Generates a random key using an arbitrary character set

randomKey.cs

void Main()
{
    var charset = "0123456789abcdefghijklmnopqrstuvwxyzABCDEFGHIJKLMNOPQRSTUVWXYZ".ToCharArray(); //which characters can appear in the string
    var length = 32; //length of the string
    Console.WriteLine(GenerateKey(charset, length));
}

///Generate a random string of arbitrary length using RNGCryptoServiceProvider.
///characterSet is the set of characters that can appear in the string.
///length is the desired length of string.
public string GenerateKey(char[] characterSet, int length)
{
    var result = string.Empty;
    using(var rng = new RNGCryptoServiceProvider())
    {
        while(result.Length < length)
        {
            var bytes = new byte[4]; //4 bytes in a uint32
            rng.GetBytes(bytes);
            var value = BitConverter.ToUInt32(bytes, 0);
            result += ConvertToBase(value, characterSet);
        }
    }
    return result.Substring(0, length);
}

///Converts a uint to an arbitrary base.
///charset is the characters used to represent the base.
///The length of charset is the base being converted to.
///For example: ConvertToBase(13, new []{'0', '1'}) would convert 13 to base 2 (binary): 1101
public string ConvertToBase(uint value, char[] characterSet)
{
    var radix = (uint)characterSet.Length;
    if(characterSet == null)
    {
        throw new ArgumentNullException("charset");
    }
    if(characterSet.Length < 2)
    {
        throw new ArgumentException("charset must have at least 2 elements");
    }
    if(characterSet.Distinct().Count() < characterSet.Length)
    {
        throw new ArgumentException("charset has duplicate characters");
    }
    var result = new List<char>();
    do
    {
        var remainder = value % radix;
        value = value / radix;
        result.Add(characterSet[remainder]);
    } while(value > 0); //repeatedly dividing will always reach 0 because: baseN is 2 or greater, and in C#, 1 divided by 2 = 0
    result.Reverse();
    return new string(result.ToArray());
}