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Int32.TryParse C# - How to keep the last integer value

I'm trying to convert a string to int with
Int32.TryParse(input, out int number);
and I want to keep the last integer value.
E.g.
If string input = "123" then int number = 123 and if string input = "" or null then int number should stay 123 till input had a new string value.
Have someone a idea?

Keep a copy of the previous value, and if the parse failed copy it back in:
var previousValue = 1;
if(!int.TryParse(input, out var number))
{
number = previousValue;
}

convert to double 5.55111512312578E-17

How to convert -5.55111512312578E-17 to 5.55?
my code:
var value=reader11["PendingQty"].ToString().Replace('-', ' ');
var a=String.Format("{0:0.00}", value);
i also Tried : value= Math.round

-5.55111512312578E-17 is equal to 0.0000000000000000555111512312578. You could get this value by doing this:
double output = Double.Parse(input, System.Globalization.NumberStyles.Float, System.Globalization.CultureInfo.InvariantCulture);
Console.WriteLine(output.ToString("F99").TrimEnd('0'));
But as far as I understood, you actually only want to first three digits, so I would do a string manipulation:
input.Substring(1,4);
This takes 4 characters, starting at the second position. If you have positive values too, simply check and read from the first digit on:
var res = "";
if (input.StartsWith("-")) {
res = input.Substring(1,4));
}
else {
res = input.Substring(0,4);
}

Input string was not in a correct format when converting to integer

In line:
int d = Convert .ToInt32 ( TxtAmount.Text);
there is an error
Input string was not in a correct format
I want to convert the number inside TxtAmount.Text. if it's a negative number to decimal or integer without - and then convert it again to string because ConvertNumbersToArabicAlphabet the parameter is string.
int d = Convert .ToInt32 ( TxtAmount.Text);
ConvertNumbersToArabicAlphabet a = new ConvertNumbersToArabicAlphabet(d.ToString());
Label2.Text = a.GetNumberAr();

I'd check whether the input is numeric first and then convert, using Math.Abs to make sure the result is always a positive number:
int result = 0;
// Does text contain numbers only (and maybe a leading "-")?
if (Regex.IsMatch(TxtAmount.Text, #"^-?[0-9]+$"))
{
// Try to parse an int from it. If successful, convert it to
// a positive number in any case (= ignore the leading "-")
if (Int32.TryParse(TxtAmount.Text, out result))
result = Math.Abs(result);
}
// In all other cases, the result is 0
return result;

int d = Convert .ToInt ( TxtAmount.Text);
You can use this if you have a Integer Value within the textbox .
If text area contain only a single character then it's not work anymore.
try to put only Integer of same type of data you want to convert in to textbox
like if you want to add a double number then you can use.
Double num = Convert .ToDouble ( TxtAmount.Text);
if your textbox is empty may sure that you have validation or check for the same:
if(TxtAmount.Text==""||TxtAmount.Text=string.Empty)
{
TxtAmount.Text=0;
}
else
{
int d = Convert .ToInt ( TxtAmount.Text);
}

If all you want is to ignore the negative sign, then you can do string manipulation like:
string value = TxtAmount.Text;
if (value.StartsWith("-"))
{
value = value.Substring(1);
}

This is because Youse textbox is empty.
check weather your textbox have values or not
if(!String.IsNullOrWhiteSpace(TxtAmount.Text))
{
int d = Convert .ToInt32 ( TxtAmount.Text);
}

Get exponential value using regular expression

I have string like this:
strings s = "1.0E-20"
Is there a way to get only -20 from this using regex?
I tried this:
(([1-9]+\.[0-9]*)|([1-9]*\.[0-9]+)|([1-9]+))([eE][-+]?[0-9]+)?
this gets me e-20 in group5 but still not just -20.

Use Regex for dealing with text, use Math(s) for dealing with numbers:
Math.Log10(Convert.ToDouble("1.0E-20")) // returns -20
To make sure your string input is a valid double use TryParse:
double d, result = 0.0;
if (Double.TryParse("1.0E-20", out d))
{
result = Math.Log10(d);
}
else
{
// handle error
}
Also, if you want to get the 1.0 (multiplier) from your input:
var d = Convert.ToDouble("1.0E-20");
var exponent = Math.Log10(d);
var multiplier = d / exponent;

No need for Regex when string methods can do wonders
string str = "1.0E-20";
str = str.Substring(str.IndexOf('E') + 1);

You can do that without Regex like:
string s = "1.0E-20";
string newStr = s.Substring(s.IndexOf('E') + 1);
Later you can parse the string to number like:
int number;
if (!int.TryParse(newStr, out number))
{
//invalid number
}
Console.WriteLine(number);
You can also use string.Split like:
string numberString = s.Split('E')[1]; //gives "-20"
Its better if you add check for string/array length when access string.Substring or accessing element 1 after split.

var x = str.IndexOf("E") != -1 ? str.Substring(str.IndexOf("E") + 1) : "1";

If you want to use regular expressions to achieve this, you should switch up your capture groups.
(([1-9]+\.[0-9]*)|([1-9]*\.[0-9]+)|([1-9]+))([eE])([-+]?[0-9]+)?
Group 6 will contain -20 with your given example with the regular expression above. Note how the parentheses have moved. We might need more information from you though. Do you have any more sample data? What's the end goal here?

How to use Substring , to return an integer of all the values after the first char

I have an string AssetNumber that have the following format C100200.
so i need to do the folloiwng:-
Get all the characters after the first (e.g. 100200 using above example)
Convert the substring to integer
Return the integer + 1
but I do not know how to use the sub-string to get all the characters after the first char, and how to convert string into int? Any help on this will be appreciated.

var result = Int32.Parse("C100200".Substring(1)) + 1;
If you would like to have default value, if you can't parse current string:
int result;
if (!Int32.TryParse("sdfsdf".Substring(1), out result)) {
result = 42;
}
result+=1;