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Can I shorten this code with a loop in C#?

I have this code written in C# but looks kind of "bad" and I would like to shorten it somehow and keep it clean and simple.
All this code works pretty fine but I want to know if there's any other way I can achieve the same thing.
EDIT: I forgot to mention that the firstLine has a bad date format attached with it, so it is like this: "This_is_my_first_line_20220126". So I split the string and then only join it with the corrected date. The problem is that I can never know how long the new string would be and I don't want to handle the code like this and go up to 100 parts.
Here's my code:
string correctDate = "26012022";
string[] lines = File.ReadAllLines("text.txt");
string firstLine = lines.FirstOrDefault();
//note: firstLine looks like this: This_is_my_first_line_20220126
string[] sub = firstLine.Split('_');
string name="";
if(sub.Length==2)
name = sub[0]+"_"+sub[1]+"_"+correctDate;
else if(sub.Length==3)
name = sub[0]+"_"+sub[1]+"_"+sub[2]+"_"correctDate;
...
else if(sub.Length==20)
name = sub[0]+"_"+ ... "_" + sub[19];
Now, my final name value should be "This_is_my_line_26012022" but I want it to depend on the length of the given string. So far I know that the maximum length would go up to 20 but I don't want my code to look like this. Can I shorten it somehow?

you can find the LastIndexOf the underscore and drop the date by using Substring:
string firstLine = "This_is_my_first_line_20220126";
string correctDate = "26012022";
string correctString = firstLine.Substring(0, firstLine.LastIndexOf("_") + 1) + correctDate;

Still a little perplexed with the split aproach, but this a way to join back all elements
string name = string.Join("_", sub.Take(sub.Length - 1).Append(correctDate));
Or use the substring method (and no need of all that split & join)
name = firstLine.Substring(0, firstLine.LastIndexOf("_") +1) + correctDate;

I forgot to mention that firstLine has a bad date format like "This_is_my_Line_20220125"
If you want to correct just the first line:
string correctDate = "26012022";
string[] lines = File.ReadAllLines("text.txt");
lines[0] = lines[0][..^8] + correctDate;
[..^8] uses C# 9's "indices and ranges" feature, that allows for a more compact way of taking a substring. It means "from the start of the string, up to the index 8 back from the end of the string".
If you get a wiggly line and possibly a messages like "... is not available in C# version X" you can use the older syntax, which would be more like lines[0] = lines[0].Remove(lines[0].Length - 8) + correctDate;
If you want to correct all lines:
string correctDate = "26012022";
string[] lines = File.ReadAllLines("text.txt");
for(int x = 0; x < lines.Length; x++)
lines[x] = lines[x][..^8] + correctDate;
If the incorrect date isn't always 8 characters long, you can use LastIndexOf('_') to locate the last _, and snip it to that point

How to slice/trim this value?

I am trying to remove the last 6 characters from item.Size because the data has a decimal place and 5 trailing 0s in the database.
sb.Append("<div>" + item.Size + " " + item.Units + " </div>");
ie. item.Size is displayed as 1.00000 and I need it to just be displayed as 1.
This is part of a StringBuilder, and as I'm new to coding, not even sure the right way to go about this.

sb.Append("<div>" + (int)item.Size + " " + item.Units + " </div>");

StringBuilder has the same formatting capabilities as String.Format when you use the AppendFormat method:
sb.AppendFormat("<div>{0:N0} {1} </div>", item.Size, item.Units);
The format string "N0" tells it to format the number with 0 decimal points. That assumes the item.Size is stored as a numerical type. If not, simply remove the part of the string you don't want:
sb.AppendFormat("<div>{0} {1}</div>", item.Size.Split('.')[0], item.Units);
Here I've used Split, assuming that the value is actually something like what you've shown in your example.

Better you use int.TryParse(or Int32.TryParse) method. because if item.Size is not convertible to int, then it wont give you any exception. you can use int or long according to your choice. So you can handle this in your code according to the if/else condition.
Sample Code:
int size;
string str = "";
if(int.TryParse(str, out size) == true)
{
}
else
{
}

String Format with undefined number of characters c#

So I'm working on formatting a string and I need to line it up in a table, but this string has an undetermined number of characters. Is there anyway to have the string be in the same spot for each column? so far I have:
ostring += "Notes\t\t"
+ " : "
+ employees[number].Notes
+ "\t\t"
+ employees[number].FirstNotes
+ "\t\t"
+ employees[number].SecondNotes;
I use a similar fashion on the other rows, but they have a pre-determined number of digits, this however doesn't so I can't use the string modifiers like I would like.
Any ideas on what I need to do?

You can use String.PadRight() to force the string to a specific size, rather than using tabs.

When you are using String.Format item format has following syntax:
{ index[,alignment][ :formatString] }
Thus you can specify alignment which indicates the total length of the field into which the argument is inserted and whether it is right-aligned (a positive integer) or left-aligned (a negative integer).
Also it's better to use StringBuilder to build strings:
var builder = new StringBuilder();
var employee = employees[number];
builder.AppendFormat("Notes {0,20} {1,10} {2,15}",
employee.Notes, employee.FirstNotes, employee.SecondNotes);

You would first have to loop over every entry to find the largest one so you know hoe wide to make the columns, something like:
var notesWidth = employees.Max(Notes.Length);
var firstNotesWidth = employees.Max(FirstNotes.Length);
// etc...
Then you can pad the columns to the correct width:
var output = new StringBuilder();
foreach(var employee in employees)
{
output.Append(employee.Notes.PadRight(notesWidth+1));
output.Append(employee.FirstNotes.PadRight(firstNotesWidth+1));
// etc...
}
And please don't do a lot of string "adding" ("1" + "2" + "3" + ...) in a loop. Use a StringBuilder instead. It is much more efficient.

Numeric fields lose leading zero while writing CSV in c#

I'm using an ASP.NET application which exports my clients data to CSV, I need my clients Phone number to be with the leading Zero.
I need the phone numbers to be without "-" and without quotations, and due to the nature of my application I cannot use 3rd party products such as EPPLUS.
I've tried to put a space and let the CSV "understand" that I need the phone number as text , but that doesn't seem right.
I would like to know how to make the excel include the leading zero , without using 3rd party products.
Thanks

Change the data that is saved in the csv with the following format:
="00023423"
CSV example:
David,Sooo,="00023423",World
This will show 00023423 in excel and not 23423.

public void CreatingCsvFiles(Client client)
{
string filePath = "Your path of the location" + "filename.csv";
if (!File.Exists(filePath))
{
File.Create(filePath).Close();
}
string delimiter = ",";
string[][] output = new string[][]{
new string[]{ "=\"" + client.phone + "\"", client.name }
};
int length = output.GetLength(0);
StringBuilder sb = new StringBuilder();
for (int index = 0; index < length; index++)
sb.AppendLine(string.Join(delimiter, output[index]));
File.AppendAllText(filePath, sb.ToString());
}
Inspired from http://softwaretipz.com/c-sharp-code-to-create-a-csv-file-and-write-data-into-it/
The important part :
"=\"" + client.phone + "\"", client.name
If the phone number is an int, of course you add .toString().

Print phone number to CSV with prepended ' (single quote), so it looks like:
"Some Name","'0000121212"
Excel should treat this 0000121212 as string then.

I believe converting the number into a formula like the accepted answer might not be a helpful solution for all.
The alternate solution I went with is to just add a tab space before the integer value.
Example:
Taking phoneNumber as a string variable which contains our int value
Solution:
"\t" + phoneNumber

If you know already how much numbers has to be inside phone you can do like this
phoneNumber.ToString("000000000000")
In this example I consider that phoneNumber is an int and required length of numbers is 12.

C# , Substring How to access last elements of an array/string using substring

I am generating 35 strings which have the names ar15220110910, khwm20110910 and so on.
The string contains the name of the Id (ar152,KHWM), and the date (20110910). I want to extract the Id, date from the string and store it in a textfile called StatSummary.
My code statement is something like this
for( int 1= 0;i< filestoextract.count;1++)
{
// The filestoextract contains 35 strings
string extractname = filestoextract(i).ToString();
statSummary.writeline( extractname.substring(0,5) + "" +
extractname.substring(5,4) + "" + extractname.substring(9,2) + "" +
extractname.substring(11,2));
}
When the station has Id containing 5 letters, then this code executes correctly but when the station Id is KHWM or any other 4 letter name then the insertion is all messed up. I am running this inside a loop. So I have tried keeping the code as dynamic as possible. Could anyone help me to find a way without hardcoding it. For instance accessing the last 8 elements to get the date??? I have searched but am not able to find a way to do that.

For the last 8 digits, it's just:
extractname.Substring(extractname.Length-8)
oh, I'm sorry, and so for your code could be:
int l = extractname.Length;
statSummary.WriteLine(extractname.substring(0,l-8) + "" +
extractname.Substring(l-8,4) + "" + extractname.Substring(l-4,2) + "" +
extractname.Substring(l-2,2));

As your ID length isn't consistent, it would probably be a better option to extract the date (which is always going to be 8 chars) and then treat the remainder as your ID e.g.
UPDATED - more robust by actually calculating the length of the date based on the format. Also validates against the format to make sure you have parsed the data correctly.
var dateFormat = "yyyyMMdd"; // this could be pulled from app.config or some other config source
foreach (var file in filestoextract)
{
var dateStr = file.Substring(file.Length-dateFormat.Length);
if (ValidateDate(dateStr, dateFormat))
{
var id = file.Substring(0, file.Length - (dateFormat.Length+1));
// do something with data
}
else
{
// handle invalid filename
}
}
public bool ValidateDate(stirng date, string date_format)
{
try
{
DateTime.ParseExact(date, date_format, DateTimeFormatInfo.InvariantInfo);
}
catch
{
return false;
}
return true;
}

You could use a Regex :
match = Regex.Match ("khwm20110910","(?<code>.*)(?<date>.{6})" );
Console.WriteLine (match.Groups["code"] );
Console.WriteLine (match.Groups["date"] );
To explain the regex pattern (?<code>.*)(?<date>.{6}) the brackets groups creates a group for each pattern. ?<code> names the group so you can reference it easily.
The date group takes the last six characters of the string. . says take any character and {6} says do that six times.
The code group takes all the remaining characters. * says take as many characters as possible.

for each(string part in stringList)
{
int length = part.Length;
int start = length - 8;
string dateString = part.Substring(start, 8);
}
That should solve the variable length to get the date. The rest of the pull is most likely dependent on a pattern (suggested) or the length of string (when x then the call is 4 in length, etc)

If you ID isn't always the same amount of letters you should seperate the ID and the Date using ',' or somthing then you use this:
for( int 1= 0;i< filestoextract.count;1++)
{
string extractname = filestoextract[i].ToString();
string ID = extractname.substring(0, extractname.IndexOf(','));
string Date = extractname.substring(extractname.IndexOf(','));
Console.WriteLine(ID + Date);
}