CSS Isolation Action Filter for .NET Framework MVC projects

CssInlineMemoryStream.cs

internal class CssInlineMemoryStream : MemoryStream
{
    private readonly Stream _stream;
    private readonly string _cssFileContent;
    public CssInlineMemoryStream(Stream stream, string cssFileContent = "")
    {
        _stream = stream;
        _cssFileContent = cssFileContent;
    }

    public override void Write(byte[] buffer, int offset, int count)
    {
        if (!string.IsNullOrEmpty(_cssFileContent))
        {
            var html = Encoding.UTF8.GetString(buffer);
            html = html.Replace("</head>", $"<style>{_cssFileContent}</style></head>");
            buffer = Encoding.UTF8.GetBytes(html);
        }
        _stream.Write(buffer, offset, buffer.Length);
    }
}

CSSIsolationActionFilter.cs

public class CSSIsolationActionFilterAttribute : ActionFilterAttribute
{
    public CSSIsolationActionFilterAttribute()
    {
    }

    public override void OnResultExecuted(ResultExecutedContext filterContext)
    {
        ViewResult viewResult = filterContext.Result as ViewResult;
        RazorView view = viewResult?.View as RazorView;
        var viewPath = view?.ViewPath;
        var cssFileContents = string.Empty;
        if (!string.IsNullOrEmpty(viewPath))
        {
            var absolutePath = filterContext.HttpContext.Server.MapPath(viewPath);
            var cssFile = absolutePath + ".css";
            if (File.Exists(cssFile))
            {
                cssFileContents = File.ReadAllText(cssFile);
            }
        }

        var filter = filterContext.HttpContext.Response.Filter;
        filterContext.HttpContext.Response.Filter = new CssInlineMemoryStream(filter, cssFileContents);
    }
}