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BigInteger.ToString in scientific notation (i.e. with E)

Double.ToString on a very large number usually returns only a number of significant digits, followed by the exponent notation with a power of ten. However, BigInteger doesn't do this, and simply returns all digits of the number.
How can I tell it to output only in the scientific format like double does instead of the full number? I can't convert it to double, because it is greater than Double.MaxValue. I could build a formatting function myself, but I would be happier if there was a built-in feature taking all cultures into account.

Like most ToString methods, BigInteger.ToString allows you to pass a format string:
var myString = myBigInteger.ToString("E"); // yields exponential notation
See the following MSDN page for a detailed description of the built-in numeric format strings available in .NET:
Standard Numeric Format Strings
Note that the Exponential Format Specifier allows you to provide an optional precision specifier.

You should simply be able to use the BigInteger.ToString() method along with the expected Exponential formatting string E to handle scientific notation :
var input = BigInteger.Parse("12498124912841982142441242424421");
var output = input.ToString("E"); // yields "1.249812E+031"

How to deal with right zeros in a string to decimal conversion?

in my website i need to read data from a XML, and some of these datas are decimal values.
Sometimes the value comes correct: 1 or 72,59and sometimes the value comes like 1.0000 or 72,590000, how is the best way to convert it to the right format:
ps: I need a string value.
Thanks!

What format are you wanting them to go to, specifically? How many decimals, etc?
If you want always 2 decimals, try a standard numeric formatting such as:
decimal value = 123.456;
Console.WriteLine("Your account balance is {0:F}.", value);
See this MSDN example for other common numeric formatting techniques.

You write that you tried
string.Format("{0:F}","1.00000");
The problem with this is that you're passing a string into the function. Numeric formatting only works on numeric data types. Convert the value to a decimal first and then format it.
Try this
public string ReformatDecimalString(string input)
{
const string formatString = //something
var decimalValue = decimal.Parse(input);
return decimalValue.ToString(formatString);
}
When you are formatting a single numeric value, it's slightly more efficient to use x.ToString(formatString) than string.Format(formatString, x). But note that the specific format string will be different in the two cases.
If your input data has decimal points (not commas) and your computer's culture uses decimal commas, you ought to be able to parse the value correctly by using CultureInfo.InvariantCulture:
var decimalValue = decimal.Parse(input, CultureInfo.InvariantCulture);

If I'm reading your answer correctly, you're trying to convert Integer values you pull from an XML file into string values without trailing zeroes ("ps: I need a string value.")
this code:
decimal test = 20.000000m
test.ToString("G29");
might do what you want

Using Decimal.ToString() without rounding up the number

I have following decimal number
string number = "60.9";
Response.Write(Convert.ToDecimal(number).ToString("#,0")); // prints 61
I want to format the value like 99,999,999... but if the value has decimal point, it is getting rounded up
Please let me know how can I print 60.9, instead of 61 with same number formatting

You can use N format. This will show the number with group separator of your locale (, for US locale for example, although you can optionally influence that), and leave decimal part as it is:
Convert.ToDecimal(number).ToString("N")
Update. After some discussion in comments the approach that meets all requirements appeared to be
Convert.ToDecimal(number).ToString("#,#.##########", CultureInfo.InvariantCulture)
Kudos to Jon.

Math.Round, keep decimal place [duplicate]

This question already has answers here:
Formatting a float to 2 decimal places
(9 answers)
Closed 9 years ago.
For example.
Math.Round(2.314, 2) //2.31
Math.Round(2.301, 2) //2.3 , but I want this as 2.30

Numbers don't have any conception of zeroes after a decimal point.
You're actually asking how to convert the number into a string with extra zeroes:
(2.301).ToString("0.00") // "2.30"
See numeric format strings for more detail.
In particular, the 0 specifier will round away from zero.

You want a string formatting of the number:
string val = Math.Round(2.301, 2).ToString("F2");
here's a post on formatting numbers in C#

2.3 and 2.30 are the same thing. If you want the string 2.30 then use .ToString("F2") on the Math.Round function.

2.3 and 2.30 is the same thing from a code perspective. You can display the trailing zero by formatting a string:
string yourString = Math.Round(2.301, 3).ToString("0.00");

The decimal is still there, you're probably just not seeing because when you look at the string representation, by default it will omit trailing zeros. You can overwrite this behavior by passing a format string to ToString():
Console.WriteLine(Math.Round(2.301, 2).ToString("N2")) // 2.30
But of course, if this is just for display purposes, you don't really need to call Math.Round:
Console.WriteLine(2.301.ToString("N2")) // 2.30
Further Reading
Standard Numeric Format Strings
Custom Numeric Format Strings

If you use decimal numbers (their literals end with m, for "money"), you get the behavior you're after. double numbers don't have a concept of significant zeroes the same way that decimals do.
Math.Round(2.314m, 2);
Math.Round(2.301m, 2);
Or if you want to change how you see the numbers, you can use a string format:
Math.Round(2.314, 2).ToString("N2");
Math.Round(2.301, 2).ToString("N2");

How do I make sure a decimal is always displayed as xxxx.xxxx positive and negative?

I have a decimal on an object in C#. I want to be able to display it as xxxx.xxxx at the moment the value is -1.61769, basically I want to round the last two digits up and make sure that it only ever has 4 decimal places after the decimal place. Im not sure if this is a Math operation (i.e. Math.Round) or is a validation operation (i.e. string.Format) or both?..
Hope someone can help...

Try this:
string result = d.ToString("0.0000");
You may also want to specify a culture:
CultureInfo cultureInfo = CultureInfo.InvariantCulture;
string result = d.ToString("0.0000", cultureInfo);
Result:
-1.6177
See it working online: ideone

That depends on whether you want at most four decimal places, or exacty four decimal places.
If you round the value, you get at most four decimal places:
value = Math.Round(value, 4);
The value 1.61799 for example would be rounded to 1.6180 and display as 1.618.
If you format the value, you get exactly four decimal places:
string formatted = value.ToString("0.0000");

you have to check with string.Format if you are looking to have a string.
check that link c# double format
if you are looking for keeping the value of the number you should use Math.round

Did you try Math.Abs()?
According to this MSDN-Page it is not possible.
Because it would display a diffierent Value.
Math.Abs(-1.61769).ToString("####.####");