Repeat

using System.Text;

namespace MyExtensions
{
    public static class StringExtensions
    {
        public static string Repeat(this string input, int count)
        {
            if (input == null)
            {
                return null; 
            }

            var sb = new StringBuilder();

            for (var repeat = 0; repeat < count; repeat++)
            {
                sb.Append(input);
            }

            return sb.ToString();
        }
    }
}
Example: 
using System;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using MyExtensions;


namespace StringExtensionsTests
{
    [TestClass]
    public class StringExtensionsTests
    {
        [TestMethod]
        public void RepeatBlank10()
        {
            var repeat10 = "".Repeat(10);
            Assert.AreEqual(0, repeat10.Length);
        }

        [TestMethod]
        public void RepeatNull10()
        {
            string nullString = null; 
// ReSharper disable once ExpressionIsAlwaysNull
            var repeat10 = nullString.Repeat(10);
            Assert.IsNull(repeat10);
        }

        [TestMethod]
        public void RepeatSingle0()
        {
            var repeat10 = "x".Repeat(0);
            Assert.AreEqual(0, repeat10.Length);

        }

        [TestMethod]
        public void RepeatSingle10()
        {
            var repeat10 = "x".Repeat(10);
            Assert.AreEqual(10, repeat10.Length);

        }

        [TestMethod]
        public void RepeatMulti10()
        {
            var repeat10 = "xxx".Repeat(10);
            Assert.AreEqual(30, repeat10.Length);

        }

    
    }
}

Description

Repeat a string N times

Details

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