This will combine multiple .cs files from a solution into a single source file.

SourceCombiner.cs

using System;
using System.Collections.Generic;
using System.Diagnostics;
using System.IO;
using System.Linq;
using System.Text;
using Microsoft.Build.Evaluation;
using Microsoft.Build.Construction;

namespace SourceCombiner
{
    public sealed class SourceCombiner
    {
        private static readonly List<string> SourceFilesToIgnore = new List<string>
        {
            "AssemblyInfo.cs"
        };

        static void Main(string[] args)
        {
            if (args == null || args.Length < 2)
            {
                Console.WriteLine("You must provide at least 2 arguments. The first is the solution file path and the second is the output file path.");
                return;
            }

            string projectFilePath = args[0];
            string outputFilePath = args[1];

            bool openFile = false;
            if (args.Length > 2)
            {
                Boolean.TryParse(args[2],out openFile);
            }

            var filesToParse = GetSourceFileNames(projectFilePath);
            var namespaces = GetUniqueNamespaces(filesToParse);

            string outputSource = GenerateCombinedSource(namespaces,filesToParse);
            File.WriteAllText(outputFilePath,outputSource);

            if (openFile)
            {
                Process.Start(outputFilePath);
            }
        }

        private static string GenerateCombinedSource(List<string> namespaces,List<string> files)
        {
            StringBuilder sb = new StringBuilder();

            sb.AppendLine(@"/*");
            sb.AppendLine($" * File generated by SourceCombiner.exe using {files.Count} source files.");
            sb.AppendLine($" * Created On: {DateTime.Now}");
            sb.AppendLine(@"*/");

            foreach (var ns in namespaces.OrderBy(s => s))
            {
                sb.AppendLine("using " + ns + ";");
            }

            foreach (var file in files)
            {
                IEnumerable<string> sourceLines = File.ReadAllLines(file);
                sb.AppendLine(@"//*** SourceCombiner -> original file " + Path.GetFileName(file) + " ***");
                var openingTag = "using ";
                foreach (var sourceLine in sourceLines)
                {
                    var trimmedLine = sourceLine.Trim().Replace("  ", " ");
                    var isUsingDir = trimmedLine.StartsWith(openingTag) && trimmedLine.EndsWith(";");
                    if (!string.IsNullOrWhiteSpace(sourceLine) && !isUsingDir)
                    {
                        sb.AppendLine(sourceLine);
                    }
                }
            }
    
            return sb.ToString();
        }

        private static List<string> GetSourceFileNames(string solutionFilePath)
        {
            List<string> files = new List<string>();
            SolutionFile solutionFile = SolutionFile.Parse(solutionFilePath);
            foreach (Project project in solutionFile.ProjectsInOrder.Select(p => new Project(p.AbsolutePath)))
            {
                foreach (ProjectItem item in project.AllEvaluatedItems.Where(item => item.ItemType == "Compile"))
                {
                    if (!SourceFilesToIgnore.Contains(Path.GetFileName(item.EvaluatedInclude)))
                    {
                        string projectFolder = Path.GetDirectoryName(project.FullPath);
                        string fullpath = Path.Combine(projectFolder, item.EvaluatedInclude);
                        files.Add(fullpath);
                    }
                }
            }

            return files;
        }


        private static List<string> GetUniqueNamespaces(List<string> files)
        {
            var names = new List<string>();
            const string openingTag = "using ";
            const int namespaceStartIndex = 6;

            foreach (var file in files)
            {
                IEnumerable<string> sourceLines = File.ReadAllLines(file);

                foreach (var sourceLine in sourceLines)
                {
                    var trimmedLine = sourceLine.Trim().Replace("  ", " ");
                    if (trimmedLine.StartsWith(openingTag) && trimmedLine.EndsWith(";"))
                    {
                        var name = trimmedLine.Substring(namespaceStartIndex, trimmedLine.Length - namespaceStartIndex - 1);

                        if (!names.Contains(name))
                        {
                            names.Add(name);
                        }
                    }
                }
            }

            return names;
        }
    }
}