How to Generate Unique Number of 8 digits?

Question

I am using this code to generate a 8 digit unique number.

byte[] buffer = Guid.NewGuid().ToByteArray();
return BitConverter.ToUInt32(buffer, 8).ToString();
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Answer 1

This method will generate a random string, it doesn't rely on the Random method and also is far better than the guid approch :

public static string gen_Digits(int length)
{
    var rndDigits = new System.Text.StringBuilder().Insert(0, "0123456789", length).ToString().ToCharArray();
    return string.Join("", rndDigits.OrderBy(o => Guid.NewGuid()).Take(length));
}

you can increase the length for less collision chance, but to have a 100% unique sequence you have to persist the old generated values and check the newly created value for uniquness.