System.Diagnostics.Process.ResolvePath C# (CSharp) Method

        private static string ResolvePath(string filename)
        {
            // Follow the same resolution that Windows uses with CreateProcess:
            // 1. First try the exact path provided
            // 2. Then try the file relative to the executable directory
            // 3. Then try the file relative to the current directory
            // 4. then try the file in each of the directories specified in PATH
            // Windows does additional Windows-specific steps between 3 and 4,
            // and we ignore those here.

            // If the filename is a complete path, use it, regardless of whether it exists.
            if (Path.IsPathRooted(filename))
            {
                // In this case, it doesn't matter whether the file exists or not;
                // it's what the caller asked for, so it's what they'll get
                return filename;
            }

            // Then check the executable's directory
            string path = GetExePath();
            if (path != null)
            {
                try
                {
                    path = Path.Combine(Path.GetDirectoryName(path), filename);
                    if (File.Exists(path))
                    {
                        return path;
                    }
                }
                catch (ArgumentException) { } // ignore any errors in data that may come from the exe path
            }

            // Then check the current directory
            path = Path.Combine(Directory.GetCurrentDirectory(), filename);
            if (File.Exists(path))
            {
                return path;
            }

            // Then check each directory listed in the PATH environment variables
            string pathEnvVar = Environment.GetEnvironmentVariable("PATH");
            if (pathEnvVar != null)
            {
                var pathParser = new StringParser(pathEnvVar, ':', skipEmpty: true);
                while (pathParser.MoveNext())
                {
                    string subPath = pathParser.ExtractCurrent();
                    path = Path.Combine(subPath, filename);
                    if (File.Exists(path))
                    {
                        return path;
                    }
                }
            }

            // Could not find the file
            throw new Win32Exception(Interop.Error.ENOENT.Info().RawErrno);
        }