BigInteger Structure

You can instantiate a BigInteger object in several ways:

• You can use the new keyword and provide any integral or floating-point value as a parameter to the BigInteger constructor. (Floating-point values are truncated before they are assigned to the BigInteger.) The following example illustrates how to use the new keyword to instantiate BigInteger values.

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  BigInteger bigIntFromDouble = new BigInteger(179032.6541);
  Console.WriteLine(bigIntFromDouble);
  BigInteger bigIntFromInt64 = new BigInteger(934157136952);
  Console.WriteLine(bigIntFromInt64);
  // The example displays the following output:
  //   179032
  //   934157136952		
  
  
  

• You can declare a BigInteger variable and assign it a value just as you would any numeric type, as long as that value is an integral type. The following example uses assignment to create a BigInteger value from an Int64.

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  long longValue = 6315489358112;      
  BigInteger assignedFromLong = longValue;
  Console.WriteLine(assignedFromLong);
  // The example displays the following output:
  //   6315489358112
  
  
  

• You can assign a decimal or floating-point value to a BigInteger object if you cast the value or convert it first. The following example explicitly casts (in C#) or converts (in Visual Basic) a Double and a Decimal value to a BigInteger.

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  BigInteger assignedFromDouble = (BigInteger) 179032.6541;
  Console.WriteLine(assignedFromDouble);   
  BigInteger assignedFromDecimal = (BigInteger) 64312.65m;      
  Console.WriteLine(assignedFromDecimal);
  // The example displays the following output:
  //   179032
  //   64312      
  
  
  

These methods enable you to instantiate a BigInteger object whose value is in the range of one of the existing numeric types only. You can instantiate a BigInteger object whose value can exceed the range of the existing numeric types in one of three ways:

• You can use the new keyword and provide a byte array of any size to the BigInteger.BigInteger constructor. For example:

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  byte[] byteArray = { 10, 9, 8, 7, 6, 5, 4, 3, 2, 1, 0};
  BigInteger newBigInt = new BigInteger(byteArray);
  Console.WriteLine("The value of newBigInt is {0} (or 0x{0:x}).", newBigInt);    
  // The example displays the following output:
  //   The value of newBigInt is 4759477275222530853130 (or 0x102030405060708090a).
  
  
  

• You can call the Parse or TryParse methods to convert the string representation of a number to a BigInteger. For example:

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  string positiveString = "91389681247993671255432112000000";
  string negativeString = "-90315837410896312071002088037140000";
  BigInteger posBigInt = 0;
  BigInteger negBigInt = 0;
  
  try {
     posBigInt = BigInteger.Parse(positiveString);
     Console.WriteLine(posBigInt);
  }
  catch (FormatException)
  {
     Console.WriteLine("Unable to convert the string '{0}' to a BigInteger value.", 
                       positiveString);
  }
  
  if (BigInteger.TryParse(negativeString, out negBigInt))
    Console.WriteLine(negBigInt);
  else
     Console.WriteLine("Unable to convert the string '{0}' to a BigInteger value.", 
                        negativeString);
  
  // The example displays the following output:
  //   9.1389681247993671255432112E+31
  //   -9.0315837410896312071002088037E+34
  
  
  

• You can call a static (Shared in Visual Basic) BigInteger method that performs some operation on a numeric expression and returns a calculated BigInteger result. The following example does this by cubing UInt64.MaxValue and assigning the result to a BigInteger.

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  BigInteger number = BigInteger.Pow(UInt64.MaxValue, 3);
  Console.WriteLine(number);
  // The example displays the following output:
  //    6277101735386680762814942322444851025767571854389858533375
  
  
  

The uninitialized value of a BigInteger is Zero.

You can use a BigInteger instance as you would use any other integral type. BigInteger overloads the standard numeric operators to enable you to perform basic mathematical operations such as addition, subtraction, division, multiplication, subtraction, negation, and unary negation. You can also use the standard numeric operators to compare two BigInteger values with each other. Like the other integral types, BigInteger also supports the bitwise And, Or, XOr, left shift, and right shift operators. For languages that do not support custom operators, the BigInteger structure also provides equivalent methods for performing mathematical operations. These include Add, Divide, Multiply, Negate, Subtract, and several others.

Many members of the BigInteger structure correspond directly to members of the other integral types. In addition, BigInteger adds members such as the following:

• Sign, which returns a value that indicates the sign of a BigInteger value.

• Abs, which returns the absolute value of a BigInteger value.

• DivRem, which returns both the quotient and remainder of a division operation.

• GreatestCommonDivisor, which returns the greatest common divisor of two BigInteger values.

Many of these additional members correspond to the members of the Math class, which provides the functionality to work with the primitive numeric types.

The following example instantiates a BigInteger object and then increments its value by one.

BigInteger number = BigInteger.Multiply(Int64.MaxValue, 3);
number++;
Console.WriteLine(number);

Although this example appears to modify the value of the existing object, this is not the case. BigInteger objects are immutable, which means that internally, the common language runtime actually creates a new BigInteger object and assigns it a value one greater than its previous value. This new object is then returned to the caller.

Note
The other numeric types in the .NET Framework are also immutable. However, because the BigInteger type has no upper or lower bounds, its values can grow extremely large and have a measurable impact on performance.

Although this process is transparent to the caller, it does incur a performance penalty. In some cases, especially when repeated operations are performed in a loop on very large BigInteger values, that performance penalty can be significant. For example, in the following example, an operation is performed repetitively up to a million times, and a BigInteger value is incremented by one every time the operation succeeds.

BigInteger number = Int64.MaxValue ^ 5;
int repetitions = 1000000;
// Perform some repetitive operation 1 million times.
for (int ctr = 0; ctr <= repetitions; ctr++)
{
   // Perform some operation. If it fails, exit the loop.
   if (! SomeOperationSucceeds()) break;
   // The following code executes if the operation succeeds.
   number++;
}

In such a case, you can improve performance by performing all intermediate assignments to an Int32 variable. The final value of the variable can then be assigned to the BigInteger object when the loop exits. The following example provides an illustration.

BigInteger number = Int64.MaxValue ^ 5;
int repetitions = 1000000;
int actualRepetitions = 0;
// Perform some repetitive operation 1 million times.
for (int ctr = 0; ctr <= repetitions; ctr++)
{
   // Perform some operation. If it fails, exit the loop.
   if (! SomeOperationSucceeds()) break;
   // The following code executes if the operation succeeds.
   actualRepetitions++;
}
number += actualRepetitions;

If you convert BigInteger values to byte arrays, or if you convert byte arrays to BigInteger values, you must consider the order of bytes. The BigInteger structure expects the individual bytes in a byte array to appear in little-endian order (that is, the lower-order bytes of the value precede the higher-order bytes). You can round-trip a BigInteger value by calling the ToByteArray method and then passing the resulting byte array to the BigInteger(Byte[]) constructor, as the following example shows.

BigInteger number = BigInteger.Pow(Int64.MaxValue, 2);     
Console.WriteLine(number);

// Write the BigInteger value to a byte array.
byte[] bytes = number.ToByteArray();

// Display the byte array.
foreach (byte byteValue in bytes)
   Console.Write("0x{0:X2} ", byteValue);
Console.WriteLine();

// Restore the BigInteger value from a Byte array.
BigInteger newNumber = new BigInteger(bytes);
Console.WriteLine(newNumber);
// The example displays the following output:
//    8.5070591730234615847396907784E+37
//    0x01 0x00 0x00 0x00 0x00 0x00 0x00 0x00 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0xFF 0x3F
//    
//    8.5070591730234615847396907784E+37

To instantiate a BigInteger value from a byte array that represents a value of some other integral type, you can pass the integral value to the BitConverter.GetBytes method, and then pass the resulting byte array to the BigInteger(Byte[]) constructor. The following example instantiates a BigInteger value from a byte array that represents an Int16 value.

short originalValue = 30000;
Console.WriteLine(originalValue);

// Convert the Int16 value to a byte array.
byte[] bytes = BitConverter.GetBytes(originalValue);

// Display the byte array.
foreach (byte byteValue in bytes)
   Console.Write("0x{0} ", byteValue.ToString("X2"));
Console.WriteLine();

// Pass byte array to the BigInteger constructor.
BigInteger number = new BigInteger(bytes);
Console.WriteLine(number);
// The example displays the following output:
//       30000
//       0x30 0x75
//       30000

The BigInteger structure assumes that negative values are stored by using two's complement representation. Because the BigInteger structure represents a numeric value with no fixed length, the BigInteger(Byte[]) constructor always interprets the most significant bit of the last byte in the array as a sign bit. To prevent the BigInteger(Byte[]) constructor from confusing the two's complement representation of a negative value with the sign and magnitude representation of a positive value, positive values in which the most significant bit of the last byte in the byte array would ordinarily be set should include an additional byte whose value is 0. For example, 0xC0 0xBD 0xF0 0xFF is the little-endian hexadecimal representation of either -1,000,000 or 4,293,967,296. Because the most significant bit of the last byte in this array is on, the value of the byte array would be interpreted by the BigInteger(Byte[]) constructor as -1,000,000. To instantiate a BigInteger whose value is positive, a byte array whose elements are 0xC0 0xBD 0xF0 0xFF 0x00 must be passed to the constructor. The following example illustrates this.

int negativeNumber = -1000000;
uint positiveNumber = 4293967296;

byte[] negativeBytes = BitConverter.GetBytes(negativeNumber); 
BigInteger negativeBigInt = new BigInteger(negativeBytes);
Console.WriteLine(negativeBigInt.ToString("N0"));

byte[] tempPosBytes = BitConverter.GetBytes(positiveNumber);
byte[] positiveBytes = new byte[tempPosBytes.Length + 1];
Array.Copy(tempPosBytes, positiveBytes, tempPosBytes.Length);
BigInteger positiveBigInt = new BigInteger(positiveBytes);
Console.WriteLine(positiveBigInt.ToString("N0")); 
// The example displays the following output:
//    -1,000,000
//    4,293,967,296      

Byte arrays created by the ToByteArray method from positive values include this extra zero-value byte. Therefore, the BigInteger structure can successfully round-trip values by assigning them to, and then restoring them from, byte arrays, as the following example shows.

BigInteger positiveValue = 15777216;
BigInteger negativeValue  = -1000000;

Console.WriteLine("Positive value: " + positiveValue.ToString("N0"));
byte[] bytes = positiveValue.ToByteArray();

foreach (byte byteValue in bytes)
   Console.Write("{0:X2} ", byteValue);
Console.WriteLine();
positiveValue = new BigInteger(bytes);
Console.WriteLine("Restored positive value: " + positiveValue.ToString("N0"));

Console.WriteLine();

Console.WriteLine("Negative value: " + negativeValue.ToString("N0"));
bytes = negativeValue.ToByteArray();
foreach (byte byteValue in bytes)
   Console.Write("{0:X2} ", byteValue);
Console.WriteLine();
negativeValue = new BigInteger(bytes);
Console.WriteLine("Restored negative value: " + negativeValue.ToString("N0"));
// The example displays the following output:
//       Positive value: 15,777,216
//       C0 BD F0 00
//       Restored positive value: 15,777,216
//       
//       Negative value: -1,000,000
//       C0 BD F0
//       Restored negative value: -1,000,000

However, you may need to add this additional zero-value byte to byte arrays that are created dynamically by the developer or that are returned by methods that convert unsigned integers to byte arrays (such as BitConverter.GetBytes(UInt16), BitConverter.GetBytes(UInt32), and BitConverter.GetBytes(UInt64)).

When parsing a hexadecimal string, the BigInteger.Parse(String, NumberStyles) and BigInteger.Parse(String, NumberStyles, IFormatProvider) methods assume that if the most significant bit of the first byte in the string is set, or if the first hexadecimal digit of the string represents the lower four bits of a byte value, the value is represented by using two's complement representation. For example, both "FF01" and "F01" represent the decimal value -255. To differentiate positive from negative values, positive values should include a leading zero. The relevant overloads of the ToString method, when they are passed the "X" format string, add a leading zero to the returned hexadecimal string for positive values. This makes it possible to round-trip BigInteger values by using the ToString and Parse methods, as the following example shows.

BigInteger negativeNumber = -1000000;
BigInteger positiveNumber  = 15777216;

string negativeHex = negativeNumber.ToString("X");
string positiveHex = positiveNumber.ToString("X");

BigInteger negativeNumber2, positiveNumber2;  
negativeNumber2 = BigInteger.Parse(negativeHex, 
                                   NumberStyles.HexNumber);
positiveNumber2 = BigInteger.Parse(positiveHex,
                                   NumberStyles.HexNumber);

Console.WriteLine("Converted {0:N0} to {1} back to {2:N0}.", 
                   negativeNumber, negativeHex, negativeNumber2);                                         
Console.WriteLine("Converted {0:N0} to {1} back to {2:N0}.", 
                   positiveNumber, positiveHex, positiveNumber2);                                         
// The example displays the following output:
//       Converted -1,000,000 to F0BDC0 back to -1,000,000.
//       Converted 15,777,216 to 0F0BDC0 back to 15,777,216.

However, the hexadecimal strings created by calling the ToString methods of the other integral types or the overloads of the ToString method that include a toBase parameter do not indicate the sign of the value or the source data type from which the hexadecimal string was derived. Successfully instantiating a BigInteger value from such a string requires some additional logic. The following example provides one possible implementation.

using System;
using System.Globalization;
using System.Numerics;

public struct HexValue
{
   public int Sign;
   public string Value;
}

public class Example
{
   public static void Main()
   {
      uint positiveNumber = 4039543321;
      int negativeNumber = -255423975;

      // Convert the numbers to hex strings.
      HexValue hexValue1, hexValue2;
      hexValue1.Value = positiveNumber.ToString("X");
      hexValue1.Sign = Math.Sign(positiveNumber);

      hexValue2.Value = Convert.ToString(negativeNumber, 16);
      hexValue2.Sign = Math.Sign(negativeNumber);

      // Round-trip the hexadecimal values to BigInteger values.
      string hexString;
      BigInteger positiveBigInt, negativeBigInt;

      hexString = (hexValue1.Sign == 1 ? "0" : "") + hexValue1.Value;
      positiveBigInt = BigInteger.Parse(hexString, NumberStyles.HexNumber);      
      Console.WriteLine("Converted {0} to {1} and back to {2}.", 
                        positiveNumber, hexValue1.Value, positiveBigInt);

      hexString = (hexValue2.Sign == 1 ? "0" : "") + hexValue2.Value;
      negativeBigInt = BigInteger.Parse(hexString, NumberStyles.HexNumber);      
      Console.WriteLine("Converted {0} to {1} and back to {2}.", 
                        negativeNumber, hexValue2.Value, negativeBigInt);
   }
}
// The example displays the following output:
//       Converted 4039543321 to F0C68A19 and back to 4039543321.
//       Converted -255423975 to f0c68a19 and back to -255423975.