Minimal Console errors out with: Object reference not set to an instance of an object.

Error

System.NullReferenceException
  HResult=0x80004003
  Message=Object reference not set to an instance of an object.
  Source=Microsoft.Owin.Hosting
  StackTrace:
   at Microsoft.Owin.Hosting.Utilities.SettingsLoader.FromConfigImplementation..ctor()
   at Microsoft.Owin.Hosting.Utilities.SettingsLoader.<LoadFromConfig>b__0()
   at System.Threading.LazyInitializer.EnsureInitializedCore[T](T& target, Func`1 valueFactory)
   at Microsoft.Owin.Hosting.Utilities.SettingsLoader.LoadFromConfig(IDictionary`2 settings)
   at Microsoft.Owin.Hosting.Engine.StartContext..ctor(StartOptions options)
   at Microsoft.Owin.Hosting.WebApp.StartImplementation(IServiceProvider services, StartOptions options, Action`1 startup)
   at Program.<Main>$(String[] args) in C:\Projects\personal\Delete\TestServer\Program.cs:line 7

Just run the following code.

using Microsoft.Owin.Hosting;
using Owin;

string BaseUri = @"http://localhost:19000";
List<string> payloads = new();
var builder = WebApp.Start(BaseUri, app =>
{
    app.Run(async context =>
    {
        using StreamReader reader = new(context.Request.Body, System.Text.Encoding.UTF8);
        context.Request.Body.Position = 0;
        payloads.Add(await reader.ReadToEndAsync());
        context.Response.ContentType = "application/json";
        await context.Response.WriteAsync(@"{""Success"":true}");
    });
});

Console.In.ReadLine();

<Project Sdk="Microsoft.NET.Sdk">

  <PropertyGroup>
    <OutputType>Exe</OutputType>
    <TargetFramework>net6.0</TargetFramework>
    <ImplicitUsings>enable</ImplicitUsings>
    <Nullable>enable</Nullable>
  </PropertyGroup>

  <ItemGroup>
    <PackageReference Include="Microsoft.AspNet.WebApi.OwinSelfHost" Version="5.2.9" />
  </ItemGroup>

</Project>

[Tratcher]

For migration you’re better off using the OWIN shim in AspNetCore to run on the servers there. That way you don’t need to bring in extra libraries like HttpListener which we know aren’t compatible.

[CZEMacLeod]

@vandanv Selfhost doesn’t work on net5/6. You can host it under kestrel in the asp.net core pipeline as shown in my example. It should also work under IIS.