Question

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What do two question marks together mean in C#?

can any one explain this syntax.

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

I want to under stand the ?? in this syntax.

Answer 1

A = B ?? C

A = C if B == NULL

A = B if B is not NULL

Below is sraightforward implementation of CompanyProductSeriesId property getter,
I believe it is self-explained:

string returnValue;

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

return returnValue;