Entity Framework使我们的编程更加容易。比如，它集成了数据验证方法，使我们可以轻松地实现数据验证。当一个类通过EF和数据库映射时，EF为该类中的每一个类成员都提供了一个"On..Changing"局部方法。通过实现这个局部方法，就可以实现数据验证。
例如，一个名字为"MyClass"的类，对应的数据库表有"MyField"字段：
public partial class MyClass{...}
EF 自动产生局部类如下:
public partial class MyClass : global::System.Data.Objects.DataClasses.EntityObject
{
public string MyField;
...
}
在这个类当中，有如下定义：
partial void OnMyFieldChanging(string value);
这个局部方法OnMyFieldChanging() 没有代码实现。到这里，你可能也会猜出来，这个局部方法就是我们谈论的关键。利用它实现数据验证，在你自己定义的局部类中实现这个局部函数，例如：
public partial class MyClass
{
...
private partial void OnMyFieldChanging(string value)
{
if (!string.IsNullOrEmpty(value.Trim()))
throw new Exception("MyField cannot be blank.");
}
}
Entity Framework makes daily life easier. For data validation, EF has a easy way for us. When we have a class that maps to database via EF, EF generates a partial class which descendents from EntityObject for the class. Within the EntityObject class, for every property, there is a partial method with a name combined with the property name and "On..Changing". This partial method is for you to do your validation.
For example, you have a class with name of "MyClass". It maps to a table with a field with name of "MyField".
public partial class MyClass{...}
EF generates:
public partial class MyClass : global::System.Data.Objects.DataClasses.EntityObject
{
public string MyField;
...
}
In this class, you will see this:
partial void OnMyFieldChanging(string value);
This OnMyFieldChanging() method is a partial method and is not implemented.
To here, you may have guessed what I am going to tell you: use this method to do the validation.
For example:
public partial class MyClass
{
...
private partial void OnMyFieldChanging(string value)
{
if (!string.IsNullOrEmpty(value.Trim()))
throw new Exception("MyField cannot be blank.");
}
}