外部联接中的内部异常

Posted 2023-02-24

【中文标题】外部联接中的内部异常

【英文标题】：Inner Exception in outer Join

【发布时间】：2011-12-22 10:23:45

【问题描述】：

我不知道为什么我的程序不起作用。请问你能帮帮我吗 ！当我运行它时，我有一个内部异常。

  var peoples = new List<People>();
   peoples.Add(new People  Model = "Mustang", FirstName = "Joe", LastName = "L'indien" );
   peoples.Add(new People  Model = "Corvet", FirstName = "Luc", LastName = "Emince" );
   peoples.Add(new People  Model = "Verron", FirstName = "pierre", LastName = "Uneed" );
   peoples.Add(new People  Model = "Viper", FirstName = "Martial", LastName = "Roste" );
   peoples.Add(new People  Model = "Camaro", FirstName = "Franck", LastName = "Vador" );

   var cars =  this._cars = new List<Car>();
   cars.Add(new Car  Model = "Mustang", Description = "Gros paté" );
   cars.Add(new Car  Model = "Mustang", Description = "Paté" );
   cars.Add(new Car  Model = "Verron", Description = "Très gros paté" );
   cars.Add(new Car  Model = "Viper", Description = "Attention paté" );
   cars.Add(new Car  Model = "Viper", Description = "Attention paté" );

    var PeopleCar = from people in peoples
                     join car in cars on people.Model equals car.Model into outer
                     from car in outer.DefaultIfEmpty()
                     select new
                     
                           people.FirstName,
                           people.LastName,                                 
                           car.Model,
                           Desc = (people == null) ? "CASSE DE MERDE" : car.Description
                      ;

  foreach (var item in PeopleCar)
  
         textBox1.AppendText(String.Format("Mr 0 1 a une voiture model 2 qui envoie du 3"
                + Environment.NewLine, item.FirstName, item.LastName, item.Model, item.Desc));
   

【问题讨论】：

欢迎来到 ***。你能给我们例外文本吗？是哪个例外？

不是文本 - 完全例外。

【参考方案1】：

在您的选择中定义字段名称：

               select new
               
                  FirstName = people.FirstName,
                  LastName = people.LastName,
                  Model = car.Model,
                  Desc = (people == null) ? "CASSE DE MERDE" : car.Description
                ;

【参考方案2】：

因为您正在执行外部连接，所以 car 可以是 null - 您应该在 select 中防范它：

           select new
           
              FirstName = people.FirstName,
              LastName = people.LastName,
              Model = car == null ? null ; car.Model,
              Desc = car == null ? null : car.Description
            ;

另外，你没有问，但你可以使用集合初始化语法：

var peoples = new List<People>()

    new People  Model = "Mustang", FirstName = "Joe", LastName = "L'indien" ),
    new People  Model = "Corvet", FirstName = "Luc", LastName = "Emince" ),
    new People  Model = "Verron", FirstName = "pierre", LastName = "Uneed" ),
    ...
;

【讨论】：

people 的空保护要么不完整，要么毫无意义。