C# 泛型 引用类型约束 值类型约束

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace referenceConstraint
{
class Myclass { }

class GenClass<T> where T : class
{
T field;
public GenClass()
{
field = null;
}
}

class Program
{
static void Main(string[] args)
{
GenClass<Myclass> g = new GenClass<Myclass>();
Console.ReadLine();

}
}
}

using System;
using System.Collections.Generic;
using System.Linq;
using System.Text;

namespace referenceConstraint
{
class Myclass { }

class GenClass<T> where T : struct
{
T field;
public GenClass(T fld)
{
field = fld;
}
}

class Program
{
static void Main(string[] args)
{
GenClass<int> g = new GenClass<int>(0);
Console.ReadLine();

}
}
}