using System;

public class Program

{

    public static void Main()

    {

        Console.WriteLine("Hello World".Repeat(3));

        Console.WriteLine("Hello World".Repeat(0).Length);

        Console.WriteLine("".Repeat(10).Length);

    }

}

​

static class Extensions

{

    public static string Repeat(this string source, int times)

    {

        return string.Create(source.Length * times, source, RepeatFromString);

    }

    private static void RepeatFromString(Span<char> result, string source)

    {

        ReadOnlySpan<char> sourceSpan = source.AsSpan();

        for (var i = 0; i < result.Length; i += sourceSpan.Length)

            sourceSpan.CopyTo(result.Slice(i, sourceSpan.Length));

    }

}