Why can't I use a Discard with C# async function in my Web API project as a fire and forget mechanism

Question

I am writing a Web API project and for one of my end points (the HandleAsync method is the implementation) I am unable to use the discard syntax (_) and I can't figure out why.

I have other places that do this same pattern: call CheckoutFileAsync and in the processNotifications delegate I am able to use _ fine when calling SendEmailAsync.

The only difference is a different entry of IfContentManagementAuthorizedAsync instead of IfFileActionAuthorizedAsync, but each has the 'same' concept of an async callback if everything is good in the authorization logic.

The compile error I receive is on this line

_ = emailService.SendEmailAsync(

and this is the error:

Cannot implicitly convert type 'System.Threading.Tasks.Task' to 'bool'

And if I hover over the _, intellisense says (parameter) bool _. If I hover over _ where this works, it says (discard) Task<bool> _.

I basically want a 'fire and forget' mechanism to send out a notification email. I don't want to process the results or wait for any delays that might occur and as I understood it, the discard syntax is the pattern I should be doing. Given the code below, do you see any reason why I couldn't use the discard syntax?

Note, I have two overloads of If*AuthorizedAsync methods, one returning Task<IActionResult> and the other returning Task<ActionResult<T>> because they are used in all my end points which vary their return type and I couldn't figure out how to make one signature work for all end points.

public async Task<IActionResult> HandleAsync( [FromQuery] FileIdParameters parameters )
{
    var fileInfo = await GetFilePropertiesAsync<DataLockerFile>( parameters.Id, null );

    if ( fileInfo == null )
    {
        return NotFound();
    }

    return await IfFileActionAuthorizedAsync(
        "Write",
        fileInfo.FolderName,
        async ( user, userEmail, canCreateFolder ) =>
            await CheckoutFileAsync(
                userEmail,
                fileInfo,
                dateTimeService.UtcNow,
                fileInfo =>
                {
                    _ = emailService.SendEmailAsync(
                        string.Join( ";", notificationEmails ),
                        emailService.DefaultFrom,
                        $"KAT Management Secure File Checkout: {fileInfo.FolderName}:{fileInfo.Name}",
                        $"{user} has checked out {fileInfo.FolderName}:{fileInfo.Name}."
                    );
                }
            )
    );
}

protected async Task<ActionResult<T>> IfFileActionAuthorizedAsync<T>(
    string requiredClaim, string folder,
    Func<string, string, bool, Task<ActionResult<T>>> validResult )
{   
    var claimResult = Foo(); // .. logic 
    return await validResult( claimResult.User, claimResult.UserEmail, claimResult.CanCreateFolder );
}
protected async Task<IActionResult> IfFileActionAuthorizedAsync(
    string requiredClaim, string folder,
    Func<string, string, bool, Task<IActionResult>> validResult )
{   
    var claimResult = Foo(); // .. logic 
    return await validResult( claimResult.User, claimResult.UserEmail, claimResult.CanCreateFolder );
}

protected async Task<IActionResult> CheckoutFileAsync(
    string userEmail,
    DataLockerBaseFile fileInfo,
    DateTime checkoutTime,
    Action<DataLockerBaseFile> processNotifications
)
{
    var fileInfo = await DoSomethingAsync(); // logic

    processNotifications( fileInfo );

    return Ok();
}

// This method is part of another class, the smtp configuration object is inject with DI
public async Task<bool> SendEmailAsync( string to, string from, string subject, string body )
{
    var message = CreateMailMessage( to, from, subject body );

    var mailClient = new SmtpClient( smtp.Server );
    mailClient.Credentials = new System.Net.NetworkCredential( smtp.UserName, smtp.Password );
    await mailClient.SendMailAsync( message );

    return true;
}

Answer 1

The IntelliSense hovering indicates the problem:

And if I hover over the _, intellisense says (parameter) bool _.

The code causing this issue is using a common pattern for sort-of-but-not-really-discarding parameter values, one of which is a bool. E.g., it probably has some lambda of the form (arg, _ /* unneeded boolean argument */) => { ... }. On a side note, the code you posted does not have this pattern and does not cause the problem; when simplifying code to post, please ensure the code you end up posting actually reproduces the problem.

This is a common pattern to indicate "this parameter isn't needed"; however, it's not an actual discard at the language level (except in certain cases in C# 9.0). Instead, it actually defines a parameter named _ of type bool.

So, the discard pattern _ = ... doesn't work for that code block. Since there's an actual parameter named _, the compiler treats it as an assignment to that parameter, and gives the error:

Cannot implicitly convert type 'System.Threading.Tasks.Task' to 'bool'

The solution is to rename the sort-of-discard parameter from _ to something like __. Then the discard pattern will work.

On a side note, "fire-and-forget" is almost always the wrong solution.