A learning test for Expression Evaluator

ExpressionTests.cs

using System;
using System.Globalization;
using ExpressionEvaluator;
using FluentAssertions;
using NUnit.Framework;

namespace Hello.Expressions
{
    [TestFixture]
    class ExpressionTests
    {
        [Test]
        public void DateTimeTest()
        {
            const string dateFormat = "yyyy-MM-dd HH:mm:ss.fff";
            var culture = CultureInfo.InvariantCulture;
            var reg = new TypeRegistry();
            reg.RegisterType<DateTime>("DateTime");
            reg.RegisterType<TimeSpan>("TimeSpan");
            reg.RegisterSymbol("InvariantCulture", culture);

            var expression = new CompiledExpression<DateTime>("DateTime.Now - TimeSpan.FromDays(1)") { TypeRegistry = reg };
            var actual = expression.Eval();
            var expected = DateTime.Now - TimeSpan.FromDays(1);
            actual.Should().BeCloseTo(expected);

            var input = String.Format("DateTime.ParseExact('{0}', '{1}', InvariantCulture)", expected.ToString(dateFormat), dateFormat);
            expression = new CompiledExpression<DateTime>(input) { TypeRegistry = reg};
            actual = expression.Eval();
            actual.Should().BeCloseTo(expected);

            reg.RegisterSymbol("d", new SimplerDate(dateFormat, culture));
            input = String.Format("d.Parse('{0}')", expected.ToString(dateFormat));
            expression = new CompiledExpression<DateTime>(input) { TypeRegistry = reg };
            actual = expression.Eval();
            actual.Should().BeCloseTo(expected);

        }

        class SimplerDate
        {
            public const string DefaultFormat = "yyyy-MM-dd HH:mm:ss.fff";
            private readonly string _format;
            private readonly IFormatProvider _culture;

            public SimplerDate(string format = null, IFormatProvider culture = null)
            {
                _format = format ?? DefaultFormat;
                _culture = culture ?? CultureInfo.InvariantCulture;
            }

            public DateTime Parse(string value)
            {
                return DateTime.ParseExact(value, _format, _culture);
            }
        }
    }
}

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