Generate specific length random id.

GenerateRandomId.cs

void Main()
{
    // conflict with following length. more than 7 is much stable.
    // 5 = 50/100000
    // 6 = 5/100000
    // 7 = 0/100000
    var rnd = new Random(Guid.NewGuid().GetHashCode());
    var coupon = new string[100000];
    var dup = 0;
    for (var j = 0; j < 100; j++)
    {
        for (var i = 0; i < coupon.Length; i++)
        {
            coupon[i] = GenerateCoupon(7, rnd);
        }
        var count = coupon.Distinct().Count();
        if (count != 100000)
        {
            count.Dump();
            dup++;
        }
        j++;
    }
    dup.Dump();
}

// https://codereview.stackexchange.com/questions/5983/random-string-generation
public static string GenerateCoupon(int length, Random random)
{
    var characters = "0123456789ABCDEFGHIJKLMNOPQRSTUVWXYZabcdefghijklmnopqrstuvwxyz";
    var result = new StringBuilder(length);
    for (var i = 0; i < length; i++)
    {
        result.Append(characters[random.Next(characters.Length)]);
    }
    return result.ToString();
}