c# - 如何从linq查询中提取条件语句？

Question

我希望能够从函数内的后续查询中提取条件语句。这是 EF 查询的 linq。我该怎么做？有没有其他方法可以应用面向对象的原则(如开放/封闭)？

    var query = new UvwRequestAssignmentManagementBO().GetAll().Where(uvw => (uvw.FK_ProcessStep == 2)
        && (uvw.FK_Entity == SecurityContext.Current.User.FK_Entity)
       && (uvw.FK_Manager == 15))
       .Select(p => new ReqSupAdminGridVm()
                {
                    NameFamily = p.NameFamily,
                    RequestDate = p.RequestDate,
                    RequestNo = p.RequestNo,
                    RequestType = p.RequestType == 1 ?"a"
                                          : (p.RequestType == 2 ? "b"
                                          : (p.RequestType == 3 ? "c" :
                                          (p.RequestType == 4 ? "d" : ""))),
                    RequestEvaluationStatus = p.RequestEvaluationStatus_Aggregation == 1 ? "a"
                                          : (p.RequestEvaluationStatus_Aggregation == 2 ? "b"
                                          : (p.RequestEvaluationStatus_Aggregation == 3 ?"c" 
                                          :(p.RequestEvaluationStatus_Aggregation == 4 ? "d" : ""))), 
                });

例如而不是写:

    RequestType = p.RequestType == 1 ?"a"
                                          : (p.RequestType == 2 ? "b"
                                          : (p.RequestType == 3 ? "c" :
                                          (p.RequestType == 4 ? "d" : ""))),

我希望能够在另一个类中编写它:

RequestType = ReqType.GetReqType(p.RequestType);

string GetReqType(int type){

           switch(type){
                      case 1:
                       return "a";
                      case 2:
                       return "b";
                          }
}

Answer 1

查看 PredicateBuilder ，它可以让你做我想你想做的事，我自己在最近的一个项目中使用过它，代码可重用且可读。

我为我的模型创建了这样的谓词:

using System;
using System.Linq;
using System.Linq.Expressions;

namespace MyModels
{
    using Predicate = Expression<Func<Component, bool>>;

    public partial class Component
    {
        public static Predicate HasKeywordContaining(string keyword)
        {
            return c => c.Keywords.Any(k => k.Value.Contains(keyword));
        }

        public static Predicate IsOwnedBy(string ownerName)
        {
            return c => c.OwnerName.Contains(ownerName);
        }

        public static Predicate HasPartNoContaining(string partNo)
        {
            return c => c.PartNo.Contains(partNo);
        }
    }
}

然后我可以使用它们来构建这样的查询:

var whereComponent = PredicateBuilder.True<Component>();

whereComponent = whereComponent.And(Component.HasKeywordContaining(keyword));
whereComponent = whereComponent.And(Component.IsOwnedBy(ownerName));

var components = from c in db.Components.Where(whereComponent)

当然，这不涉及客户端过滤。