This snippet takes an arithmetic expression as a string and attempts to evaluate it

Calc.cs

using System;
using System.Linq;

public class Calc
{
    /// <summary>
    /// Takes a mathematical expression and tries to evaluate it
    /// </summary>
    /// <param name="expression">The mathematical expression</param>
    /// <returns>The result of evaluating the string</returns>
    public double DoCalc(string expression)
    {
        string[] parts = expression.Split(' ');
        while (parts.Contains("/"))
        {
            parts = Division(parts);
        }
        while (parts.Contains("*"))
        {
            parts = Multiplication(parts);
        }
        while (parts.Contains("+"))
        {
            parts = Addition(parts);
        }
        while (parts.Contains("-"))
        {
            parts = Subtraction(parts);
        }
        return double.Parse(parts[0]);
    }

    public string[] Division(string[] parts)
    {
        int index = Array.IndexOf(parts, "/");
        if (index != 0 || index != -1)
        {
            double value = double.Parse(parts[index - 1]) / double.Parse(parts[index + 1]);
            string[] result = new string[parts.Length - 2];
            for (int i = 0; i < index - 1; i++)
            {
                result[i] = parts[i];
            }
            for (int j = parts.Length; j > index + 1; j--)
            {
                result[j - 3] = parts[j - 1];
            }
            result[index - 1] = value.ToString();
            return result;
        }
        else
        {
            return parts;
        }
    }


    public string[] Multiplication(string[] parts)
    {
        int index = Array.IndexOf(parts, "*");
        if (index != 0 || index != -1)
        {
            double value = double.Parse(parts[index - 1]) * double.Parse(parts[index + 1]);
            string[] result = new string[parts.Length - 2];
            for (int i = 0; i < index - 1; i++)
            {
                result[i] = parts[i];
            }
            for (int j = parts.Length; j > index + 1; j--)
            {
                result[j - 3] = parts[j - 1];
            }
            result[index - 1] = value.ToString();
            return result;
        }
        else
        {
            return parts;
        }
    }

    public string[] Addition(string[] parts)
    {
        int index = Array.IndexOf(parts, "+");
        if (index != 0 || index != -1)
        {
            double value = double.Parse(parts[index - 1]) + double.Parse(parts[index + 1]);
            string[] result = new string[parts.Length - 2];
            for (int i = 0; i < index - 1; i++)
            {
                result[i] = parts[i];
            }
            for (int j = parts.Length; j > index + 1; j--)
            {
                result[j - 3] = parts[j - 1];
            }
            result[index - 1] = value.ToString();
            return result;
        }
        else
        {
            return parts;
        }
    }

    public string[] Subtraction(string[] parts)
    {
        int index = Array.IndexOf(parts, "-");
        if (index != 0 || index != -1)
        {
            double value = double.Parse(parts[index - 1]) - double.Parse(parts[index + 1]);
            string[] result = new string[parts.Length - 2];
            for (int i = 0; i < index - 1; i++)
            {
                result[i] = parts[i];
            }
            for (int j = parts.Length; j > index + 1; j--)
            {
                result[j - 3] = parts[j - 1];
            }
            result[index - 1] = value.ToString();
            return result;
        }
        else
        {
            return parts;
        }
    }
}