在通用可空T上实施使用 [英] On Generic Nullable<T> implementation use

问题描述

我正在通过考试参考70-483:用C#编程"进行操作.由Wouter de Kort撰写的"Generic Nullable< T"示例是"Generic Nullable< T"示例.实施" (清单2-13).我对如何使用–使用–结构感到困惑.以下 是那个清单.任何点击-技巧-资源指示都将不胜感激.

struct Nullable< T>其中T:struct
{
私人布尔hasValue;
私有T值；
公共Nullable(T值)
{
this.hasValue = true;
this.value = value;
}

公共布尔HasValue {获取{return this.hasValue; }}

公共T值
{
get
{
如果(！this.HasValue)抛出新的ArgumentException();
返回this.value;
}
}

public T GetValueOrDefault()
{
返回this.value;
}
}

I am working through "Exam Ref 70-483: Programming in C#" by Wouter de Kort and in the generics section is an example "Generic Nullable<T> implementation" (Listing 2-13). I am confused as to how to consume – use – the struct. Below is that listing. Any hits – tips – directions to resources, would be greatly appreciated.

struct Nullable<T> where T : struct
{
private bool hasValue;
private T value;
public Nullable(T value)
{
this.hasValue = true;
this.value = value;
}

public bool HasValue { get { return this.hasValue; } }

public T Value
{
get
{
if (!this.HasValue) throw new ArgumentException();
return this.value;
}
}

public T GetValueOrDefault()
{
return this.value;
}
}

谢谢，

山姆

推荐答案

Nullable< T>是系统类型.您不会在C#中直接使用它.相反，您只是使用T?其中T是任何值类型.这是Nullable T的别名.因此诠释?变为Nullable< int>和DateTime?变为Nullable< DateTime>.

Nullable<T> is a system type. You don't use it directly in C#. Instead you simply use T? where T is any value type. This is an alias for Nullable<T>. Hence int? becomes Nullable<int> and DateTime? becomes Nullable<DateTime>.

主要用法是从数据库中检索可为空的数据，但它也用于指示可选值.例如，您可能有一个存储在数据库中的Employee，而他们可能对主管有FK.但是，并不是每个员工都可以有一名主管. 因此，数据库将允许FK为空.如果您想映射到该表，则可以执行以下操作.

The primary usage is for retrieving nullable data from the database but it is also used to indicate optional values. As an example you might have an Employee stored in a database and they may have a FK to a supervisor. But not every employee may have a supervisor. So the database would allow a null for the FK. If you wanted to map to this table you'd do something like this.

public class Employee
{
   public string Name { get; set; }
   public int Id { get; set; }

   public int? SupervisorId { get; set; }
}

//Later when wanting to use the value
var employee = new Employee();
...

//Using Value will throw exception if HasValue is false
if (employee.Supervisor.Id.HasValue)
   GetSupervisor(employee.SupervisorId.Value);

//Alternative approach
GetSupervisor(employee.SupervisorId.GetValueOrDefault());

另一个示例是可选参数，但我倾向于不同意这种方法.

Another example is for optional parameters but I tend to disagree with this approach.

void Foo ( string message, int? optionalValue = null )
{
   if (optionalValue.HasValue)
      //Use optional value
}

迈克尔·泰勒
http://www.michaeltaylorp3.net

Michael Taylor
http://www.michaeltaylorp3.net

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