check to see if value is null or not

How can I rewrite the condition below to determine if one or the other string is null or not?  If both are null or empty throw an error message My first option is to use userEmailAddress but if it is null User SubBusEmailaddress.... please help Codes if (x.UserEmailAddress != string.Empty) {     UserEmailAddress = x.UserEmailAddress; }    else if (x.SubBusEmailAddress != string.Empty) {     UserEmailAddress = x.SubBusEmailAddress; }