Question

I tried to connect to a HTTPS page. In C# i can write a line that asks C# to connect to HTTPS easily with the line

ServicePointManager.ServerCertificateValidationCallback = delegate { return true; };

How do I adapt this to f#?

open System.Net
open System.IO

let url = "https://..."

let mutable request = HttpWebRequest.Create(url)
request.Method <- "GET"  
request.ContentType <- "multipart/form-data"

ServicePointManager.ServerCertificateValidationCallback = fun -> true ???? 

let mutable resp = request.GetResponse()

let fn =
    for i = 1 to 10 do
        request <- WebRequest.Create(url)
        resp <- request.GetResponse()

Answer 1

ServicePointManager.ServerCertificateValidationCallback <-
  System.Net.Security.RemoteCertificateValidationCallback(fun _ _ _ _ -> true)

The following works too:

System.Net.ServicePointManager.ServerCertificateValidationCallback <- 
  (fun _ _ _ _ -> true) //four underscores (and seven years ago?)

RemoteCertificateValidationCallback has the following signature:

public delegate bool RemoteCertificateValidationCallback(
    Object sender,
    X509Certificate certificate,
    X509Chain chain,
    SslPolicyErrors sslPolicyErrors
)

Since pattern matching occurs for function arguments, and you’re ignoring all four parameters, you can substitute the wildcard pattern (underscore) for each, which is an idiomatic way to indicate a parameter is unused.