这行代码在做什么？ （编辑 .txt 文件中的一行 - ASP.NET MVC / C#）

Question

Below i have an action result that is suppose to perform a delete function on a specific line within a text file when a condition is met in order to execute a particular if statement. Right now i believe i understand the majority of the lines and their specific functions, however i am unsure about a specific line and variable being created.

Here is the ActionResult receiving an integer parameter containing a ID value:

1. public ActionResult DeleteItemLine(int id) //recieve id of the delete button clicked
2.         {
3.             string strFilePath = "~/App_Data/item.txt"; //fetch file path (access file)
4.             string strSearchText = id.ToString(); //assign the passed id to 'strSearchText' and convert to string
5.             string strOldText; //create string variable 'strOldText' This will be the string responsible for holding each line within the text file at a particular time
6.             
7.             string n = ""; //NOT SURE WHAT THE PURPOSE OF THIS IS
8.             StreamReader sr = System.IO.File.OpenText(Server.MapPath(strFilePath)); //open the text file specified by 'strFilePath'
9.             while ((strOldText = sr.ReadLine()) != null) //run through entire text file, line-by-line, until the last line is reached (a null is encountered)
10.             {
11.
12.                 string[] x = strOldText.Split(','); //create an array 'x' of type string, make this array split each word within a line, in the text file, when a comma is encountered
13. 
14.                 if (!x[1].Contains(strSearchText)) //if the Primary Key within the item matches the id of the edit button pressed (strSearchText), then execute the following code
15.                 {
16.                     n += strOldText + Environment.NewLine; //NOT SURE HOW THIS WORKS
17.                 }
18.             }
19.             sr.Close(); //closes the StreamReader
20.             System.IO.File.WriteAllText(Server.MapPath(strFilePath), n); //writes the updated text file to the specified directory containing the text file originally opened and read
21. 
22.             return RedirectToAction("Index"); //not important right now
23.         }

I am struggling to understand what the purpose of the variable 'n' is, and why it is assigned "". (Line 7), and then also i am not sure how the delete function works within the if statement on line 16. I have no idea why n is being incremented with strOldText.

If anyone is able to explain this to me, i would greatly appreciate it. Thank you so much!

Answer 1

You misunderstand line 14. The exclamation mark means 'not' . So the while loop iterates over every line of the input file, adding it to the output except if the second field of the line maches the search string. So it is effectively removing lines by not adding them to the output.

Line 7 is just initializing the variable that holds the output with an empty string. This is to prevent NullReferenceException in the case that the input is empty. It also prevents a compiler warning if you use an uninitialized variable.

Answer 2

Line 7

This is assigned to using an empty string as if it was just var n; it'll cause an error that basically says it cannot determine the type of n . So you assign an empty string to it. This also prevents another error where the variable isn't assigned to (though, this error would only occur if you did string n; ).

The variable itself is used to append text to. See line 16. If the text contains the search item, it'll append strOldText to n with a newline.

Line 16

Here, n isn't being incremented. It is being appended to as the value of n is string . What += does is add the new text to the string without overriding the existing string . So n is having whatever the current value of strOldText is added to it.

I'm also not sure what you mean about "delete function" as nothing in that snippet is deleting anything.

Hope I helped shed light on the code.

EDIT:

Here's some documentation to help:

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/operators/addition-operator

https://docs.microsoft.com/en-us/dotnet/csharp/language-reference/keywords/var