Question

It is all in the title basically. It is simple, but I don’t know why my While loop is failing sometimes. Every now and then I would get a list that is of length 2 instead of 3.

Here is my C# code:

public List<int> generateRequiredSecretCode()
{
    List<int> placeHolder = new List<int>();
    Random random = new Random();
    int randomNo = random.Next(0, 10);
    while (!placeHolder.Contains(randomNo) && placeHolder.Count != 3)
    {
        placeHolder.Add(randomNo);
        randomNo = random.Next(0, 10);
    }
    return placeHolder;
}

Summary of my aim: I want a List of integers that is of length 3 and where each number in the list is between 0 and 9 and is unique

Answer 1

!placeHolder.Contains(randomNo) is your problem here because the while ends if the list contains the randomNo.
Check that !placeHolder.Contains(randomNo) in an inner if like this:

while (placeHolder.Count != 3)
{
    if( !placeHolder.Contains(randomNo) )
      placeHolder.Add(randomNo);
    randomNo = random.Next(0, 10);
}