Accessing HttpClientHandler in the same place as HttpClient is created - .net-core

Question

I have a service requesting an URL and validating the server SSL certificate. The code has been running smoothly with HttpWebRequest in full .NET framework, but now I want to migrate it to HttpClient and .NET Core. I can get the certificate like this (the approach is recommended in multiple blog posts and stack overflow answers):
X509Certificate2 cert = null;
var httpClient = new HttpClient(new HttpClientHandler
{
ServerCertificateCustomValidationCallback = (request, certificate, chain, errors) =>
{
cert = certificate;
return true;
}
});
httpClient.GetAsync(...);
The issue here is that I constantly create new HttpClient instances, which isn't recommended. I want to move to HttpClientFactory, why I add the following in my setup code:
services
.AddHttpClient("sslclient", x =>
{
...
})
.ConfigurePrimaryHttpMessageHandler(() => new HttpClientHandler
{
ServerCertificateCustomValidationCallback = (request, certificate, chain, errors) =>
{
return true;
}
});
The challenge now is that the code creates the client no longer has access to ServerCertificateCustomValidationCallback:
var httpClient = httpClientFactory.CreateClient("sslclient");
Anyone know how to solve this?

Answer 1

Someone at Reddit suggested the following solution. Once the call to AddHttpClient has been made, it is no longer possible to modify the HttpClientHandler. It is possible to share a resource, though:
var certificates= new ConcurrentDictionary<string, X509Certificate2>();
services.AddSingleton(certificates);
services
.AddHttpClient("sslclient", x =>
{
...
})
.ConfigurePrimaryHttpMessageHandler(() => new HttpClientHandler
{
ServerCertificateCustomValidationCallback = (request, certificate, chain, errors) =>
{
certificates.TryAdd(request.RequestUri.Host, new X509Certificate2(certificate));
return true;
}
});
In the code making the HTTP request, you'd need to inject the certificates dictionary as well. Once the request has been made, you can check for a certificate in the dictionary:
var response = await httpClient.GetAsync(url);
if (certificates.ContainsKey(uri.Host))
{
// Happy days!
}