Why can't lamdas infer types when an out param is present?

[Brondahl]

If I want to pass something resembling a .TryParse() lamda into a method, then I have to define all the types on the lamda, in a way that I don't have to the rest of the time.

public delegate bool TryFunc<in TSource, TResult>(TSource input, out TResult? result);

public static IEnumerable<TResult> TrySelect<TSource, TResult>(
        this IEnumerable<TSource> source,
        TryFunc<TSource, TResult> trySelector)
{}


// why do I have to do this?
listOfIntegerStrings
    .TrySelect((string input, out int result) => int.TryParse(input, out result)
    .ToList();

// rather than this, which seems superficially sufficient?
listOfIntegerStrings
    .TrySelect((input, out result) => int.TryParse(input, out result)
    .ToList();

Obviously in that particular case I could have just passed the method group, rather than define a lamda, but if that isn't an option it's annoying 😁

I assume that there is in fact a reason; so I'm just curious about what it is that makes it necessary / makes allowing the "easier" option so hard?

[HaloFour]

I believe that is slated for C# 14 due out this fall: https://learn.microsoft.com/en-us/dotnet/csharp/whats-new/csharp-14#simple-lambda-parameters-with-modifiers

[Brondahl]

Nice! Thanks :)

[Brondahl]

lol. Love that TryParse is the cited use case on that too! 😁

[LeQuackers]

@HaloFour FYI it isn't working in the preview currently

Also, duplicate of #9522