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Unnecessary NullReferenceException? [duplicate]

This question already has answers here:
What is a NullReferenceException, and how do I fix it?
(27 answers)
Closed 3 years ago.
Why do I get a NullReferenceException in the following code?
private string FileR ( string name )
{
string[] content = ReadSite(name, Protocol.read, url);
Request_Template newCon;
string[] final = new string[content.Length];
for (int i = 0; i < content.Length; i++)
{
if (content[i].Equals(null))
{
return "content [" + i + "] returns null";
}
newCon = JsonSerializer.Deserialize<Request_Template>(content[i]);
if (newCon.Sender.Contains(myAccount.Username))
{
newCon.Sender = "Me";
}
string sender = newCon.Sender;
string message = newCon.Message;
final[i] = sender + ":\t" + message;
}
string nFinal = string.Concat(final);
Thread.Sleep(10);
return nFinal;
}
string[] ReadSite(string filename, Protocol p, string uri)
{
Read_Template temp = new Read_Template
{
Chat = filename,
Code = key1
};
string myUrl = JsonSerializer.Serialize(temp);
WebClient web = new WebClient();
Stream stream = web.OpenRead(uri + "/" + myUrl);
int length = 0;
while (new StreamReader(stream).ReadLine() != null)
{
length++;
}
string[] content = new string[length];
using (StreamReader reader = new StreamReader(stream))
{
for (int i = 0; i < length; i++)
{
content[i] = reader.ReadLine();
}
}
return content;
}
I've tried using the debugging tools with no avail. When I ran the code, it said that the error came from string [] final = new string [content.Length];
and for (int i = 0; i < content.Length; i++).That lead me to assume that content was null. But when I used the watch window and it said that the variable content cannot be determined. How do I fix this?

I strongly suspect that the problem is actually here:
if (content[i].Equals(null))
If content[i] is genuinely null, then calling content[i].Equals will throw a NullReferenceException. That test should be written as:
if (content[i] is null)`
Or (for C# 6 or older):
if (content[i] == null)`
Now if ReadSite didn't have a bug in, you shouldn't need that check at all - because ReadSite should return an array of non-null string references. However, the way that you're populating the array is broken. You're currently:
Creating a StreamReader around the stream
Reading from the reader until you run out of data
Creating a new array of the "right" length
Creating another StreamReader around the same stream - which is by now at the end
Reading from the reader however many times you read a line originally
Because the stream is already at the end, ReadLine() is going to return null on every iteration.
Instead, you should just read once, populating a list as you go, like this:
List<string> content = new List<string>();
using (var stream = web.OpenRead(uri + "/" + myUrl))
{
using (var reader = new StreamReader(stream))
{
string line;
while ((line = reader.ReadLine()) is object)
{
content.Add(line);
}
}
}
return content;
That's returning a List<string>, so you'd want to change your ReadSite return type to List<string> or perhaps IReadOnlyList<string>. If you really need it to return an array, you could return content.ToArray() instead.
(Ideally, move to using HttpClient as well, but that's a different story.)

private string FileR ( string name )
{
// ReadSite always returns a content that has length 0 or more. string[] content will never equals null.
// TEST: string[] test = new string[0]; test == null -> False
string[] content = ReadSite(name, Protocol.read, url);
Request_Template newCon;
string[] final = new string[content.Length];
// if content.Length == 0, this loop will never occur and you wont get any NREs.
for (int i = 0; i < content.Length; i++)
{
// It is highly unlikely that a reader.ReadLine() would generate a null but
// just in case it does, use == null instead of .Equals() method. When content[i] == null, you cannot use .Equals() method since nulls dont have Equals method.
if (content[i].Equals(null))
{
return "content [" + i + "] returns null";
}
// If you have checked that content[i] is Not empty space or null, you might
//end up with newCon == null if Deserialization fails. Cover this around try / catch.
newCon = JsonSerializer.Deserialize<Request_Template>(content[i]);
// If deserialization fails, this will throw NRE because nulls wont have
// Sender as a property or array. Check if newCon == null or not. Also,
// check if Sender was correctly initialized as an array/list.. as it could
// be null. nulls wont have Contains as a method.
if (newCon.Sender.Contains(myAccount.Username))
{
newCon.Sender = "Me";
}
string sender = newCon.Sender;
string message = newCon.Message;
// This should work correctly as its dependent on content.length. If the
// loop is happening, then there is at least one final element that can be updated.
final[i] = sender + ":\t" + message;
}
string nFinal = string.Concat(final);
Thread.Sleep(10);
return nFinal;
}

Reading a Line from text file and return back

I am developing a C# application in which I need to read a line from a text file and return back to first of line.
As file size may be too large I can't copy it into an array .
I tried this code
StreamReader str1 = new StreamReader(#"c:\file1.txt");
StreamReader str2 = new StreamReader(#"c:\file2.txt");
int a, b;
long pos1, pos2;
while (!str1.EndOfStream && !str2.EndOfStream)
{
pos1 = str1.BaseStream.Position;
pos2 = str2.BaseStream.Position;
a = Int32.Parse(str1.ReadLine());
b = Int32.Parse(str2.ReadLine());
if (a <= b)
{
Console.WriteLine("File1 ---> " + a.ToString());
str2.BaseStream.Seek(pos2, SeekOrigin.Begin);
}
else
{
Console.WriteLine("File2 ---> " + b.ToString());
str1.BaseStream.Seek(pos1, SeekOrigin.Begin);
}
}
When I debuged the program I found out str1.BaseStream.Position and str2.BaseStream.Position are same in every loop , so nothing will change.
Is there any better way ?
Thanks

You can use ReadLines for large file, it is deferred execution and does not load the whole file into memory, so you can manipulate lines in IEnumerable type:
var lines = File.ReadLines("path");
If you are in old .NET version, below is how to build ReadLines by yourself:
public IEnumerable<string> ReadLine(string path)
{
using (var streamReader = new StreamReader(path))
{
string line;
while((line = streamReader.ReadLine()) != null)
{
yield return line;
}
}
}

Another way Which I prefer to use.
Create a Function like this:
string ReadLine( Stream sr,bool goToNext)
{
if (sr.Position >= sr.Length)
return string.Empty;
char readKey;
StringBuilder strb = new StringBuilder();
long position = sr.Position;
do
{
readKey = (char)sr.ReadByte();
strb.Append(readKey);
}
while (readKey != (char)ConsoleKey.Enter && sr.Position<sr.Length);
if(!goToNext)
sr.Position = position;
return strb.ToString();
}
Then , Create a stream from file for It's argument
Stream stream = File.Open("C:\\1.txt", FileMode.Open);

StreamReader case sensitive

My program currently reads a text file and compares it with the value in a text box and then tells me how many matches, this currently works.
My query is that it is case sensitive. Is there any way to make it so it doesn't matter whether it is in upper or lower case?
This is my code below:
if (!String.IsNullOrEmpty(CustodianEAddress.Text))
{
for (AddressLength1 = 0; AddressLength1 < Length; AddressLength1++)
{
List<string> list1 = new List<string>();
using (StreamReader reader = new StreamReader(FileLocation))
{
string line1;
//max 500
string[] LineArray1 = new string[500];
while ((line1 = reader.ReadLine()) != null)
{
list1.Add(line1); // Add to list.
if (line1.IndexOf(cust1[AddressLength1].ToString()) != -1)
{
count1++;
LineArray1[count1] = line1;
}
}
reader.Close();
using (System.IO.StreamWriter filed =
new System.IO.StreamWriter(FileLocation, true))
{
filed.WriteLine("");
filed.WriteLine("The email address " +
cust1[AddressLength1].ToString() + " was found " + count1 +
" times within the recipient's inbox");
}
string count1a;
count1a = count1.ToString();
}
}
}
else
{
MessageBox.Show("Please Enter an Email Address");
}
So basically, I need to compare the value in cust1[AddressLength1] with any values found in an array which is in the text file.

String.Compare() takes in an optional parameter that let's you specify whether or not the equality check should be case sensitive.
Edited in response to code being posted
Compare and Index of both take in an optional enumeration, StringComparison. If you choose StringComparison.OrdinalIgnoreCase then case will be ignored.

Here's a quick way to compare two strings without checking case:
string a;
string b;
string.Compare(a, b, true);
The true here is passed as the value of the ignoreCase parameter, meaning that upper and lower-case letters will be compared as if they were all the same case.
EDIT:
I've cleaned up your code a bit, and also put in the compare function. I included comments where I changed stuff:
// Not needed: see below. List<string> list1 = new List<string>();
using (StreamReader reader = new StreamReader(FileLocation))
{
string line1;
//max 500
List<string> LineArray1 = new List<string>();
while ((line1 = reader.ReadLine()) != null)
{
// list1.Add(line1); // Add to list.
// By adding to the list, then searching it, you are searching the whole list for every single new line - you're searching through the same elements multiple times.
if (string.Compare(line1, cust1[AddressLength1].ToString(), true) == 0)
{
// You can just use LineArray1.Count for this instead. count1++;
LineArray1.Add(line1);
}
}
// Not needed: using() takes care of this. reader.Close();
using (System.IO.StreamWriter filed =
new System.IO.StreamWriter(FileLocation, true))
{
filed.WriteLine(); // You don't need an empty string for a newline.
filed.WriteLine("The email address " +
cust1[AddressLength1].ToString() + " was found " + LineArray1.Count +
" times within the recipient's inbox");
}
string count1a;
count1a = LineArray1.Count.ToString();
}

The fact you are reading from a file or not it does not matter, when compare
use the static string Comapare function:
public static int Compare(
string strA,
string strB,
bool ignoreCase
)
and pass true as a last parameter.

Extracting text from a file where date -time is the index

I have got around 800 files of maximum 55KB-100KB each where the data is in this format
Date,Time,Float1,Float2,Float3,Float4,Integer
Date is in DD/MM/YYYY format and Time is in the format of HH:MM
Here the date ranges from say 1st May to 1June and each day, the Time varies from 09:00 to 15:30.
I want to run a program so that, for each file, it extracts the data pertaining to a particular given date and writes to a file.
I am trying to get around, to form a to do a search and extract operation. I dont know, how to do it, would like to have some idea.
I have written the code below:
static void Main(string[] args)
{
string destpath = Directory.GetCurrentDirectory();
destpath += "\\DIR";
DirectoryInfo Dest = Directory.CreateDirectory(destpath);
DirectoryInfo Source = new DirectoryInfo(Directory.GetCurrentDirectory() + "\\IEOD");
FileInfo[] fiArr = Source.GetFiles("*.csv");
Console.WriteLine("Search Date:");
string srchdate = Console.ReadLine();
String FileNewLine;
String FileNewdt;
FileInfo r;
foreach (FileInfo f in fiArr)
{
r = new FileInfo(destpath + "\\" + f.Name);
r.Create();
StreamWriter Sw = r.AppendText();
StreamReader Sr = new StreamReader(f.FullName);
while (Sr.Peek() >= 0)
{
FileNewLine = Sr.ReadLine();
FileNewdt = FileNewLine.Substring(0,10);
if (String.Compare(FileNewdt, srchdate, true) == 0)
{
//write it to a file;
Console.WriteLine(FileNewLine);
}
}
}
Console.ReadKey();
}
As of now, it should write into the Console. The writing with the help of StreamWriter will be done later, but I am facing a runtime error. It says, " 'C:\Documents and Settings\Soham Das\Desktop\Test\DIR\ABAN.csv' because it is being used by another process."
Here ABAN is a newly created file, by the code. The problem is faced at StreamWriter Sw = r.AppendText()
Help appreciated.
Thanks
Soham

Now that you have edited the question to show that the delimiter is actually a comma instead of a slash (which would have conflicted with the date format) this becomes a lot easier. I've re-posted the answer from last night below.
// This would come from Stream.ReadLine() or something
string line = "02/06/2010,10:05,1.0,2.0,3.0,4.0,5";
string[] parts = line.Split(',');
DateTime date = DateTime.ParseExact(parts[0], "dd/MM/yyyy", null);
TimeSpan time = TimeSpan.Parse(parts[1]);
date = date.Add(time); // adds the time to the date
float float1 = Single.Parse(parts[2]);
float float2 = Single.Parse(parts[3]);
float float3 = Single.Parse(parts[4]);
float float4 = Single.Parse(parts[5]);
int integer = Int32.Parse(parts[6]);
Console.WriteLine("Date: {0:d}", date);
Console.WriteLine("Time: {0:t}", date);
Console.WriteLine("Float1: {0}", float1);
Console.WriteLine("Float2: {0}", float2);
Console.WriteLine("Float3: {0}", float3);
Console.WriteLine("Float4: {0}", float4);
Console.WriteLine("Integer: {0}", integer);
Obviously you can make it more resilient by adding error handling, using TryParse, etc. But this should give you a basic idea of how to manipulate strings in .NET.

So 800 files with around 100KB sums up to 80 KBytes. So why don't built up a little class like
public class Entry
{
public DateTime Date {get; set;}
public float Float1 {get; set;}
public int Integer1 {get; set;}
public Entry(string values)
{
//ToDo: Parse single line into properties
// e.g. use String.Split, RegEx, etc.
}
}
Also you should take care about implementing GetHashCode() and Equals() (there is a good explanation in the book Essential C#). And you should add the interface IComparable to that class which just makes somethine like
public int CompareTo(Entry rhs)
{
return this.Date.CompareTo(rhs.Date);
}
If you got this you can easily do the following:
var allEntries = new SortedList<Entry>();
string currentLine = null;
using (var streamReader = new StreamReader("C:\\MyFile.txt"))
while ((currentLine = streamReader.ReadLine()) != null)
{
try
{
var entry = new Entry(currentLine);
allEntries.Add(entry);
}
catch (Exception ex)
{
//Do whatever you like
//maybe just
continue;
//or
throw;
}
}
So what's missing is to read in all the files (instead of a single one). But this can be done by another loop on Directory.GetFiles() which maybe itself is looped through a Directory.GetDirectories().
After reading all the files into your List you can do whatever LINQ query comes to your mind.

Delete specific line from a text file?

I need to delete an exact line from a text file but I cannot for the life of me workout how to go about doing this.
Any suggestions or examples would be greatly appreciated?
Related Questions
Efficient way to delete a line from a text file (C#)

If the line you want to delete is based on the content of the line:
string line = null;
string line_to_delete = "the line i want to delete";
using (StreamReader reader = new StreamReader("C:\\input")) {
using (StreamWriter writer = new StreamWriter("C:\\output")) {
while ((line = reader.ReadLine()) != null) {
if (String.Compare(line, line_to_delete) == 0)
continue;
writer.WriteLine(line);
}
}
}
Or if it is based on line number:
string line = null;
int line_number = 0;
int line_to_delete = 12;
using (StreamReader reader = new StreamReader("C:\\input")) {
using (StreamWriter writer = new StreamWriter("C:\\output")) {
while ((line = reader.ReadLine()) != null) {
line_number++;
if (line_number == line_to_delete)
continue;
writer.WriteLine(line);
}
}
}

The best way to do this is to open the file in text mode, read each line with ReadLine(), and then write it to a new file with WriteLine(), skipping the one line you want to delete.
There is no generic delete-a-line-from-file function, as far as I know.

One way to do it if the file is not very big is to load all the lines into an array:
string[] lines = File.ReadAllLines("filename.txt");
string[] newLines = RemoveUnnecessaryLine(lines);
File.WriteAllLines("filename.txt", newLines);

Hope this simple and short code will help.
List linesList = File.ReadAllLines("myFile.txt").ToList();
linesList.RemoveAt(0);
File.WriteAllLines("myFile.txt"), linesList.ToArray());
OR use this
public void DeleteLinesFromFile(string strLineToDelete)
{
string strFilePath = "Provide the path of the text file";
string strSearchText = strLineToDelete;
string strOldText;
string n = "";
StreamReader sr = File.OpenText(strFilePath);
while ((strOldText = sr.ReadLine()) != null)
{
if (!strOldText.Contains(strSearchText))
{
n += strOldText + Environment.NewLine;
}
}
sr.Close();
File.WriteAllText(strFilePath, n);
}

You can actually use C# generics for this to make it real easy:
var file = new List<string>(System.IO.File.ReadAllLines("C:\\path"));
file.RemoveAt(12);
File.WriteAllLines("C:\\path", file.ToArray());

This can be done in three steps:
// 1. Read the content of the file
string[] readText = File.ReadAllLines(path);
// 2. Empty the file
File.WriteAllText(path, String.Empty);
// 3. Fill up again, but without the deleted line
using (StreamWriter writer = new StreamWriter(path))
{
foreach (string s in readText)
{
if (!s.Equals(lineToBeRemoved))
{
writer.WriteLine(s);
}
}
}

Read and remember each line
Identify the one you want to get rid
of
Forget that one
Write the rest back over the top of
the file

I cared about the file's original end line characters ("\n" or "\r\n") and wanted to maintain them in the output file (not overwrite them with what ever the current environment's char(s) are like the other answers appear to do). So I wrote my own method to read a line without removing the end line chars then used it in my DeleteLines method (I wanted the option to delete multiple lines, hence the use of a collection of line numbers to delete).
DeleteLines was implemented as a FileInfo extension and ReadLineKeepNewLineChars a StreamReader extension (but obviously you don't have to keep it that way).
public static class FileInfoExtensions
{
public static FileInfo DeleteLines(this FileInfo source, ICollection<int> lineNumbers, string targetFilePath)
{
var lineCount = 1;
using (var streamReader = new StreamReader(source.FullName))
{
using (var streamWriter = new StreamWriter(targetFilePath))
{
string line;
while ((line = streamReader.ReadLineKeepNewLineChars()) != null)
{
if (!lineNumbers.Contains(lineCount))
{
streamWriter.Write(line);
}
lineCount++;
}
}
}
return new FileInfo(targetFilePath);
}
}
public static class StreamReaderExtensions
{
private const char EndOfFile = '\uffff';
/// <summary>
/// Reads a line, similar to ReadLine method, but keeps any
/// new line characters (e.g. "\r\n" or "\n").
/// </summary>
public static string ReadLineKeepNewLineChars(this StreamReader source)
{
if (source == null)
throw new ArgumentNullException(nameof(source));
char ch = (char)source.Read();
if (ch == EndOfFile)
return null;
var sb = new StringBuilder();
while (ch != EndOfFile)
{
sb.Append(ch);
if (ch == '\n')
break;
ch = (char)source.Read();
}
return sb.ToString();
}
}

Are you on a Unix operating system?
You can do this with the "sed" stream editor. Read the man page for "sed"

What?
Use file open, seek position then stream erase line using null.
Gotch it? Simple,stream,no array that eat memory,fast.
This work on vb.. Example search line culture=id where culture are namevalue and id are value and we want to change it to culture=en
Fileopen(1, "text.ini")
dim line as string
dim currentpos as long
while true
line = lineinput(1)
dim namevalue() as string = split(line, "=")
if namevalue(0) = "line name value that i want to edit" then
currentpos = seek(1)
fileclose()
dim fs as filestream("test.ini", filemode.open)
dim sw as streamwriter(fs)
fs.seek(currentpos, seekorigin.begin)
sw.write(null)
sw.write(namevalue + "=" + newvalue)
sw.close()
fs.close()
exit while
end if
msgbox("org ternate jua bisa, no line found")
end while
that's all..use #d