I think you're missing a "=":

string PagerDomain = @ConfigurationSettings.AppSettings["PagerDomain"];
if ((PagerDomain == null) || PagerDomain.Length <=0 )

>Could someone explain to me why the following expression works:
>
>

>string PagerDomain = ConfigurationSettings.AppSettings"PagerDomain"];
>if ((PagerDomain != null) && PagerDomain.Length >0 )
>

>
>But this one gives the error:
>

>string PagerDomain =
>@ConfigurationSettings.AppSettings["PagerDomain"];
>if ((PagerDomain = null) || PagerDomain.Length <=0 )
>

>
>The error I get is "Operatore || cannot be applied to operand of type 'string' and 'bool'.