C#中的where泛型约束

这个关于泛型约束的东西我看了几天了。一直没打看懂，我的领悟能力有点差，刚才突然明白了一点，泛型约束的意思就是说：

类的泛型，只能是where字句后面所写的接口或类。

这么说好像也有点不大明白，举个例子。

我有一个接口，如下：

///<summary>
/// 国籍的接口
///</summary>
public interface INationality
{
    string Nationality
    {
        set;
        get;
    }
    string GetNationality();
}

然后该接口有两个实现，如下：

///<summary>
/// 中国人
///</summary>
public class Chinese : INationality
{
    private string _Nationality;
    public string Nationality
    {
        set
        {
            _Nationality = value;
        }
    }

    public string GetNationality()
    {
        return string.IsNullOrEmpty(_Nationality) ? "Default." : _Nationality;
    }
}

///<summary>
/// 美国人
///</summary>
public class American : INationality
{
    private string _Nationality;
    public string Nationality
    {
        set { _Nationality = value; }
    }

    public string GetNationality()
    {
        return string.IsNullOrEmpty(_Nationality) ? "Default." : _Nationality;
    }
}

然后创建一个泛型类，带有泛型约束的类，如下：

///<summary>
///
///</summary>
///<typeparam name="T"></typeparam>
public class PrintNationality<T> where T : INationality, new()
{
    T item = new T();
    public void Print()
    {
        Console.WriteLine(string.Format("Nationality:{0}", item.GetNationality()));
    }
}

由于有where字句的泛型约束，所以，创建PrintNationality<T>的对象时，T的类型只能是继承子INationality接口的类。

public class Program
{
    static void Main(string[] args)
    {
        PrintNationality<Chinese> _c = new PrintNationality<Chinese>();
        PrintNationality<American> _a = new PrintNationality<American>();
        //PrintNationality<Object> _o = new PrintNationality<Object>(); 此句是错误的，因为泛型类型必须是继承自INationality接口的类
        _c.Print();
        _a.Print();
        Console.ReadKey();
    }
}

以上的代码运行结果：

Nationality:Chinese.
Nationality:American.

http://luacloud.com/2011/where-generic-constraint.html