关于C#：属性里的??

What is ?? in my Property?

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Possible Duplicate:
What do two question marks together mean in C#?

任何人都能解释这种语法吗？

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protected string CompanyProductSeriesId
{
get
{
return Request.QueryString["CPGId"]
?? (ViewState["CPSId"] == null
?""
: ViewState["CPGId"].ToString());
}
}

我想站在下面？？在这种语法中。

??is the空coalesce社。P＞＜／

它返回第一操作数from the left the that is not null。P＞＜／

A = B ?? C

A = C if B == NULL

A = B if B is not NULL

执行sraightforward below is property of CompanyProductSeriesId吸气剂，我是自explained：恩P＞＜／

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string returnValue;

if (Request.QueryString["CPGId"] != null)
{
returnValue = Request.QueryString["CPGId"];
}
else
{
if (ViewState["CPSId"] == null)
{
returnValue ="";
}
else
{
returnValue = ViewState["CPGId"].ToString());
}
}

return returnValue;

？is called？空- coalescing社，从MSDNP＞＜／

"The ?? operator is called the null-coalescing operator and is used to
define a default value for nullable value types or reference types. It
returns the left-hand operand if the operand is not null; otherwise it
returns the right operand."

does that help？P＞＜／

？可空类型的作品？操作者只在线类int？DateTime，？等。P＞＜／

实例：P＞＜／

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int? a = 5;
int b = a ?? 0; // out put will be 5

int? c = null;
int d = c ?? 0; // output will be 0;

在这房子，它会返回Request.QueryString["CPGId"]if that is，or，它将返回null，(ViewState["CPSId"] == null ?"" : ViewState["CPGId"].ToString())。P＞＜／