C# Generic method for random number generation

Question

I see that there is a similar question for C++. Does anyone know why this method works when the method is non-generic, but as soon as I make it generic, the random number portion of code fails? Error: Cannot implicitly convert type int to 'T'. If I can't use generics, I will have to rewrite the same function over and over for each different length of array.

public void fillGenericArray<T>(T[] inputArray) where T : IComparable
{
    var randomNumb1 = new Random();

    for (int i = 0; i < inputArray.Length - 1; i++)
    {
        Console.WriteLine($"{inputArray[i] = randomNumb1.Next(1, 501)},");
    }
}

Answer 1

I had to look twice at this, but here's the issue:

Because inputArray is an 'array of type T'

then even though i is an int the expression

inputArray[i] 

returns a type T not a type int.

And so, conversely, a type T must be assigned to it.

A generic method like this might achieve your goal:

public static void fillGenericArray<T>(T[] inputArray)
{
    for (int i = 0; i < inputArray.Length; i++)
    {
        // Where T has a CTor that takes an int as an argument
        inputArray[i] = (T)Activator.CreateInstance(typeof(T), Random.Next(1, 501));
    }
}

(Thanks to this SO post for refreshing my memory about instantiating T with arguments.)

You could also use Enumerable.Range() to get the same result without writing a method at all:

// Generically, for any 'SomeClass' with a CTor(int value)
SomeClass[] arrayOfT = 
    Enumerable.Range(1, LENGTH).Select(i => new SomeClass(Random.Next(1, 501)))
    .ToArray();

(Slightly Modified with help from this SO post ) - see the answer using Enumerable.Range().

Here is a test runner:

class Program
{
    static Random Random { get; } = new Random();
    const int LENGTH = 10;
    static void Main(string[] args)
    {

        Console.WriteLine();
        Console.WriteLine("With a generic you could do this...");


        SomeClass[] arrayOfT;
        arrayOfT = new SomeClass[LENGTH];
        fillGenericArray<SomeClass>(arrayOfT);
        Console.WriteLine(string.Join(Environment.NewLine, arrayOfT.Select(field=>field.Value)));


        Console.WriteLine();
        Console.WriteLine("But perhaps it's redundant, because Enumerable is already Generic!");

        arrayOfT = Enumerable.Range(1, LENGTH).Select(i => new SomeClass(Random.Next(1, 501))).ToArray();
        Console.WriteLine(string.Join(Environment.NewLine, arrayOfT.Select(field => field.Value)));

        // Pause
        Console.WriteLine(Environment.NewLine + "Any key to exit");
        Console.ReadKey();
    }
    public static void fillGenericArray<T>(T[] inputArray)
    {
        for (int i = 0; i < inputArray.Length; i++)
        {
            inputArray[i] = (T)Activator.CreateInstance(typeof(T), Random.Next(1, 501));
        }
    }
    class SomeClass
    {
        public SomeClass(int value)
        {
            Value = value;
        }
        public int Value { get; set; }
    }
}

Clone or Download this example from GitHub.

Answer 2

There is no reason to use generics. Just replace T with int and you will have function that does what you want (based on your question and comment below it).

EDIT: From your comment it seems you misunderstand the purpose of generics. The non-generic function WILL work for all lengths of the array.

And to answer why the change to generics fails. You are trying to assign int to generic type T which can be anything and compiler will not allow such a cast.