How to convert numbers between hexadecimal and decimal

To convert from decimal to hex do...

string hexValue = decValue.ToString("X");

To convert from hex to decimal do either...

int decValue = int.Parse(hexValue, System.Globalization.NumberStyles.HexNumber);

or

int decValue = Convert.ToInt32(hexValue, 16);

Hex -> decimal:

Convert.ToInt64(hexValue, 16);

Decimal -> Hex

string.format("{0:x}", decValue);

It looks like you can say

Convert.ToInt64(value, 16)

to get the decimal from hexdecimal.

The other way around is:

otherVar.ToString("X");

If you want maximum performance when doing conversion from hex to decimal number, you can use the approach with pre-populated table of hex-to-decimal values.

Here is the code that illustrates that idea. My performance tests showed that it can be 20%-40% faster than Convert.ToInt32(...):

class TableConvert
  {
      static sbyte[] unhex_table =
      { -1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1
       ,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1
       ,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1
       , 0, 1, 2, 3, 4, 5, 6, 7, 8, 9,-1,-1,-1,-1,-1,-1
       ,-1,10,11,12,13,14,15,-1,-1,-1,-1,-1,-1,-1,-1,-1
       ,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1
       ,-1,10,11,12,13,14,15,-1,-1,-1,-1,-1,-1,-1,-1,-1
       ,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1,-1
      };

      public static int Convert(string hexNumber)
      {
          int decValue = unhex_table[(byte)hexNumber[0]];
          for (int i = 1; i < hexNumber.Length; i++)
          {
              decValue *= 16;
              decValue += unhex_table[(byte)hexNumber[i]];
          }
          return decValue;
      }
  }