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.NET Forum / Languages / C# / November 2006passing object to method doesn't update object
I was under the assumption that if you pass an object as a param to a method and inside that method this object is changed, the object will stay changed when returned from the method because the object is a reference type? my code is not proving that. I have a web project i created from a web service that is my object: public class ExcelService : SoapHttpClientProtocol { public ExcelService(); ... In my code in initialize the object varialbe to null and then pass it to a method that instantiates it, but when the method returns the varaible is still null ExcelService es = null; string sessionId = OpenSession(es); es equals null here!!!!! thanks > I was under the assumption that if you pass an object as a param to a method > and inside that method this object is changed, the object will stay changed [quoted text clipped - 16 lines] > > es equals null here!!!!! Yes is does. Your is a classic misunderstanding of the difference between "pass reference by value" and "pass by reference". Read Jon Skeet's article on parameter passing and pay particular attention to that bit: http://www.yoda.arachsys.com/csharp/parameters.html Hi TS, Yes, I agree with Bruce, that you have to pass the parameter by reference instead of pass reference by value. You can try the following when defining your method. OpenSession(ref ExcelService ExcelS) When calling, you can use OpenSession(ref es); HTH. Kevin Yu Microsoft Online Community Support ================================================== Get notification to my posts through email? Please refer to http://msdn.microsoft.com/subscriptions/managednewsgroups/default.aspx#notif ications. Note: The MSDN Managed Newsgroup support offering is for non-urgent issues where an initial response from the community or a Microsoft Support Engineer within 1 business day is acceptable. Please note that each follow up response may take approximately 2 business days as the support professional working with you may need further investigation to reach the most efficient resolution. The offering is not appropriate for situations that require urgent, real-time or phone-based interactions or complex project analysis and dump analysis issues. Issues of this nature are best handled working with a dedicated Microsoft Support Engineer by contacting Microsoft Customer Support Services (CSS) at http://msdn.microsoft.com/subscriptions/support/default.aspx. ================================================== (This posting is provided "AS IS", with no warranties, and confers no rights.) TS, If it is a webservice, then you are not "passing" the actual object, but a proxy xml serialized representation of the object. Expecting a webservice to handle this as if it were a reference type is not feasible. Peter  SignatureCo-founder, Eggheadcafe.com developer portal: http://www.eggheadcafe.com UnBlog: http://petesbloggerama.blogspot.com > I was under the assumption that if you pass an object as a param to a method > and inside that method this object is changed, the object will stay changed [quoted text clipped - 18 lines] > > thanks TS, In my idea are you writing it yourself. (In slightly other words bellow) You are passing a reference to "zero objects" to a method, and when you are changing things in your method those "zero objects" will not be changes. If you want to sent a to be used reference than you have to send that reference and not the "zero objects". Cor >I was under the assumption that if you pass an object as a param to a >method and inside that method this object is changed, the object will stay [quoted text clipped - 19 lines] > > thanks More precisely speaking, a reference object is basically a pointer to the memory location that holds the actual data. When you pass a reference object by value to a function this pointer is copied to a new pointer instance that will only be valid inside the function. Normally this doesn't make much of a difference to passing it by reference, since you will still be making changes to the same data in memory, only through two different pointers (the original one if passed by ref, the copy if passed by value). When you pass a null reference by value, the pointer points to no data yet, so when it is copied you receive a new null pointer. Initializing this pointer won't change the fact that your original pointer still points to nothing, that's why your object is still null after the function returns. If you initialize your object with a new instance first, pass that to the function and alter its properties, you should see the changes you are looking for, although the same can be achieved by simply passing it as a reference. Sincerely, Kevin Wienhold Cor Ligthert [MVP] schrieb: > TS, > [quoted text clipped - 31 lines] > > > > thanks OK, this is the one that did it for me. i understand the error of my thinking. I was remembering what i read in the help system: ms-help://MS.VSCC.2003/MS.MSDNQTR.2003FEB.1033/csref/html/vclrfPassingMethodParameters.htm#vclrfpassingmethodparameters_referencetypes Passing Reference-Type Parameters A variable of a reference type does not contain its data directly; it contains a reference to its data. When you pass a reference-type parameter by value, it is possible to change the data pointed to by the reference, such as the value of a class member. However, you cannot change the value of the reference itself; that is, you cannot use the same reference to allocate memory for a new class and have it persist outside the block. To do that, pass the parameter using the ref (or out) keyword. For simplicity, the following examples use ref. Example 4: Passing Reference Types by Value The following example demonstrates passing a reference-type parameter, myArray, by value, to a method, Change. Because the parameter is a reference to myArray, it is possible to change the values of the array elements. However, the attempt to reassign the parameter to a different memory location only works inside the method and does not affect the original variable, myArray. ... So my original assumption was correct, but as you pointed out, i instantiated my object to null, so the reference i sent was null and you cannot do anything with a null reference. If i first instantiate the object and then pass it by value, changing the object in the method has an effect on the object. Thanks for clearing the fog on that one! TS > More precisely speaking, a reference object is basically a pointer to > the memory location that holds the actual data. When you pass a [quoted text clipped - 58 lines] >> > >> > thanks You assumption is correct in a sense that you can use this variable to access the object: call its methods, access members. But you cannot assign whole another object to this variable. Well, you can, but it won't be visible outside the function. > I was under the assumption that if you pass an object as a param to a method > and inside that method this object is changed, the object will stay changed [quoted text clipped - 18 lines] > > thanks Free MagazinesGet these publications absolutely FREE for up to 12 months. There are no hidden fees and no obligation. Simply choose a title, complete the application form and submit it. Read more ...
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