Transforms an index.html file to update the base path href

IndexFileBasePathTransformer.cs

public class IndexFileBasePathTransformer
{
    private static readonly ReaderWriterLockSlim Rw = new ReaderWriterLockSlim();
    private static string _indexHtml = "";
    private static bool _transformed;

    public static void Transform(string contentRootPath, string basePath, ILogger logger)
    {
        Rw.EnterUpgradeableReadLock();
        if (!_transformed)
        {
            Rw.EnterWriteLock();
            logger.LogInformation("Index file not transformed");
            logger.LogInformation($"Base path provided is [{basePath}]");

            basePath = string.IsNullOrWhiteSpace(basePath) ? "/" : $"{basePath}/";

            var indexFilePath = Path.Combine(contentRootPath, "wwwroot/index.html");

            // indexFilePath is not a user provided string
            #pragma warning disable SCS0018
            var indexHtml = File.ReadAllText(indexFilePath);

            indexHtml = Regex.Replace(indexHtml, "(.*<base href=)(\".*\")(>)", $"$1\"{basePath}\"$3");

            File.WriteAllText(indexFilePath, indexHtml);
            #pragma warning restore SCS0018

            _indexHtml = indexHtml;
            _transformed = true;
            Rw.ExitWriteLock();
        }
        else
        {
            logger.LogInformation("Index file transformed");    
        }

        Rw.ExitUpgradeableReadLock();
    }

    public static bool IsTransformed
    {
        get
        {
            Rw.EnterReadLock();
            var isTransformed = _transformed;
            Rw.ExitReadLock();

            return isTransformed;
        }
    }

    public static string CachedIndex
    {
        get
        {
            Rw.EnterReadLock();
            if (!_transformed)
            {
                throw new InvalidOperationException("Cannot get an Index that has not been transformed yet.");
            }

            var indexHtml = _indexHtml;
            Rw.ExitReadLock();

            return indexHtml;
        }
    }
}

Startup.cs

public void Configure(IApplicationBuilder app, IWebHostEnvironment env, ILoggerFactory loggerFactory)
{
   app
      .UseMiddleware<ApiErrorHandlingMiddleware>()
      .UseDefaultFiles(new DefaultFilesOptions()
      {
          DefaultFileNames = new[] {"index.html"}
      })
      .Use(async (context, next) =>
      {
          if (!IndexFileBasePathTransformer.IsTransformed)
          {
              IndexFileBasePathTransformer.Transform(env.ContentRootPath, context.Request.PathBase, logger);
          }

          await next();
      })
      .UseStaticFiles()
      .Use(async (context, next) =>
      {
          if (!context.Request.Path.Value.Contains("/api"))
          {
              await context.Response.WriteAsync(IndexFileBasePathTransformer.CachedIndex);
              return;
          }

          await next();
      })
      .UseRouting()
       .UseCors("default")
       .UseAuthentication()
       .UseAuthorization()
       .UseEndpoints(endpoints =>
       {
           endpoints.MapControllers();
       });
}