LINQ查询中的空引用

Question

我将尽我所能来表达这句话。 我目前正在尝试获取一个名为Program的值，但是我的LINQ查询遇到了问题

Interactions = new BindableCollection<InteractionDTO>(_client.Interactions.
    Select(x => new InteractionDTO
    {         
       Id = x.Id,
       ClientName = x.Person.CorrespondenceName,
       Indepth = x.Indepth,
       Program = x.Allocations.FirstOrDefault(y => y.Interaction_Id == x.Id).Program.Value,
       Category = x.Allocations.FirstOrDefault().Category.Value,
       ActivityDate = x.ActivityDate,
       Type = x.Type,
       Subject = x.Subject,
       LoanApplicationProvided = x.LoanApplicationProvided,
       BusinessPlanProvided = x.BusinessPlanProvided
    }));

我得到的错误是Object reference not set to an instance of an object.为Object reference not set to an instance of an object.

当我在下面注释掉它时，它起作用但Program未通过。

Program = x.Allocations.FirstOrDefault(y => y.Interaction_Id == x.Id).Program.Value,
Category = x.Allocations.FirstOrDefault().Category.Value

我从LINQ查询中获得的目标是：必须查找交互，然后从InterActionAllocations获取Program / CategoryID ，然后从InteractionPrograms获取“值”。

Program.cs

// Primary Keys -------------------------------------------------------
public int Id { get; set; }

[InverseProperty("Interaction")]
public virtual ICollection<InteractionAllocation> Allocations { get; set; }

InteractionAllocation.cs

// Associations -------------------------------------------------------
public int Interaction_Id { get; set; }

[ForeignKey("Interaction_Id")]
public virtual Interaction Interaction { get; set; }

public int? Category_Id { get; set; }

[ForeignKey("Category_Id")]
public InteractionCategory Category { get; set; }

Answer 1

似乎您的互动中至少有一个没有相应的程序。 您需要子查询来支持这种情况。 一种简单的方法是在调用FirstOrDefault 之前将程序转换为程序值：

Program = x.Allocations.Where(y => y.Interaction_Id == x.Id)
    .Select(y => y.Program.Value)
    .FirstOrDefault(),

Answer 2

我认为解决此问题的最佳方法是将Program和Category存储为中间结果，然后使用条件运算符（ ? ）。 与综合语法中的let关键字一起使用时效果更好：

from i in _client.Interactions
let program = x.Allocations.Where(y => y.Interaction_Id == x.Id)
                           .Select(a => a.Program).FirstOrDefault()
let cat = x.Allocations.Select(a => a.Category).FirstOrDefault()
select new InteractionDTO
    {         
       Id = x.Id,
       ClientName = x.Person.CorrespondenceName,
       Indepth = x.Indepth,
       Program = program == null ? null : program.Value,
       Category = cat == null ? null : cat.Value,
       ActivityDate = x.ActivityDate,
       Type = x.Type,
       Subject = x.Subject,
       LoanApplicationProvided = x.LoanApplicationProvided,
       BusinessPlanProvided = x.BusinessPlanProvided
    }