Question

Possible Duplicate:
Ternary Expression with Interfaces as a Base Class

Classes:

interface ISms {
  void f_SendSms();
}

class SmsVodafone : ISms {
  public void f_SendSms(){
        // ...
  }
}

class SmsClickatell : ISms {
  public void f_SendSms(){
        // ...
  }
}

This works:

ISms sms = null;
if (string.IsNullOrEmpty(_bilgi.M_Originator))
{
    sms = new SmsVodafone();
}
else
{
    sms = new SmsClickatell();
}

This works too:

ISms sms = null;
sms = string.IsNullOrEmpty(_bilgi.M_Originator)
        ? (ISms) new SmsVodafone()
        : new SmsClickatell();

This doesn’t work:

ISms sms = null;
sms = string.IsNullOrEmpty(_bilgi.M_Originator)
          ? new SmsVodafone()
          : new SmsClickatell();

Why?

Answer 1

In a ternary expression a ? b : c, both expressions b and c must be convertible to the same type, which must be one of b‘s type or c‘s type.

SmsVodafone is not convertible to SmsClickatell, and SmsClickatell is not convertible to SmsVodafone. That’s why you get an error.