Editor script that adds System.Numerics assembly reference to Unity-generated C# projects

AddSystemNumericsToCsProjectPostprocessor.cs

using System.Collections.Generic;
using System.IO;
using System.Linq;
using System.Xml.Linq;
using UnityEditor;

namespace Loom.Unity3d.Internal.Editor
{
    /// <summary>
    /// Adds System.Numerics assembly reference to Unity-generated C# projects.
    /// </summary>
    internal class AddSystemNumericsToCsProjectPostprocessor : AssetPostprocessor
    {
        public static void OnGeneratedCSProjectFiles()
        {
            string currentDirectory = Directory.GetCurrentDirectory();
            string[] projectFiles = Directory.GetFiles(currentDirectory, "*.csproj");

            foreach (string file in projectFiles)
            {
                AddSystemNumericsToProjectFile(file);
            }
        }

        private static void AddSystemNumericsToProjectFile(string file)
        {
            XDocument project = XDocument.Load(file);
            XElement projectRootElement = project.Root;
            XNamespace xmlns = projectRootElement.Name.NamespaceName;

            IEnumerable<XElement> existingReferences =
                projectRootElement
                    .Elements(xmlns + "ItemGroup")
                    .Elements(xmlns + "Reference");

            // Bail out of System.Numerics is already present
            if (existingReferences.FirstOrDefault(reference => reference.Attribute("Include").Value == "System.Numerics") != null)
                return;

            // Get mscorlib.dll path for reference
            XElement mscorlibReferenceElement =
                projectRootElement
                    .Elements(xmlns + "ItemGroup")
                    .Elements(xmlns + "Reference")
                    .FirstOrDefault(a => a.Attribute("Include").Value == "mscorlib");

            string mscorlibAssemblyPath =
                mscorlibReferenceElement
                    .Element(xmlns + "HintPath")
                    .Value;

            // Constuct and add System.Numerics reference
            string systemNumericsAssemblyPath = Path.Combine(Path.GetDirectoryName(mscorlibAssemblyPath), "System.Numerics.dll");
            XElement systemNumericsReferenceElement = new XElement(xmlns + "Reference");
            systemNumericsReferenceElement.SetAttributeValue("Include", "System.Numerics");
            XElement systemNumericsReferenceHintPathElement = new XElement(xmlns + "HintPath");
            systemNumericsReferenceHintPathElement.Value = systemNumericsAssemblyPath;
            systemNumericsReferenceElement.AddFirst(systemNumericsReferenceHintPathElement);

            mscorlibReferenceElement.Parent.Add(systemNumericsReferenceElement);

            project.Save(file);
        }
    }
}