Syntax meaning

Question

Hi Given the following code example. public string AddSomethingMethod(string templateName) return GetString(new object[] { templateName }); From what I know object[] is an array of objects. However the reference being passed in is of type string (singular) and not an array of strings. The GetString method accepts a parameter of params object[] args. And the logic does the following:- for(int i = 0; i string str1 = args[i] as string; if(str1 != null) || str1.Length > 0x400 args[i] = str1.Substring(0 0x3fd) + ...; return args; From this I gather that the string being passed in is broken down into chars hence why the object array is used? Regards Steven

Vulpes · Answer

Well taking this step by step the AddSomethingMethod returns a string. The GetString method sounds like it returns a string so it sounds OK so far. As string is implicitly convertible to object (all types are) then passing GetString an array of objects (args) with a single string element is no problem either. The code in the GetString method then seems to be trimming each string in args so that it doesn't exceed 1024 characters in length. However there are two things I don't understand: 1. Like you I don't understand why the parameter to GetString is an object[] rather than a string[] because the snippet you've shown will only work for strings anyway and so a cast is needed. 2. I don't understand the line : return args; The return type of GetString is evidently string rather than object[] and there's no need to return 'args' anyway because (as it's a reference type) any changes made to it will automatically be persisted to the calling code in any case.

Vulpes · Answer

Yes that expression creates a new object and passes it to the GetString method but as it's not been assigned to a variable the caller will have no access to that object when the method returns unless it's used as the return value itself.

S · Answer

Understood thanks kindly Vulpes. And in this case where the expression new object[] { templateName } is being used instead of a reference variable then a completely new object\instance reference will be created? thus why returning args in my original code snippet would make sense as there is no direct reference between the two object\instance variables anymore?

Vulpes · Answer

I'm talking there about passing an object (i.e. a reference type) by value. In that case it's the reference that gets copied not the object itself. Suppose you do this using a variable rather than an expression. Then both the parameter and the original variable will refer to the same object. So if you've made any changes to that object within the method those changes will also apply to the object referred to by the original variable as it's the same object.

S · Answer

Vulpes Could you slightly elaborate on this older post please... Now if you had passed an object variable to GetString then that variable would still point to the same object as it did before the method was called inclusive of the changes. This I assume refers to passing a value type object by value? Regards

Vulpes · Answer

That's right Steven the string parameter is passed by value. In fact you can always tell immediately in C# whether a parameter is being passed by reference or not because if it's the latter then it must be preceded by 'ref' (or 'out') at the call site as well as within the method's signature. Also for a parameter to be passed by reference you must use a variable - you can't use a literal.

S · Answer

Vulpes One last ask on this topic if I may please:- Given a generic statement like the following:- List string1 = GetInitializedList (SomeName 1); Which category for Passing Parameters would this fall under? as the SomeName value has been explicitly entered when the GetInitializedList method is invoked. Given the SomeName value of type string (reference-type) is passed then it would be reference-type by value? Thanks

S · Answer

Perfect thanks kindly Vulpes case closed : )

Vulpes · Answer

I'll just repeat all that with a correction and some amplifcations highlighted: All arguments in C# are passed by value unless otherwise stated with the ref or out modifiers. Passing a value-type When these type of arguments are passed by value a copy of the value is created when passed to the method. Passing a reference-type When these type of arguments are passed  by value a copy of the reference is created and not the actual value. Therefore any changes made in the called method are reflected in the calling method. Passing a value-type with the ref modifier declared Changing a value in the called method changes the value in the calling method. Passing a reference-type with the ref modifier declared Not only are any changes to the object in the called method persisted in the calling method but you can even change to a different object entirely. Difference between out and ref Out is the same as ref except that the variable passed in need not have been assigned to at that time but the method guarantees that it will have been assigned to by the time the method returns .

S · Answer

Hi Vulpes Just for my understanding and\or clarification:- All arguements in C# are passed by value unless otherwise stated with the ref modifier. Passing by value-type When these type of arguements are passed by value a copy of the value is created when passed to the method. Passing a reference-type When these type of arguements are passed  by value a copy of the reference is created and not the actual value. Therefore any changes made in the called method are not reflected in the calling method. With the ref modifier declared Changing a value in the called method changes the value in the calling method. With regards to the AddSomethingMethod I changed the Return type to be void. I used a try\catch for when attempting to add the templateName parameter to a collection. and finally used a throw new Exception passing in the GetString method which takes the new object[] { templateName } expression. The GetString method has a Return type of string and returns a string.Format which has the received args parameter reference passed to it. Regards Steven

Vulpes · Answer

When you pass an object 'by value' to a method then the parameter receives a reference to the object not a copy of it. So if any changes are made to the object's state within the method then these changes will still be there when the method returns. Now if you had passed an object variable to GetString then that variable would still point to the same object as it did before the method was called inclusive of the changes. However it's just dawned on me that we're not in fact passing an object variable here but the object array created by the constructor expression: new object[] { templateName } Whilst this object array will have been changed there will be no way to access it or the changes from the calling code. As the method doesn't know how it's going to be called it might therefore make sense to return 'args' to the caller. However it still doesn't explain how the return type is (apparently) string but an object array is being returned instead :-/

S · Answer

Thanks Vulpes. Ill look into it further. Could you elaborate on the following please:-  there's no need to return 'args' anyway because (as it's a reference type) any changes made to it will automatically be persisted to the calling code in any case Regards