如何在实体框架中具有映射到可空数据库列的值类型？

Question

我有以下实体模型:

public class Todo  
{  
    [Required]  
    public int ID { get; set; }  
    public int OrderId { get; set; } //Not required  
    public string Description { get; set; }  
    public bool Finished { get; set; }  
    public DateTime CreationDate { get; set; }  
    public int Priority { get; set; } //Not required  
    public string CreatedBy { get; set; }  
    public bool Deleted { get; set; }  
}

在相应的数据库表中,所有字段都创建为"not null".我想允许一些字段为空.我该怎么做呢？

Answer 1

在数据库端,您必须更改要作为可选字段,以便它们可以为null.ALTER TABLE语句可以解决这个问题.

ALTER TABLE Todo
ALTER COLUMN OrderId int NULL

ALTER TABLE Todo
ALTER COLUMN Priority int NULL

在应用程序方面,您需要使用可空类型.试试这个:

public class Todo
{
    [Required]
    public int ID { get; set; }
    public int? OrderId { get; set; } //Not required
    public string Description { get; set; }
    public bool Finished { get; set; }
    public DateTime CreationDate { get; set; }
    public int? Priority { get; set; } //Not required
    public string CreatedBy { get; set; }
    public bool Deleted { get; set; }
}

可空类型是常规值类型的变体,区别在于它可以为null.在您的代码中,您可以使用HasValue属性测试null :

int? foo= 42;
Console.WriteLine(foo.HasValue); // prints True
Console.WriteLine(foo.Value); // prints 42
int? bar = null;
Console.WriteLine(bar.HasValue); // prints False
Console.WriteLine(bar.Value); // throws InvalidOperationException

该类型的所有运算符都被提升,这意味着您仍然可以使用它们进行算术运算:

int? foo = 23;
int? bar = 17;
int? foobar = foo + bar;
Console.WriteLine(foobar); // Prints 40
int? baz = null;
int? foobaz = foo + baz + bar; // If any of them is null, the result will be null.
Console.WriteLine(foobaz); // Prints null