C# 8 port of F# brace matching

CurlyBraces.cs

// 
// C# 8 port of F# brace matching.
//
using System;
using System.Linq;
using Microsoft.VisualStudio.TestTools.UnitTesting;

namespace BraceMatching
{
    using ParseState = Nullable<int>; // we get to cheat a bit because the C# compiler will "map" over nullable types implicitly
    using ParseResult = Boolean;

    [TestClass]
    public class CurlyBraces
    {
        public ParseResult AreCurlyBracesMatched(string input)
        {
            static ParseState OpenBrace(ParseState state) =>
                state + 1;
            static ParseState ClosingBrace(ParseState state) =>
                state - 1 as int? switch
                {
                    int n when n >= 0 => n,
                    _ => null,
                };

            return input
                .Aggregate(
                    0 as int?,
                    (result, character) => character switch
                    {
                        '{' => OpenBrace(result),
                        '}' => ClosingBrace(result),
                        _ => throw new ArgumentException("Invalid character")
                    }
                ) switch
                {
                    0 => true,
                    _ => false
                };
        }

        [DataTestMethod]
        [DataRow("{}", true)]
        [DataRow("{{}}", true)]
        [DataRow("{{{{}", false)]
        [DataRow("{}}}}", false)]
        [DataRow("{", false)]
        [DataRow("}", false)]
        // [DataRow("}{", true)] fails in original algorithm?
        public void Test_AreCurlyBracesMatched(string input, bool expected)
        {
            bool result = AreCurlyBracesMatched(input);
            Assert.AreEqual(expected, result);
        }
    }
}