c# - 如何在 orderby 子句中使用带有动态参数的 LINQ

Question

我在使用 orderby linq 表达式中的动态参数时遇到问题

1. SearchExp函数

   public Expression<Func<EmailAflAwmMessageDM, bool>> SearchXpr(string param, string q)
   {
       if (param == "to")
           return e => e.to_msg.Contains(q);
       else if (param == "from")
           return e => e.from_msg.Contains(q);
       else if (param == "cc")
           return e => e.cc_msg.Contains(q);
       else if (param == "bcc")
           return e => e.bcc_msg.Contains(q);
       else if (param == "subject")
           return e => e.subject.Contains(q);
       else
           return e => e.body_text.Contains(q);
   }
   

2. filterExp 函数

      public Expression<Func<EmailAflAwmMessageDM, bool>> FiltertXpr(string filter, string value)
      {
       if (filter == "attachments")
       return e => e.attachments == value;
       else if (filter == "flagged")
       return e => e.flagged == value;
       else
       return e => e.seen == value;
       }
   

3. IQueryable 函数

      private IQueryable SearchFilter(string param,string q,string filter,
          string value,string sort,string dir)
      {
         var searchXpr = SearchXpr(param, q);
         var filterXpr = FiltertXpr(filter, value);
         var emailmessage = 
         db.EmailAflAwmMessage.
         Where(filterXpr).Where(searchXpr)
         .OrderByDescending(a => a.msg_date).Select(a =>
         new
         {
          a.subject,
          a.msg_date,
         });
   
          return emailmessage;
         }
   

上面的代码可以工作，但我需要动态方式的 OrderBy 。 因为我有 2 个参数 sort( 表示其参数名称) 和 dir ( 表示升序或降序) 就像我想要 orderby( 参数名称) dir 请帮助我，我感谢您的宝贵时间和建议，并以简单的方式向我建议任何替代方案。谢谢。

Answer 1

我建议您阅读Expression's tree's ，下面的代码是用于 didatic 的，但我认为这会对您有所帮助:

  public static class ExpressionBuilder
    {
        private static readonly MethodInfo ToStringMethod = typeof(object).GetMethod("ToString");
        private static readonly MethodInfo StringContainsMethod = typeof(string).GetMethod("Contains");

        public static Func<T, object> Selector<T>(string prop)
        {
            var type = typeof(T);
            var param = Expression.Parameter(type);
            return Expression.Lambda<Func<T, object>>(Expression.Property(param, type.GetProperty(prop)), param).Compile();
        }

        public static Expression<Func<T, bool>> BuildFilterPredicate<T>(string q)
        {
            var query = Expression.Constant(q);
            var type = typeof(T);
            var lbdSelector = Expression.Parameter(type);
            var predicates = type.GetProperties().SelectMany(p => PredicateContainsBuilder(lbdSelector, p, query)).ToList();
            Expression body = predicates[0];
            body = predicates.Skip(1).Aggregate(body, Expression.OrElse);
            return Expression.Lambda<Func<T, bool>>(body, lbdSelector);
        }

        private static IEnumerable<MethodCallExpression> PredicateContainsBuilder(Expression lbdSelector, PropertyInfo prop, Expression query)
        {

            if (prop.PropertyType.IsClass)
                return new List<MethodCallExpression> { Expression.Call(Expression.Call(Expression.Property(lbdSelector, prop), ToStringMethod), StringContainsMethod, query) };

            var properties = prop.PropertyType.GetProperties();
            return properties.Select(p => Expression.Call(Expression.Call(Expression.Property(lbdSelector, p), ToStringMethod), StringContainsMethod, query)).ToList();
        }
    }

所以现在你在你的方法中执行此操作:

Note:

• I supose that the entity is EmailMessage so i use that to generate the predicate;
• It doesn't search in depth;
• it will search in all properties and doesn't use the string param to define what property to match;

private IQueryable SearchFilter(string param,string q,string filter,string value,string sort,string dir)
    {
        var emailMessage = db.EmailAflAwmMessage
                            .Where(ExpressionBuilder.BuildFilterPredicate<EmailMessage>(q))
                            .OrderBy(ExpressionBuilder.Selector<EmailMessage>(sort))
                            .Select(m=> new{m.subject,m.msg_date});        
    return emailmessage;
    }