/// Updates a .wxs or other text file containing a /// <?define version="..."?> preprocessor directive /// and increments the least-significant version /// grouping present and applicable to Major Upgrades.

App.config

<?xml version="1.0" encoding="utf-8" ?>
<configuration>
    <startup> 
        <supportedRuntime version="v4.0" sku=".NETFramework,Version=v4.5" />
    </startup>
</configuration>

VersionUpdater.cs

using System;
using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;

namespace VersionUpdater
{
    class VersionUpdater
    {
        /// <summary> 
        /// 
        /// Updates a .wxs or other text file containing a 
        /// <?define version="..."?> preprocessor directive 
        /// and increments the least-significant version 
        /// grouping present and applicable to Major Upgrades. 
        /// 
        /// <?define version="3.0.0"?> becomes <?define version="3.0.1"?> 
        /// <?define version="3.0.0.0"?> becomes <?define version="3.0.1.0"?> 
        /// <?define version="3.0"?> becomes <?define version="3.1"?> 
        /// <?define version="3"?> becomes <?define version="4"?> 
        /// 
        /// For reference within the WiX project as in this example: <Product Version="$(var.version)" /> 
        /// 
        /// In Votive, a pre-build event of "...VersionUpdater.exe" "$(ProjectDir)Product.wxs" updates 
        /// the version number for each build. 
        /// 
        /// </summary> 
        /// <param name="args">A single command-line parameter</param> 
        static void Main(string[] args)
        {
            if (args.Length < 1)
            {
                Console.WriteLine("Error: Expected the path to a file to update as an argument");
                Console.ReadKey();
                Environment.Exit(1);
            }

            if (!File.Exists(args[0]))
            {
                Console.WriteLine("Error: File {0} not found", args[0]);
                Console.ReadKey();
                Environment.Exit(2);
            }

            using (var fs = File.Open(args[0], FileMode.Open, FileAccess.ReadWrite, FileShare.None))
            {
                using (StreamReader sr = new StreamReader(fs))
                {
                    string data = sr.ReadToEnd(); // Read in the entire file 

                    fs.Seek(0, SeekOrigin.Begin); // Reset to the beginning of the file 

                    System.Text.RegularExpressions.Regex regEx = new System.Text.RegularExpressions.Regex(@"\<\?define\s+version\s*\=\s*\""(?<majorMinor>(\d*\.){0,2})(?<version>\d*)(.\d*)?\""\s*\?\>");

                    var versionMatch = regEx.Match(data);
                    var version = versionMatch.Groups["version"];
                    var majorMinor = versionMatch.Groups["majorMinor"];

                    if (!version.Success || !majorMinor.Success)
                    {
                        Console.WriteLine(
                            "Error: Version match not found.  " +
                            "Expected <?define version=\"w.x.y\"?> (Recommended), " +
                            "<?define version=\"w\"?>, <?define version=\"w.x\"?>, " +
                            "or <?define version=\"w.x.y.z\"?>",
                            args[0]);
                        Console.ReadKey();
                        Environment.Exit(3);
                    }

                    int versionNumber;

                    int.TryParse(version.Value, out versionNumber);

                    Console.WriteLine("Updating {0}{1} to {0}{2}",
                        majorMinor.Value,
                        versionNumber,
                        versionNumber + 1);

                    versionNumber++;

                    string newData = data.Substring(0, version.Index) + versionNumber.ToString() +
                        data.Substring(version.Index + version.Length);

                    using (StreamWriter sw = new StreamWriter(fs))
                    {
                        sw.Write(newData);
                        sw.Flush();
                        fs.SetLength(newData.Length);
                        fs.Flush();
                    }
                }
            }
        }
    }
}