Query on Optional Parameter

Question

In Wrox book I found the following statement and example for optional parameter the caller must provide a value for the first parameter but can exclude the initial and last. However if the caller provides a value for last it must also provide a value for the initial. If a value for the initial is passed a value for the last name is still optional however. public void displayName(string first string initial =  string last = ) { Console.WriteLine(first +   + initial +   + last); } So I wrote the example to test the same. public string printFullName(string firstNamestring middleName=string lastName=) { return FullName =>+firstName + middleName + lastName; } My caller is the below code string fullName = p.printFullName(firstName:FirstNamelastName:LastName); Console.WriteLine(Testing Named Parameter: +fullName); As per the above statement from the book if I pass the lastName and not the middleName the compiler has to throw an error. But I am getting the output as FirstNameLastName without any error. Am I understanding anything wrong here. I thought it this way as well. Bcos there is a default value for middlename it doesnt throw the error. but I think providing default value for the parameter represents the optional parameter. Is it not? If so what is the meaning of the statement from the book above? Note: I tried giving just firstName and middleName without lastName then also the system outputs the message. But as per the statement from the book its acceptable. Thanks Geetha

Vulpes · Answer

If you ignore the fact that you can use 'named' parameters then what it says in the book is in fact correct. In other words this isn't allowed: displayName(John  Smith); but this is: displayName(John  Smith); and so is this: displayName(John D); I remember the C# design team saying when optional parameters were introduced that they didn't want to allow 'comma counting' code' such as you see in COM interop using VB.NET. If therefore you don't want to have to specify the 'initial' parameter you have to use a named parameter for the 'last' argument: displayName(John last: Smith);

Jignesh Trivedi · Answer

hi Optional parameter has a default value as a part of its definition so when you are not passing any value it will take default value. In your case only firstname is compulsory middle name and last name is optional parameter. so if you are not passing either of middle name or last name function will take its default value. I think this is correct behavior of optional parameter http://msdn.microsoft.com/en-us/library/dd264739.aspx Hope this will help you.