C# Nullable 类型

Nullable types are one of those great little inventions to help you with the impedance mismatch between object-oriented applications and relational databases.  Let's take a fake Order Class that maps to an Orders Table in your database.  The class might look like this:

 

Order Class

using System;
using System.Collections;

public class Order
{
    private int _id;
    private string _number = string.Empty;
    private decimal _amount;
    private DateTime _shipDate;
    
    public int ID
    {
        get { return _id;; }
    }
    
    public string Number
    {
        get { return _number; }
        set { _number = value; }
    }
    
    public decimal Amount
    {
        get { return _amount; }
        set { _amount = value; }
    }
    
    public DateTime ShipDate
    {
        get { return _shipDate; }
        set { _shipDate = value; }
    }
    
    public Order() {}
}

Now if an order has not shipped, the ShipDate for the order will be null in the Orders Table and you would expect the ShipDate to be null (Nothing in VB) in the Order object as well.  However, DateTime is a struct (value type) and cannot be null.  If not assigned, ShipDate will default to System.DateTime.MinValue.  This is the mismatch.

We can get around this in C# 2.0 by making _shipDate a nullable DateTime type.  You can make it a nullable type by declaring it either:

• Nullable<DateTime> _shipDate;
• DateTime? _shipDate;

with the Nullable structure being defined as follows:

 

Nullable Types

struct Nullable<T>
{
    public bool HasValue;
    public T Value;
}

Now the new class is defined as follows:

 

Order Class With Nullable ShipDate

using System;
using System.Collections;

public class Order
{
    private int _id;
    private string _number = string.Empty;
    private decimal _amount;
    private DateTime? _shipDate;
    
    public int ID
    {
        get { return _id;; }
    }
    
    public string Number
    {
        get { return _number; }
        set { _number = value; }
    }
    
    public decimal Amount
    {
        get { return _amount; }
        set { _amount = value; }
    }
    
    public DateTime? ShipDate
    {
        get { return _shipDate; }
        set { _shipDate = value; }
    }
    
    public Order() {}
}

Checking For Null

If _shipDate is not assigned now, it will have a null value and you can determine if it has been assigned by the following statements:

if (order.ShipDate != null) ...

or

if (order.ShipDate.HasValue) ...

 

InvalidOperationException

If _shipDate has not been assigned and you try to extract the DateTime value from it, you will get an InvalidOperationException:

DateTime shipDate = Order.ShipDate.Value; // throws exception if not assigned

 

C# Nullable Types will play a big role in defining business objects and data transfer objects and how they map to relational databases.