TryParseOrElseExtension.cs

namespace Extensions
{
    public static class TryParseOrElseExtension
    {
        public static int TryParseOrElse(this string value, int orElse)
        {
            return int.TryParse(value, out int result) ? result : orElse;
        }
    }
}

TryParseOrElseExtensionTest.cs

using NUnit.Framework;
using Extensions;

[TestFixture]
public class TryParseOrElseExtensionTest
{
    [Test]
    public void ShouldReturnTheValueIfTheStringIsAValidNumber()
    {
        int result = "10".TryParseOrElse(1);

        Assert.IsTrue(result == 10);
    }

    [Test]
    public void ShouldReturnOrElseValueIfTheStringIsNotAValidNumber()
    {
        int result = "whatever".TryParseOrElse(100);

        Assert.IsTrue(result == 100);
    }
}