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How can I store 256-bit integer on unity?

I tried using the BigInteger implementation for Unity, but it still overflows(Or maybe I used in a wrong way? Im not sure also, it can only do 20 characters of ints which is like 64 bits).... This is how my thing works, I have a Hexadecimal which is has 64 characters, and then for me to do arthmetic computations, I want to convert it first into a decimal form and store it in variable.
public BigInteger x = 0;
and then here where it overflows... HexToDecimal is a function that takes a string of Hexadecimal and returns the decimal form of that.
x = HexToDecimal(hex);
a sample output of HexToDecimal is
105627842363267744400190144423808258002852957479547731009248450467191077417570
that's the ideal size of a number I want to store.
It works if I used very small numbers like hundreds thousands or something. but BitInteger kinda limits it to 20 characters only cause I tried declaring a variable like this, just to know the where BitInteger limits me
public BigInteger x = 10000000000000000000
when I add another "0" there, its throws an error stating that integral is too large

The way you've instantiated your BigInteger is a convenience method for smaller numbers (see "instantiating a BigInteger" -- I can't seem to link to it directly). This means that you're actually creating an int32 or an int64 and then converting it to a BigInteger (so it has to be able to fit into the limited size of those types).
To truly take advantage of BigInteger's arbitrary size, you probably want to use BigInteger.Parse(String). This method will return a BigInteger for a numeric String (and it must be a numeric string as defined by the current system culture -- nothing else except a possible leading negative symbol) This method should work perfectly fine in Unity as it's part of the C# standard lib.
So, for your HexToDecimal example, assuming it returns a string you'd use it like
x = BigInteger.Parse(HexToDecimal(hex));

Parsing in CSharp - how to understand it

It's just few days ago that I jumped into learning C# and I already have one problem with understanding basics.. Maybe it's just the language barrier (I'm not English native speaker). Please, could you explain me how to understand parsing? For example: while creating a very simple calculator I wanted to read the first input number (which is a variable a). I use this code:
float a = float.Parse(Console.ReadLine());
and the same with b for the other number:
float b = float.Parse(Console.ReadLine());
I learnt that the float is a data type for decimals numbers so what exactly does this particular Parse() stands for?
Obviously, I tried to run the application without parsing and it wouldn't work because it reads it as string, but why? Thank you..

Console.ReadLine() returns a string, which represents a piece of text. So, from the computer's point of view, what you have after calling Console.ReadLine() is a piece of text. It may or may not contain the text "6.0", but from the computer's point of view, it is just a piece of text. As such, you cannot use it to add, subtract etc.
Using the float.Parse(...) method, you tell the computer: "This piece of text actually represents a floating point number, could you please read the text and give me back a number, so that I can start doing math with it?".

The method you are using, float.Parse() is just one of many such methods that take a String input value, and attempt to convert it into the target type, here a float.
There is a safer alternative, however, and it is TryParse():
float a;
if (float.TryParse(Console.ReadLine(), out a))
{
//do something with your new float 'a'
}
In either case, your are asking the framework to inspect the value you provide, and attempt to make a conversion into the requested type. This topic can be quite deep, so you'll want to consult MSDN for the specifics.

Console.ReadLine reads text that the user inputs and returns it to the program so that you may do with it what you want. Therefore, the ReadLine method returns a string.
If you want to work with a decimal (check the decimal class instead of float), you need to convert the string, which is a character sequence, to a number of your desired type, that's where float.Parse comes in:
float.Parse accepts a string and if possible, returns a float value.
Almost every type contains the Parse method which is used to transform a string into the calling one.

How to use float.Parse to get decimal from string like "5/2"

If I had a string "5/2", how could I use float.Parse to get 2.5? When I do it inside Unity3D I get an Invalid Format error. It works for whole numbers, like "5" would get 5, but I'm making a graphing calculator and a lot of times the slope of line is a fraction.

It is not a valid number, it is an expression which needs to be evaluated. you can do that using DataTable.Compute. You can evaluate more complex expressions too using this technique.
var result = new DataTable().Compute("5/2",null);
Note: Datatable is expensive, so you can create a instance or static member which holds the reference of DataTable for you.
Read more about compute in MSDN.

You'll need to split the string, parse the values individually and then do the division. So :
string[] tokens = input.Split('/');
float result = float.Parse(tokens[0])/float.Parse(tokens[1]);
Of course, you should add error handling to this, but that is "Proof of Concept" quality code.

sql type float, real, decimal?

well in my database i had a colum for price of one product
i had it as float, my problem is if i saved it since my c# application
as 10.50 .. in a query it returns 10,50 and if i update i get a error
10,50 cant convert to float ... or something so..
and if i saved it as decimal, in queries inside sql management .. are ok..
but in my c# application... i get the same error..
10.50 retuns as 10,50 i dont know why, and how to solved it.. my unique solution is saved it
as varchar...

That's a localisation problem of some sort. 10,50 is the "European" way of writing ten and a half. If you're getting that from your select statements then your database is probably configured incorrectly.

Generally speaking you should use the same type throughout your layers. So if the underlying types in the database are x, you should pass around those data with identical types in c#, too.
What type you choose depends on what you are storing--you shouldn't be switching around types just to get something to "work". To that end, storing numeric data in a non-numeric type (e.g. varchar) will come back to bite you very soon. It's good you've opened this question to fix that!
As others have miraculously inferred, you are likely running into a localization issue. This is a great example of why storing numbers as strings is a problem. If you properly accept user input in whatever culture/localization they want (or you want), and get it into a numeric-type variable, then the rest (talking to the DB) should be easy. More so, you should not do number formatting in the database if you can help it--that stuff is much better placed at the front end, closer to the users.

I think your setting in windows regional and language for decimal symbol is wrong.please set it to dot and again test it.

This may help out for temporary use but I wouldn't recommend it for permanent use:
Try making it so that just before you save the file, convert the number to a string, replace the commas with periods (From , to .) and then save it into the database as the string, hopefully it should see that it is in the correct format and turn it into what the database sees as "Decimal" or "Floating".
Hope this helps.

Yep, localization.
That said, I think your pice is being stored on a "money" field in SQLServer (I'm assuming it's SQLServer you're using). If that was a float in the DB, it would return it with a normal decimal point, and not the European money separator ",".
To fix:
Fist DO NO USE FLOAT in your c# code, unless you absolutely require a floating point number. Use the decimal type instead. That's not just in this case, but in all cases. Floating point numbers are binary (base-2), not decimal (base-10), so what you see in the interface is only a decimal approximation of the actual number. The result is that frequently (1 == 1) evaluates as false!
I've run into that problem myself, and it's maddening if you don't know that can happen. Always use decimal instead of float in c#.
Ok, after you've fixed that, then do this to get the right localization:
using System.Globalization;
...
NumberFormatInfo ni = new NumberFormatInfo();
ni.CurrencyDecimalSeparator = ",";
decimal price = decimal.Parse(dbPriceDataField, ni);
Note that "dbPriceDataField" must be a string, so you may have to do a ".ToString()" on that db resultset's field.
If you end up having to handle other "money" aspects of that money field, like currency symbols, check out: http://msdn.microsoft.com/en-us/library/system.globalization.numberformatinfo.aspx
If you need more robust error handling, either put that decimal.Parse in a try/catch, or use decimal.TryParse.
EDIT --
If you know what culture (really, country), the db is set to, you can do this instead:
using System.Globalization;
...
CultureInfo ci = new CultureInfo("fr-FR"); // fr-FR being "french France"
decimal price = decimal.Parse(dbprice, ci.NumberFormat);

Such problems were faced by me in my Web Apps... but i found the solution like I was fetching my price value in textbox. So I was have database attached with that. So when you attached your database with textbox... When you right click textbox and click Edit DataBinding.... in that you have to provide.... type like in Bind Property..... {0:N2}
This will work only for web apps or websites... not for desktop applications...

Suggest a good method for having a number with high precision in C#

I am programming on a project which I should store the key of the user to the initial configuration of a machine, I want to write it in C#.
I have an initial configuration which consists of two number R and X0, R = 3.9988 and X0 = 0.5. I want to add the user key to these numbers. for example:
Key: hos110 =>
R = 3.9988104111115049049048
X0 = 0.5104111115049049048
104111115049049048 are ASCII codes of the key which are concatenated.
How can I store these numbers?
Is there a better method for doing this?
Update: How about MATLAB?

You're not really "adding" numbers. You are concatenating strings.
Store them as strings. You can't get much more precise than that.
If you need to perform any arithmetic operations, it is easy enough to convert them to a decimal number on the fly.

I don't really follow why you're using a key as part of a number, but leaving that aside... System.Decimal (aka decimal) seems like the right tool for the job here.

If you need infinite precision you need something that is called BigInteger. However these classes are usually only used for scientific calculations (and usually unsuited for stroring the data) which doesn't really seem to match your code sample. If you need to do only general calculations use Strings and then convert them to Decimal for the calculations.
However if you are looking for such a BigInterger Class you can find one here.
.Net 4.0 will have a BigInteger built-in-class in the class libraries named System.Numerics.BigInteger.

Well, depending on the precision you are trying to achieve, you can probably save these as a pair of decimal values.
However, if this is an ASCII code, you may just want to save these as a string directly. This will avoid the numerical precision issues, especially if you're going to pull off the 104111... prior to using this information.

It seems that you are storing a "key", so why not use a String then?

Floating point numbers are inherently imprecise. I'm not sure what this 'initial configuration' is or why it's a float, but you're not going to be able to tack on a 'user key' (whatever that may be) and recover it later. Store the user key separately, in a string or something.

If these 'numbers' have no numeric value, i.e. you will not use them for mathematical computation then there is no need to store them in a numeric datatype. You can store them as strings.