Flurl and untrusted certificates

The most typical way to do this is to create a custom factory:

public class UntrustedCertClientFactory : DefaultHttpClientFactory
{
    public override HttpMessageHandler CreateMessageHandler() {
        return new HttpClientHandler {
            ServerCertificateCustomValidationCallback = (_, _, _, _) => true
        };
    }
}

Then register it somewhere in your app startup:

FlurlHttp.ConfigureClient("https://theapi.com", cli =>
    cli.Settings.HttpClientFactory = new UntrustedCertClientFactory());

Flurl reuses the same HttpClient instance per host by default, so configuring this way means that every call to theapi.com will allow the use of the untrusted cert. The advantage of this over passing an HttpClient to a FlurlClient constructor is that it keeps this configuration "off to the side" and works when you use Flurl in the more typical/less verbose way:

await "https://theapi.com/endpoint".GetJsonAsync();

Here is my setup for Flurl, which works with untrusted certificates:

HttpClientHandler httpClientHandler = new HttpClientHandler();
httpClientHandler.ServerCertificateCustomValidationCallback = (message, cert, chain, 
  errors) => true;
HttpClient httpClient = new HttpClient(httpClientHandler);
httpClient.BaseAddress = new Uri("https://myaddress.com");
var flurlClient = new FlurlClient(httpClient);

var apiInfo = await flurlClient.Request("apiInfo").GetJsonAsync<ApiInfoDto>();

I have created custom HttpClientHandler which accepts every certificate in ServerCertificateCustomValidationCallback. Of course, you can use other logic in this handler.

Update: With this setup, you cannot use Flurl extensions for URL (you cannot write "http://myadress.com/apiInfo".GetJsonAsync<ApiInfoDto>(). You have to create Flurl client as seen above and use Flurl client for your calls as demonstrated also in mine code. The usage is the same as Flurl extensions for URL.

An inline solution to accept any certificate is:

var myString = await "https://some-server-with-an-invalid-cert.net"
    .AppendPathSegment("/some-file.txt")
    .WithClient(new FlurlClient(new HttpClient(new HttpClientHandler
              {
                  ServerCertificateCustomValidationCallback = (message, cert, chain,
                                                               errors) => true
              })))
    .GetStringAsync();

With WithClient() you can pass a client configured different than the default client. In some cases you would not want to change the default client, but apply properties, e.g. the certificate validation only to this specific case.