var db = new ExamdaDbContext(EnmDb.ExamSystem_Set);
            IQueryable<ExpressionClassModel> cities = db.ExpresstionClass;
            ParameterExpression param = Expression.Parameter(typeof(ExpressionClassModel), "c");
            Expression left = Expression.Property(param, typeof(ExpressionClassModel).GetProperty("Name"));
            Expression right = Expression.Constant("心情");
            Expression filter = Expression.Equal(left, right);
            var result = db.ExpresstionClass.Where(Expression.Lambda<Func<ExpressionClassModel, bool>>(filter, param));