自定义MVC数据验证属性，如何通过反射动态创建表达式树

 public class OnlyOneAttribute : ValidationAttribute

    {

        public OnlyOneAttribute()

        {



        }



        protected override ValidationResult IsValid(object value, ValidationContext validationContext)

        {

            using (var context = new EfDbContext())

            {

                //验证属性父对象的类型

                var type = validationContext.ObjectType;

                //获取该类型的仓储类型

                var typeRepository = typeof (EfRepository<>);

                var genericType = typeRepository.MakeGenericType(type);



                //获取该类型的仓储对象  EfRepository<Student>

                var o = Activator.CreateInstance(genericType, context);

                //FirstOrDefault参数的表达树  需要修改的地方

                var expressionType = typeof (Expression<Func<Student, bool>>);

                //获取对应参数的FirstOrDefault方法

                var method = genericType.GetMethod("FirstOrDefault",new Type[] { expressionType });



                //表达式  需要修改的地方

                Expression<Func<Student, bool>> func = (s) => s.UserName == value.ToString();

                var result = method.Invoke(o, new object[] {func});



                if (result != null)

                {

                    var errorMessage = FormatErrorMessage(validationContext.DisplayName);



                    return new ValidationResult(errorMessage);

                }

            }

            return ValidationResult.Success;

        }

    }