c# to f# when 3rd party lib wants a byref as an out parameter

I’m trying to convert this method into F#

private static bool VerifySignedJwt(ECDsa eCDsa, string token) {
    var tokenHandler = new JwtSecurityTokenHandler();
    var claimsPrincipal = tokenHandler.ValidateToken(token, new TokenValidationParameters {
        ValidIssuer = "me",
        ValidAudience = "you",
        IssuerSigningKey = new ECDsaSecurityKey(eCDsa)
    }, out var parsedToken);
    return claimsPrincipal.Identity.IsAuthenticated;
}

I’m translating this great piece of work by Scott Brady in order to create and validate JWT tokens for a web app, the creation part went without a hitch. On with the Validation part and the reference to a byref required in the JwtSecurityTokenHandler.ValidateToken method has stumped me. https://www.scottbrady91.com/C-Sharp/JWT-Signing-using-ECDSA-in-dotnet-Core

If I follow the Microsoft docs below; I get an error saying a “type instantiation involves a byref type. This is not permitted by the rules of Common IL.” When trying to declare a byref as:

let _h (x: byref<'T>) = ()

https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/byrefs I have read the Microsoft docs on byrefs but am left wondering if this relates to .Net core or F#4.5 or does dotnet core supersede all features including byrefs in F#4.5? Either way I can’t seem to instantiate a SecurityToken at all, the "out var parsedToken" is a SecurityToken type, let alone a byref one to use as an output to this 3rd party library, or I can’t seem to declare a byref SecurityToken directly in the call to JwtSecurityTokenHandler.ValidateToken.

If I use the advice posted in this SO post Using c# delegates with f# functions

let function_1 (x:double) (y:double byref) = 
    y <- 6.0

I get the same error “type instantiation involves a byref type. This is not permitted by the rules of Common IL.”

I have used Byrefs before in F# but these were simple integer types and worked without issue. My project is .Net core 2.1

Any help appreciated.

Solution

Since the out parameter in the ValidateToken() call is the last one, you can simply omit it, and treat it as a second return value:

let claimsPrincipal, parsedToken = tokenHandler.ValidateToken (token, tvp)

(Here tvp would be your TokenValidationParameters instance.)

EDIT: Since you don't seem to be using the second return value, I figure your function could look as follows:

let verifySignedJwt ecdsa token =
    let tokenHandler = JwtSecurityTokenHandler ()
    let tvp = TokenValidationParameters (ValidIssuer = "me",
                                         ValidAudience = "you",
                                         IssuerSigningKey = ECDsaSecurityKey ecdsa)
    let claimsPrincipal, _ = tokenHandler.ValidateToken (token, tvp)
    claimsPrincipal.Identity.IsAuthenticated