Possible Duplicate:
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.
?? is the null-coalesce operator.
It returns the first operand from the left that is not null.
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;
?? is called null-coalescing operator, From MSDN -
"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?
In this case, it will return Request.QueryString["CPGId"], or, if that is null, it will return (ViewState["CPSId"] == null ? "" : ViewState["CPGId"].ToString()).
?? Operater works only on Nullable Datatype like int?, DateTime? etc.
Example :
int? a = 5;
int b = a ?? 0; // out put will be 5
int? c = null;
int d = c ?? 0; // output will be 0;
