StreamReader.ReadLine（）出现意外结果

Question

I am currently testing an application that stores the user's contact details in a file. This information is also stored in a local compact database as the primary method - storing their details in this file is a backup in case those details are lost.

The file I'm using for testing has my personal data in it, so I hope you understand that I have replaced the lines with placeholders! The structure of this file is as follows (minus the first line):

File: 


Business Name
Mr.
Joe
Bloggs
user@email.com

Address Line 1
Address Line 2
City
Postcode
Country        




07777123456

Below, I have some code that reads this file and stores each line as a variable. The structure of the file will never change, hence the very simple code:

public static bool RestoreBusinessTable(out string title, out string busName, out string mobileNumber, out string firstName, out string lastName)
        {    
            string email = "", referral = "", contactNo, addressLine1 = "", addressLine2 = "", city = "", postcode = "", country = "", district = "";
            busName = null;
            mobileNumber = null;
            firstName = null;
            lastName = null;
            title = null;

            try
            {
                if (!File.Exists(fileName))
                    return false;
                StreamReader sr = new StreamReader(fileName);
                string work;
                work = sr.ReadLine();   // Empty line
                work = sr.ReadLine();   // Empty line
                busName = sr.ReadLine();
                title = sr.ReadLine();
                firstName = sr.ReadLine();
                lastName = sr.ReadLine();
                email = sr.ReadLine();
                referral = sr.ReadLine();
                addressLine1 = sr.ReadLine();
                addressLine2 = sr.ReadLine();
                city = sr.ReadLine();
                postcode = sr.ReadLine();
                country = sr.ReadLine();
                work = sr.ReadLine();  // Empty line
                work = sr.ReadLine();   // Empty line
                contactNo = sr.ReadLine();
                district = sr.ReadLine();
                mobileNumber = sr.ReadLine();
                sr.Close();
                // Add to database here
                return true;
            }
            catch
            {
                return false;
            }
        }

When running this code, I noticed that busName , title , firstName , and lastName all had a value of 07777123456 . The data looked like this:

07777123456
07777123456
07777123456
07777123456
user@email.com

Address Line 1
Address Line 2
City
Postcode
Country




07777123456

I do not have any asynchronous processes or threads which are writing to the file at the same time. Could anyone shed any light as to what is happening here, and why the first four lines would appear as the mobile number?

Answer 1

One way to do that would be for the calling code to supply the same variable's/field's address to the various out parameters:

string tmp;
RestoreBusinessTable(out tmp, out tmp, out tmp, ...);

Here the same address is passed in each location, so no matter whether your code assigns to title , busName , etc it is writing to the same actual location .

Since mobileNumber is assigned last, the value assigned for the mobile number will be the value that appears for all the values.

The key point here is that title , busName , etc are not each a reference to a string - because of the out (or ref , equally) they are each a reference to a reference to a string .