Difference between the if statement and "?/:" operator

I have this piece of code here. In case ServiceType.Register: I have what appears to be two equalivenat statements one using reguarl "if" statement and one using the triany operator "?/:". The "if" statement VS reports no error. However with this line:

IsXML == true ? PopulateRegister(ParseType.Xml) : PopulateRegister(ParseType.Str);

VS is erroring out saying:

Only assignment, call, increment, decrement, await, and new object expressions can be used as a statement

Anyone know why regular if statement has no error but if you use "?/:" (one liner) throws an error? Pic is also attached.

 switch (this.ServiceType)
        {
            case SerivceType.Login:
                PopulateLogin();
                break;
            case SerivceType.Register:

                if (IsXML == true)
                    PopulateRegister(ParseType.Xml);
                else
                    PopulateRegister(ParseType.Str);

                IsXML == true ? PopulateRegister(ParseType.Xml) : PopulateRegister(ParseType.Str);

                break;
            case SerivceType.Verify:
                PopulateVerify();
                break;

        }