Program.cs

using System;

namespace ExpressionParser
{
   public class Program
   {
      private static Dictionary<string, Type> variablesTypes = new Dictionary<string, Type>();
      private static Dictionary<string, object> variables = new Dictionary<string, object>();

      public static void Main(string[] args)
      {

         variablesTypes.Add("var1", typeof (int));
         variablesTypes.Add("var2", typeof (int));
         variablesTypes.Add("var3", typeof(string));

         variables = new Dictionary<string, object>();

         variables.Add("var1", 5);
         variables.Add("var2", 4);
         variables.Add("var3", "Eeeemily");

         RunTest("CONCAT('HELLO ', 'WORLD ', {var3}, ' ',{var1})");
         RunTest("1 + 2 * 3");
         RunTest("(1 + 2) * 3");
         RunTest("-(1 + 2) * 3");
         RunTest("-(1 + 2) * -3");
      }

      private static void RunTest(string expression)
      {
         Console.WriteLine($"Test: '{expression}'");
         var expressionParser = new ExpressionParser(variablesTypes);
         var x = expressionParser.Evaluate(expression, variables);
         Console.WriteLine($"{x.GetType().ToString()} : {x.ToString()}");
         Console.WriteLine();
      }
   }
}