Simple math calculator

Calc.csproj

<?xml version="1.0" encoding="utf-8"?>
<Project ToolsVersion="14.0" DefaultTargets="Build" xmlns="http://schemas.microsoft.com/developer/msbuild/2003">
  <Import Project="packages\Antlr4.CodeGenerator.4.6.4\build\Antlr4.CodeGenerator.props" Condition="Exists('packages\Antlr4.CodeGenerator.4.6.4\build\Antlr4.CodeGenerator.props')" />
  <Import Project="$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props" Condition="Exists('$(MSBuildExtensionsPath)\$(MSBuildToolsVersion)\Microsoft.Common.props')" />
  <PropertyGroup>
    <Configuration Condition=" '$(Configuration)' == '' ">Debug</Configuration>
    <Platform Condition=" '$(Platform)' == '' ">AnyCPU</Platform>
    <ProjectGuid>{EE3E54A6-D9F2-47D9-A017-0E3FCCB12700}</ProjectGuid>
    <OutputType>Exe</OutputType>
    <RootNamespace>Calc</RootNamespace>
    <AssemblyName>Calc</AssemblyName>
    <TargetFrameworkVersion>v4.6.1</TargetFrameworkVersion>
    <FileAlignment>512</FileAlignment>
    <AutoGenerateBindingRedirects>true</AutoGenerateBindingRedirects>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Debug|AnyCPU' ">
    <PlatformTarget>AnyCPU</PlatformTarget>
    <DebugSymbols>true</DebugSymbols>
    <DebugType>full</DebugType>
    <Optimize>false</Optimize>
    <OutputPath>bin\Debug\</OutputPath>
    <DefineConstants>DEBUG;TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <PropertyGroup Condition=" '$(Configuration)|$(Platform)' == 'Release|AnyCPU' ">
    <PlatformTarget>AnyCPU</PlatformTarget>
    <DebugType>pdbonly</DebugType>
    <Optimize>true</Optimize>
    <OutputPath>bin\Release\</OutputPath>
    <DefineConstants>TRACE</DefineConstants>
    <ErrorReport>prompt</ErrorReport>
    <WarningLevel>4</WarningLevel>
  </PropertyGroup>
  <ItemGroup>
    <Reference Include="Antlr4.Runtime, Version=4.6.0.0, Culture=neutral, PublicKeyToken=09abb75b9ed49849, processorArchitecture=MSIL">
      <HintPath>packages\Antlr4.Runtime.4.6.4\lib\net45\Antlr4.Runtime.dll</HintPath>
      <Private>True</Private>
    </Reference>
    <Reference Include="System" />
    <Reference Include="System.Core" />
    <Reference Include="Microsoft.CSharp" />
  </ItemGroup>
  <ItemGroup>
    <Compile Include="Program.cs" />
  </ItemGroup>
  <ItemGroup>
    <None Include="App.config" />
    <Antlr4 Include="Calc.g4">
      <Generator>MSBuild:Compile</Generator>
      <CustomToolNamespace>Calc</CustomToolNamespace>
    </Antlr4>
    <None Include="packages.config" />
  </ItemGroup>
  <Import Project="$(MSBuildToolsPath)\Microsoft.CSharp.targets" />
  <Target Name="EnsureNuGetPackageBuildImports" BeforeTargets="PrepareForBuild">
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</Project>

Calc.g4

grammar Calc;

/*
 * Parser Rules
 */
expr
    : expr PLUS expr     #addExpr
    | expr ASTERISK expr #mulExpr
    | LPR expr RPR       #parExpr
    | NUMBER             #numExpr
    ;

/*
 * Lexer Rules
 */
WS: ' ' -> channel(HIDDEN);

fragment DIGIT: [0-9];
NUMBER: DIGIT+;

LPR: '(';
RPR: ')';
PLUS: '+';
ASTERISK: '*';

packages.config

<?xml version="1.0" encoding="utf-8"?>
<packages>
  <package id="Antlr4" version="4.6.4" targetFramework="net461" developmentDependency="true" />
  <package id="Antlr4.CodeGenerator" version="4.6.4" targetFramework="net461" developmentDependency="true" />
  <package id="Antlr4.Runtime" version="4.6.4" targetFramework="net461" />
</packages>

Program.cs

using System;
using Antlr4.Runtime;
using Antlr4.Runtime.Misc;

namespace Calc
{
    class Program
    {
        static void Main(string[] args)
        {
            var inputStream = new AntlrInputStream(Console.ReadLine());
            var lexer = new CalcLexer(inputStream);
            var token = new CommonTokenStream(lexer);
            var parser = new CalcParser(token);

            var ctx = parser.expr();
            var visitor = new CalcVisitor();
            Console.WriteLine(visitor.Visit(ctx));

            Console.ReadLine();
        }
    }

    class CalcVisitor : CalcBaseVisitor<int>
    {
        public override int VisitAddExpr([NotNull] CalcParser.AddExprContext context)
            => Visit(context.expr(0)) + Visit(context.expr(1));

        public override int VisitMulExpr([NotNull] CalcParser.MulExprContext context)
            => Visit(context.expr(0)) * Visit(context.expr(1));

        public override int VisitParExpr([NotNull] CalcParser.ParExprContext context)
            => Visit(context.expr());

        public override int VisitNumExpr([NotNull] CalcParser.NumExprContext context)
            => int.Parse(context.NUMBER().GetText());
    }
}