Modifying Request body using Owin Middleware

Question

I am trying to sanitize the requests body coming to an Asp.net (.Net 4.6) application with WebAPI 2 using OWIN/Katana. However, after modifying the body I am still getting the original text inside the API method.

Example input whwc8'-alert(123)-'mg763 After Sanitizing It will look like whwc8-alert|123|-mg763

(I know that this is not the best way to prevent XSS, I am properly encoding the text at the client side. This is just because my customer is hell-bent on disabling the set of special characters that they have in a list).

My Startup class:

Startup.cs

public class Startup
    {
        private static ILog Log;
        public void Configuration(IAppBuilder app)
        {
            app.Use<SanitizerMiddleware>();
            app.MapSignalR();
            app.UseWebApi(WebApiConfig.Register());
        }
    }

The sanitizer Middle-ware

public class SanitizerMiddleware : OwinMiddleware
    {
        OwinMiddleware _next;
        public SanitizerMiddleware(OwinMiddleware next) :base(next)
        {
            _next = next;
        }

        public async override Task Invoke(IOwinContext context)
        {
            await Sanitize(context);
            await Next.Invoke(context);
           
        }
        public async Task Sanitize(IOwinContext context)
        {
            if (context.Request.Method == "POST")
            {
                StreamReader reader = new StreamReader(context.Request.Body);
                string text = reader.ReadToEnd();
                text = new HtmlSanitizer().Sanitize(text).Replace("'", "`").Replace("(", "|").Replace(")", "|");
                byte[] requestData = Encoding.UTF8.GetBytes(text);
                context.Request.Body=new MemoryStream(requestData);
                
            }
        }