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how i can get the string after specific character in my case :?

str="Brand : TROLLBEADS";
int length = str.Length;
length = length - 6;
str = str.Substring(6, length);
i want to display "TROLLBEADS"
and want to discard other remaining string

You can split the string using : delimiter if it is fixed
var result = str.Split(':')[1].Trim();
or if your string can have multiple : in that case
var result = str.Substring(str.IndexOf(":") + 1).Trim();

Split is nice, but a bit too much. You can just specify the start position for substring.
string str="Brand : TROLLBEADS";
string val = str.Substring(str.IndexOf(":") + 1);
Console.WriteLine(val);

how to do substring in reverse mode in c#

My string is "csm15+abc-indiaurban#v2". I want only "indiaurban" from my string. what I am doing right now is :
var lastindexofplusminus = input.LastIndexOfAny(new char[]{'+','-'});
var lastindexofattherate = input.LastIndexOf('#');
string sub = input.Substring(lastindexofplusminus,lastindexofattherate);
but getting error "Index and length must refer to a location within the string."
Thanks in Advance.

You should put the length in the second argument (instead of passing another index) of the Substring you want to grab. Given that you know the two indexes, the translation to the length is pretty straight forward:
string sub = input.Substring(lastindexofplusminus + 1, lastindexofattherate - lastindexofplusminus - 1);
Note, +1 is needed to get the char after your lastindexofplusminus.
-1 is needed to get the Substring between them minus the lastindexofattherate itself.

You can simple reverse the string, apply substring based on position and length, than reverse again.
string result = string.Join("", string.Join("", teste.Reverse()).Substring(1, 10).Reverse());
Or create a function:
public static string SubstringReverse(string str, int reverseIndex, int length) {
return string.Join("", str.Reverse().Skip(reverseIndex - 1).Take(length));
}
View function working here!!

You can use LINQ:
string input = "csm15+abc-indiaurban#v2";
string result = String.Join("", input.Reverse()
.SkipWhile(c => c != '#')
.Skip(1)
.TakeWhile(c => c != '+' && c != '-')
.Reverse());
Console.WriteLine(result); // indiaurban

I don't know what is identify your break point but here is sample which is working
you can learn more about this at String.Substring Method (Int32, Int32)
String s = "csm15+abc-indiaurban#v2";
Char charRange = '-';
Char charEnd = '#';
int startIndex = s.IndexOf(charRange);
int endIndex = s.LastIndexOf(charEnd);
int length = endIndex - startIndex - 1;
Label1.Text = s.Substring(startIndex+1, length);

Assuming that your string is always in that format
string str = "csm15+abc-indiaurban#v2";
string subSTr = str.Substring(10).Substring(0,10);

Extract text with random numbers inside using C#

In C#, if I have a long string that will always contains somewhere a format such as W##S##Q## where # can be any number, how can I get that sequence of W##S##Q## extracted from the string. Bare in mind that the string may have more before or after but I am only interested in that sequence.
Regards.

Wobbles comment is correct, a regular expression such as #"W\d{2}S\d{2}Q\d{2}" will do what you need. \d matches any any decimal digit and the {2} afterwards tells it to match exactly twice.
This fiddle gives an example of how you would extract the string you want from a longer string.

Wobbles is right, Regular Expressions are the best way to do it generally. For your specific example, if you know in advance that the W,S,Q portions are always going to be in the same place you could use:
string testString = "WSomethingW01S02Q03SomethingElse";
bool TheRightString = false;
string WNumString = string.Empty;
string SNumString = string.Empty;
string QNumString = string.Empty;
int StartPosition = 0;
do
{
StartPosition = testString.IndexOf('W', StartPosition);
WNumString = testString.Substring(StartPosition, 3);
SNumString = testString.Substring(StartPosition + 3, 3);
QNumString = testString.Substring(StartPosition + 6, 3);
StartPosition += 1;
if (SNumString.StartsWith("S") && QNumString.StartsWith("Q"))
TheRightString = true;
} while (TheRightString == false);
Console.WriteLine(WNumString + SNumString + QNumString);
Console.ReadKey();

How to Remove the last char of String in C#?

I have a numeric string, which may be "124322" or "1231.232" or "132123.00".
I want to remove the last char of my string (whatever it is).
So I want if my string is "99234" became "9923".
The length of string is variable. It's not constant so I can not use string.Remove or trim or some like them(I Think).
How do I achieve this?

YourString = YourString.Remove(YourString.Length - 1);

var input = "12342";
var output = input.Substring(0, input.Length - 1);
or
var output = input.Remove(input.Length - 1);

newString = yourString.Substring(0, yourString.length -1);

If this is something you need to do a lot in your application, or you need to chain different calls, you can create an extension method:
public static String TrimEnd(this String str, int count)
{
return str.Substring(0, str.Length - count);
}
and call it:
string oldString = "...Hello!";
string newString = oldString.TrimEnd(1); //returns "...Hello"
or chained:
string newString = oldString.Substring(3).TrimEnd(1); //returns "Hello"

If you are using string datatype, below code works:
string str = str.Remove(str.Length - 1);
But when you have StringBuilder, you have to specify second parameter length as well.
That is,
string newStr = sb.Remove(sb.Length - 1, 1).ToString();
To avoid below error:

How to Remove Last digit of String in .Net?

I want to eliminate B if i scan Serial number bar code: 21524116476CA2006765B
Output: 21524116476CA2006765

string foo = "21524116476CA2006765B";
string bar = foo.Substring(0, Math.Max(foo.Length - 1, 0));

If you can't be certain that the input barcode will always end with a B, do something like this:
char[] barcodeEnd = { 'B' };
string input = "21524116476CA2006765B";
string output = input.TrimEnd(barcodeEnd);
PUt the first line somewhere it'll only get run once, so you aren't constantly creating new char[] arrays. You can add more characters to the array if you're looking for other specific characters.

string s = "21524116476CA2006765B";
string b = s.Substring(0, s.Length - 1);

string value= "21524116476CA2006765B";
string bar = value.Substring(0, 19);

string str = "Your String Whatever it is";
str = str.Substring(0, s.Length - 1);