Mark Ashley Bell

02 Aug 2021

Shooting yourself in the foot with the C# default literal and nullable

One of the things I love about C# is that the language designers are constantly refining it, so we regularly get new syntax to play with. I'm generally keen to adopt these new features, as they normally help us to write cleaner, more concise code.

However, I'd like to highlight something which caught me out: using the default literal in conjunction with nullable values.

It's not the fault of the language feature itself, but rather a case of me blindly applying it without really thinking about it first—which is always dangerous!

The default literal

We have had default value expressions—e.g. default(int)—for a long time* now. Here's a contrived example which assigns the default value for int (0) to the variable if the boolean setValue is false:

var setValue = false;
var intValue = 123;
var valueOrDefault = setValue ? intValue : default(int);

As of C# 7.1, the default literal can infer the type:

var valueOrDefault = setValue ? intValue : default;

Note that the type is inferred from the type returned by the other branch of the condition (value), not from the type of the variable you're assigning to; it couldn't possibly be anyway in this case, as it's declared with var.

How I shot myself in the foot...

I was building a really simple cookie-based auth system in a .NET Core web app. Within the Login controller method, I added the following:

// Preceding code omitted for brevity

var principal = new ClaimsPrincipal(identity);

var authenticationProperties = new AuthenticationProperties {
    IsPersistent = model.RememberMe,
    ExpiresUtc = model.RememberMe 
        ? DateTimeOffset.UtcNow.AddDays(7) 
        : default
};

await HttpContext.SignInAsync(principal, authenticationProperties);

I expect some of you will already have spotted the mistake... but if not, read on!

AuthenticationProperties.ExpiresUtc determines how long the auth cookie hangs around in the client's browser. if the user checks the "Remember Me" checkbox, I want it to expire after 7 days; if not, the expiry time should be left as the default.

ExpiresUtc is a DateTimeOffset?, so the default value would be null. This will cause the cookie to be set as a session cookie, which expires when the browser is closed.

However, although that is the way the code reads, it isn't the way it behaves!

As I've already mentioned, the default literal infers the type from the value returned by the other branch of the ternary conditional, which in this case is DateTimeOffset.UtcNow.AddDays(7).

This is not a nullable DateTimeOffset?, but a basic DateTimeOffset: the default of which is DateTimeOffset.MinValue.

So... if the user doesn't check "Remember Me", my code sets ExpiresUtc to 01/01/0001 00:00:00 +00:00. This means that the auth cookie is immediately expired, and—even though their login was actually successful—the user is redirected straight back to the login page.

The right way

The correct way to do this is to use a default value expression, specifying the type:

var authenticationProperties = new AuthenticationProperties {
    IsPersistent = model.RememberMe,
    ExpiresUtc = model.RememberMe 
        ? DateTimeOffset.UtcNow.AddDays(7) 
        : default(DateTimeOffset?)
};

Now we're defaulting to a DateTimeOffset? value, which means the default is actually null... not an offset value thousands of years in the past. 😉

This was incredibly frustrating to debug, so I thought I'd write about it in case it helps anyone else with a similar issue. Happy coding!

*Does anyone know which version of C# they were introduced in? If you do, please get in touch!

Questions or comments? Get in touch @markashleybell, or email [Turn On Javascript To View Email Address].

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© Mark Ashley Bell 2018