Using Roslyn Analyzers in combination with Unity

nuget.config

<?xml version="1.0" encoding="utf-8"?>
<configuration>
	<config>
		<add key="repositoryPath" value=".\Assets\Nuget" />
	</config>
</configuration>

PostProcessAssets.cs

// Put this file inside Assets/Editor/

// This script generates the appropriate .csproj and csc.rsp files in order to 
// also include your Roslyn Analyzers in the Visual Studio Project

using System.IO;
using System.Text;
using UnityEditor;
using UnityEngine;

[InitializeOnLoad]
public class PostProcessAssets : AssetPostprocessor
{
	private const string ITEM_GROUP_SEARCH = @"<ItemGroup>";
	private const string ANALYZER_DIR = @"\Assets\Nuget\"; // Use the same path as defined in 'nuget.config' here

	private const string ITEM_GROUP_STRING = "<ItemGroup>\r\n";
	private const string ITEM_GROUP_END_STRING = "  </ItemGroup>\r\n  ";
	private const string ANALYZER_STRING_START = "    <Analyzer Include=\"";
	private const string ANALYZER_STRING_END = "\"/>\r\n";

	private static void OnGeneratedCSProjectFiles()
	{
		string dir = Directory.GetCurrentDirectory();
		string[] analyzerPaths = Directory.GetFiles(dir + ANALYZER_DIR, "*.dll", SearchOption.AllDirectories);
		// Shorten analyzerPaths to be relative
		for (int i = 0; i < analyzerPaths.Length; i++)
		{
			int assetsIndex = analyzerPaths[i].IndexOf("\\Assets\\");
			if (assetsIndex > 0)
				analyzerPaths[i] = analyzerPaths[i].Substring(assetsIndex + 1); // + 1 to cut off first \
		}

		// Add analyzers to csproj files
		string[] csprojFiles = Directory.GetFiles(dir, "*.csproj");
		foreach (string file in csprojFiles)
			CsprojAddAnalyzers(file, analyzerPaths);

		// Add analyzers to csc.rsp
		string[] cscRspFiles = Directory.GetFiles(dir + "\\Assets\\", "csc.rsp");
		if (cscRspFiles.Length == 0)
		{
			cscRspFiles = new string[1];
			cscRspFiles[0] = dir + "\\Assets\\csc.rsp";
		}
		else if (cscRspFiles.Length > 1)
			Debug.LogWarning("More than one csc rsp file defined in assets folder.");

		RspAddAnalyzer(cscRspFiles[0], analyzerPaths);
	}
	
	private static void CsprojAddAnalyzers(string filePath, string[] analyzerPaths)
	{
		string fileContent = File.ReadAllText(filePath);
		int firstItemGroupIndex = fileContent.IndexOf(ITEM_GROUP_SEARCH);
		if (firstItemGroupIndex < 0)
		{
			Debug.LogWarning("Could not find ItemGroup to add analyzer to csproj file.");
			return;
		}

		StringBuilder stringBuilder = new StringBuilder(fileContent, 0, firstItemGroupIndex, fileContent.Length);

		stringBuilder.Append(ITEM_GROUP_STRING);
		foreach (string analyzer in analyzerPaths)
		{
			stringBuilder.Append(ANALYZER_STRING_START);
			stringBuilder.Append(analyzer);
			stringBuilder.Append(ANALYZER_STRING_END);
		}
		stringBuilder.Append(ITEM_GROUP_END_STRING);

		stringBuilder.Append("");
		stringBuilder.Append(fileContent, firstItemGroupIndex, fileContent.Length - firstItemGroupIndex);

		File.WriteAllText(filePath, stringBuilder.ToString());
	}

	private static void RspAddAnalyzer(string filePath, string[] analyzerPaths)
	{
		StringBuilder stringBuilder = new StringBuilder();

		foreach (string analyzer in analyzerPaths)
		{
			stringBuilder.Append("-a:");
			stringBuilder.AppendLine(analyzer);
		}

		File.WriteAllText(filePath, stringBuilder.ToString());
	}
}