right转换成MemberExpression 有的可能，有的应该不能吧?
            //la6 = n => n.id == refClass;这一句的时候la.Body.Right是UnaryExpression并不是MemberExpression, 所以m_exp=null，后面if(m_exp.Expression==null)就会异常。
            //MemberInitExpression m_exp = B_exp.Right as MemberInitExpression;
            string valueClassName = string.Empty;
            if (m_exp.Expression == null)
            {
                Console.WriteLine("数据为空");
            }
            else
            {
                valueClassName = m_exp.Expression.GetType().Name;
                Console.WriteLine("数据Expression类名：" + valueClassName);
            }
            Console.WriteLine("值为：" + Eval(m_exp));
            Console.WriteLine("\n\n*********************************************");
        }
        public static object Eval(MemberExpression member)
        {
            if(member.NodeType == ExpressionType.MemberAccess)
            {
                if(member.Member.DeclaringType == typeof(MyStudent))
                {
                    if(member.Type == typeof(MyStudent))
                    {
                        //if(member.Expression)
                    }
                }
            }
            //当最右边的n.new_id这个MemberExpression时（实际是FieldExpression)则打印显示如下：
            Console.WriteLine("member.NodeType=" + member.NodeType);
            Console.WriteLine("member.Member.DeclaringType=" + member.Member.DeclaringType); //是表达式右边的Member的类型 h.id2时则是StudentHelper
            Console.WriteLine("member.Member.DeclaringType.Name=" + member.Member.DeclaringType.Name);
            Console.WriteLine("member.Type=" + member.Type);
            Console.WriteLine("member.Expression=" + member.Expression);
            //member.NodeType=MemberAccess
            //member.Member.DeclaringType=钩子.MyStudent
            //member.Member.DeclaringType.Name=MyStudent
            //member.Type=System.Int32
            //member.Expression=n
            //上面member.Type其实是FieldExpression或PropertyExpression从Expression继承过来并且override的属性），代表这个表达式所代表的静态类型

            //为了分析方便，将la2   n => n.id2 == h.id2 的显示也列在下面：
            //*****************字段*********************
            //Expression类名:FieldExpression
            //数据Expression类名：FieldExpression
            //member.NodeType=MemberAccess
            //member.Member.DeclaringType=钩子.StudentHelper
            //member.Member.DeclaringType.Name=StudentHelper
            //member.Type=System.Int32
            //member.Expression=value(钩子.Form1+<>c__DisplayClass8).h
            //值为：2
            //为了分析方便，将la3 = n => n.id3 == StudentHelper.id3的显示也列在下面：
            //*****************静态属性*********************
            //Expression类名:PropertyExpression
            //数据为空
            //member.NodeType=MemberAccess
            //member.Member.DeclaringType=钩子.StudentHelper
            //member.Member.DeclaringType.Name=StudentHelper
            //member.Type=System.Int32
            //member.Expression=
            //值为：10
            UnaryExpression cast = null;
            object obj = null;
            if(member.Member.DeclaringType.Name.Contains("MyStudent"))
            {
                cast = Expression.Convert(member, typeof(int));
                var deg = Expression.Lambda<Func<int>>(cast).Compile();
                //MyStudent mst = new MyStudent();mst.id=100;
                int rv = deg();
                obj = rv;
                //obj = Expression.Lambda<Func<int>>(cast).Compile().Invoke();//出现该变量n未定义
            }
            else
            {
                cast = Expression.Convert(member, typeof(object));
                obj = Expression.Lambda<Func<object>>(cast).Compile().Invoke();
            }           
            
            return obj;
        }

RefClas refClass = new RefClas(20,21);
Expression<Func<MyStudent, bool>> la6 = n => n.id == refClass;
var dla6 = la6.Compile();
MyStudent myStudent = new MyStudent();
myStudent.id = 20; //21
Console.WriteLine("dla6=" + dla6(myStudent));