clr via c# 可空值类型

1,在CLR中引入了可空值类型结构 System.Nullable<T>-----注意这个是值类型(struct).

  注解:[System.Runtime.InteropServices.ComVisible(true)]---控制member对于Com的可访问性.意思是是否可以由COm来访问.

  注解:system.SerializableAttribute----表明对象是否可以序列化.

[System.Runtime.InteropServices.ComVisible(true)]
[System.AttributeUsage(System.AttributeTargets.Class | System.AttributeTargets.Delegate | System.AttributeTargets.Enum | System.AttributeTargets.Struct, Inherited=false)]
public sealed class SerializableAttribute : Attribute

   Serializable---举例

  1.1创建类及相关的方法

 [Serializable]
    public class SerializeTest
    {
        public int member1;
        public string member2;
        public string member3;
        public double member4;

        // A field that is not serialized.
        [NonSerialized()] public string member5;

        public SerializeTest()
        {

            member1 = 11;
            member2 = "hello";
            member3 = "hello";
            member4 = 3.14159265;
            member5 = "hello world!";
        }

        public void Print()
        {

            Console.WriteLine("member1 = '{0}'", member1);
            Console.WriteLine("member2 = '{0}'", member2);
            Console.WriteLine("member3 = '{0}'", member3);
            Console.WriteLine("member4 = '{0}'", member4);
            Console.WriteLine("member5 = '{0}'", member5);
        }

1.2 进行测试代码

使用Soap.SoapFormatter,进行序列化和反序列化一个类的实列.

 public static void Go()
        {
            // Creates a new SerializeTest object.
            SerializeTest obj = new SerializeTest();

            Console.WriteLine("Before serialization the object contains: ");
            obj.Print();

            // Opens a file and serializes the object into it in binary format.
            Stream stream = File.Open("data.xml", FileMode.Create);
            SoapFormatter formatter = new SoapFormatter();

            //BinaryFormatter formatter = new BinaryFormatter();

            formatter.Serialize(stream, obj);
            stream.Close();

            // Empties obj.
            obj = null;

            // Opens file "data.xml" and deserializes the object from it.
            stream = File.Open("data.xml", FileMode.Open);
            formatter = new SoapFormatter();

            //formatter = new BinaryFormatter();

            obj = (SerializeTest)formatter.Deserialize(stream);
            stream.Close();

            Console.WriteLine("");
            Console.WriteLine("After deserialization the object contains: ");
            obj.Print();
        }

查看 data.xml文件

<SOAP-ENV:Envelope xmlns:xsi="http://www.w3.org/2001/XMLSchema-instance" xmlns:xsd="http://www.w3.org/2001/XMLSchema" xmlns:SOAP-ENC="http://schemas.xmlsoap.org/soap/encoding/" xmlns:SOAP-ENV="http://schemas.xmlsoap.org/soap/envelope/" xmlns:clr="http://schemas.microsoft.com/soap/encoding/clr/1.0" SOAP-ENV:encodingStyle="http://schemas.xmlsoap.org/soap/encoding/">
<SOAP-ENV:Body>
<a1:SerializeTest id="ref-1" xmlns:a1="http://schemas.microsoft.com/clr/nsassem/ClrFromCSharp_2_2.LearnAttribute/ClrFromCSharp_2_2%2C%20Version%3D1.0.0.0%2C%20Culture%3Dneutral%2C%20PublicKeyToken%3D11fccde6e91ad1e9">
<member1>11</member1>
<member2 id="ref-3">hello</member2>
<member3 href="#ref-3"/>
<member4>3.14159265</member4>
</a1:SerializeTest>
</SOAP-ENV:Body>
</SOAP-ENV:Envelope>

查看结果--member5没有值.

Before serialization the object contains:
member1 = '11'
member2 = 'hello'
member3 = 'hello'
member4 = '3.14159265'
member5 = 'hello world!'

After deserialization the object contains:
member1 = '11'
member2 = 'hello'
member3 = 'hello'
member4 = '3.14159265'
member5 = ''

2,Nullrable<T>结构的本质

基本属性

Nullable<T> 结构的两个基本成员是 HasValue 和 Value 属性。 如果 Nullable<T> 对象的 HasValue 属性为 true，则可以使用 Value 属性访问对象的值。 如果 falseHasValue 属性，则对象的值是不确定的，尝试访问 Value 属性将引发 InvalidOperationException。

装箱和拆箱

• 装箱:如果HasValue=True ,则装箱Null,否则,装箱其基础值<T>,而非Nullrable<T>.
• 拆箱:如果HasValue=True,则将其基础值返回,否则返回null;

3,空接合操作符号??

string filename=GetFileName()??"Untitled";//如果前面的方法是空,则赋值后面的值.

4,可空值调用GetType(),返回的是<T>的类型,而不是Null able<T>.