Interview calculator

calculator.cs

// Entry point name must be "Solution"
using System;
using System.Linq;
using System.Collections.Generic;

public static class Solution
{
    private static void Main()
    {
        Console.WriteLine("*** Test Results ***");
        Tests.TestAll();
    }
}

public class Calculator {
    private string _input;
    private IEnumerable<string> _tokens;
    public Calculator(string input) {
        _input = input;
        _tokens = Utils.SplitStringToTokens(_input);
    }
    
    public int GetResult() {
        string firstNumber = _tokens.ElementAt(0);
        //string op = _tokens.ElementAt(1);
        string secondNumber = _tokens.ElementAt(2);
        
        var first = Utils.NumberFromString(firstNumber);
        var second = Utils.NumberFromString(secondNumber);
        
        return first + second;
    }
}

public static class Utils {
    public static int NumberFromString(string input) {
        return int.Parse(input);
    }
    
    public static IEnumerable<string> SplitStringToTokens(string input) {
        string nextToken = string.Empty;
        foreach (var currentChar in input) {
            if (Char.IsNumber(currentChar)) {
                nextToken += currentChar;
                continue;
            }
            if (!string.IsNullOrEmpty(nextToken)) {
                yield return nextToken;
                nextToken = string.Empty;
            }
            yield return currentChar.ToString();
        }
        yield return nextToken;
    }
}

public static class Tests {
    public static void TestAll() {
        Test1();
    }
    
    public static void Test1() {
        var input = "1+2";
        const int EXPECTED = 3;
        AssertEqual(EXPECTED, new Calculator(input).GetResult());
    }
    
    private static void AssertEqual(object expected, object actual) {
        if (!expected.Equals(actual)) {
            Console.WriteLine("X Expected {0}, actual result: {1}", expected, actual);
        } else {
            Console.WriteLine("  {0} == {1}", expected, actual);
        }
    }
}