When developing web applications, you may encounter the following CORS error:

Access to XMLHttpRequest at 'http://xxx.xxx' from origin 'http://localhost:8002' has been blocked by CORS policy: Response to preflight request doesn't pass access control check: Rediretc is not allowed for a preflight request.

This typically occurs when your frontend application attempts to make requests to a different domain or port than the one it was served from.

Approach 1: Manual Header Configuration

You can manually add required CORS headers in your application code:

// In your controller or global.asax
HttpContext.Current.Response.AddHeader("Access-Control-Allow-Origin", "*");
HttpContext.Current.Response.AddHeader("Access-Control-Allow-Methods", "GET, POST, PUT, DELETE, OPTIONS");
HttpContext.Current.Response.AddHeader("Access-Control-Allow-Headers", "Origin, X-Requested-With, Content-Type");
HttpContext.Current.Response.AddHeader("Access-Control-Allow-Credentials", "true");

Alternatively, configure headers through web.config:

<system.webServer>
  <httpProtocol>
    <customHeaders>
      <add name="Access-Control-Allow-Origin" value="*" />
      <add name="Access-Control-Allow-Headers" value="Content-Type" />
      <add name="Access-Control-Allow-Methods" value="GET, POST, PUT, DELETE, OPTIONS" />
    </customHeaders>
  </httpProtocol>
  <handlers>
    <remove name="ExtensionlessUrlHandler-Integrated-4.0" />
    <remove name="OPTIONSVerbHandler" />
    <remove name="TRACEVerbHandler" />
    <add name="ExtensionlessUrlHandler-Integrated-4.0" 
         path="*." 
         verb="*" 
         type="System.Web.Handlers.TransferRequestHandler" 
         preCondition="integratedMode,runtimeVersionv4.0" />
  </handlers>
</system.webServer>

Approach 2: Using Microsoft.AspNet.WebApi.Cors Package

A more structured approach involves enabling CORS via configuration settings.

Add origins list to web.config:

<appSettings>
  <!-- CORS Origins -->
  <add key="AllowedCorsOrigins" value="http://localhost:8002,http://localhost:9536" />
</appSettings>

In WebApiConfig.cs, register the CORS functionality:

public static class WebApiConfig
{
    public static void Register(HttpConfiguration configuration)
    {
        var corsOrigins = ConfigurationManager.AppSettings["AllowedCorsOrigins"];
        var corsPolicy = new EnableCorsAttribute(corsOrigins, "*", "*")
        {
            SupportsCredentials = true
        };
        
        configuration.EnableCors(corsPolicy);

        configuration.MapHttpAttributeRoutes();

        configuration.Routes.MapHttpRoute(
            name: "DefaultRoute",
            routeTemplate: "api/{controller}/{identifier}",
            defaults: new { identifier = RouteParameter.Optional }
        );
    }
}

To use this method, install the Microsoft.AspNet.WebApi.Cors NuGet package first.