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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"

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");

Use scientific notation only if needed

I want to parse a double value into a string. I want my number to have a specified number of digits (that I won't know until runtime). If this number can be expressed with a non-zero value in number of digits this is what I want. If the number comes out as zero's like this I want it expressed in scientific notation.
Some examples will make this more clear, this assumes I wanted 3 digits:
Value: .2367 Output: "0.23"
Value: .00367 Output: "3.67E-3"
Value: 22.3 Output: "22.3"
Value: 3364.0 Output: "3.36E3"
My work around solution would use the ToString() method and the N numeric format string and if it results in zero's revert to the E format string, but this feels like reinventing the wheel. Does anyone know if a built in method to do this?

Have you looked at using the General Number Format Specifier?
The general ("G") format specifier converts a number to the most
compact of either fixed-point or scientific notation, depending on the
type of the number and whether a precision specifier is present.
Some samples from the documentation:
double number;
number = .0023;
Console.WriteLine(number.ToString("G", CultureInfo.InvariantCulture));
// Displays 0.0023
number = 1234;
Console.WriteLine(number.ToString("G2", CultureInfo.InvariantCulture));
// Displays 1.2E+03
number = Math.PI;
Console.WriteLine(number.ToString("G5", CultureInfo.InvariantCulture));
// Displays 3.1416

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("####.####");

Globalized custom number formatting - Variable decimal points

I'm trying to alter the existing number formatting in my company's application to make it more readable for international users. This is a stock trading application, so most stock prices come in with numbers precise to 2 decimal points, like so-> 17.23 We could also get ticks in that have precision out to 4 decimal points, so a penny stock might be 0.0341. The original string format that we were using for stocks was "#,##0.00##" Which would give us the format we wanted (essentially trimming '0's). The problem here is the ',' and '.' are forced onto the user, where in many other countries the thousands separator is '.' and the decimal point is ','. Boss man doesn't want to use "N4" for all numbers, even though this would resolve the globalization issue. Is it possible to have a globalized custom string format?
Other options besides writing some middle man code to internationalize numbers formatted the original way or another string.format method?

Check this out:
float value = 0.0341f;
string output = string.Empty;
CultureInfo brazil = new CultureInfo("pt-BR");
CultureInfo usa = new CultureInfo("en-US");
output = value.ToString("C4", brazil.NumberFormat); //output is R$ 0,0341
output = value.ToString("C4", usa.NumberFormat); //output is $0.0341

The , and . in the custom format string already translate into the correct separators for the current culture. You should not need to change your code. See MSDN.