C# – How to improve this C# randomising method

c++randomsorting

I think I've settled on this as the most simple and unit-testable method for randomising a list, but would be interested to hear of any improvements.

public static IList<T> RandomiseList<T>(IList<T> list, int seed)
{
    Random random = new Random(seed);
    List<T> takeFrom = new List<T>(list);
    List<T> ret = new List<T>(takeFrom.Count);

    while (takeFrom.Count > 0)
    {
        int pos = random.Next(0, takeFrom.Count - 1);
        T item = takeFrom[pos];
        takeFrom.RemoveAt(pos);
        ret.Add(item);
    }

    return ret;
}

Best Solution

You want a shuffle, and the best way to do that is the Fisher-Yates shuffle:

public static IList<T> Randomise<T>(IList<T> list, int seed) 
{
    Random rng = new Random(seed); 

    List<T> ret = new List<T>(list);      
    int n = ret.Length;            
    while (n > 1) 
    {
        n--;                         
        int k = rng.Next(n + 1);  
        // Simple swap of variables
        T tmp = list[k];
        ret[k] = ret[n];
        ret[n] = tmp;
    }
    return ret;
}
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