//A.比如下面一句，直接定義int為null是錯(cuò)誤的，錯(cuò)誤提示為無(wú)法將null轉(zhuǎn)化成int，因?yàn)楹笳呤遣豢梢詾閚ull的值類型。
private int getNum = null;

//B.如果修改為下面的寫法就可以初始指為null，在特定情況下?等同于基礎(chǔ)類型為Nullable。
private int? getNum = null;
private Nullable<int> getNumNull = null;

string strParam =Request.Params["para"];  
if ( strParam== null )  
 {  
    strParam= "";  
 }

string strParam=Request.Params["para"] == null ? "":Request.Params["para"];

int? x = null;//定義可空類型變量
int? y = x ?? 1000;//使用合并運(yùn)算符，當(dāng)變量x為null時(shí)，預(yù)設(shè)賦值1000
Console.WriteLine(y.ToString()); //1000

//A.定義getNum為null，輸出結(jié)果為0
private int? getNum = null;
Console.WriteLine(getNum ?? 0);

//B.定義getNum為1，輸出結(jié)果為1
 private int getNum = 1;
 Console.WriteLine(getNum ?? 0);

void Main()
{

    //首先我們定義全局變量Person對(duì)象,有可能p為null的情況下取值。
    Person p = null;
    string telePhoneNo = string.Empty;  //定義telePhoneNo 

    //取Person的telePhoneNo 
    //A.初始寫法，if條件判斷
    if (p != null)
    {
        if (p.Secret != null)
        {
            telePhoneNo = p.Secret.TelePhoneNo;
        }
    }

    //B.三元運(yùn)算符寫法，單問(wèn)號(hào)?
    telePhoneNo = p != null ? (p.Secret != null ? p.Secret.TelePhoneNo : "") : "";

    //C. NULL檢查運(yùn)算符?.
    telePhoneNo = p?.Secret?.TelePhoneNo ?? "";
    Console.Write(telePhoneNo);
}

public class SecretByMySelf
{
    public string Password { get; set; }
    public string TelePhoneNo { get; set; }
}

public class Person
{
    public string Name { get; set; }
    public SecretByMySelf Secret { get; set; }
}

int? count = customers?[0]?.Orders?.Count(); 
 // null if customers, the first customer, or Orders is null