C#•3y ago

How to let compiler know that List is not null?

Hi community!

I have the following extension method:, which checks if the source is not null and if it contains any elements:

public static bool SafeAny<T>(this IEnumerable<T> source) => source != null && source.Any();

public static bool SafeAny<T>(this IEnumerable<T> source) => source != null && source.Any();

public static bool SafeAny<T>(this IEnumerable<T> source) => source != null && source.Any();

public static bool SafeAny<T>(this IEnumerable<T> source) => source != null && source.Any();

I'd like to inform the compiler that, when returning true, the source is not null. Is this possible to achieve with C#?

Thank you!

Nelson SousaOP•11/21/23, 11:35 AM

Note: I know we could use myList!, but that's developer work instead of compiler actually knowing

Thinker•11/21/23, 11:42 AM

~~Where are you getting myListmyList from? The compiler is likely inferring this for a reason~~

Joschi•11/21/23, 12:11 PM

I think you are looking for the NotNullWhenAttribute NotNullWhenAttribute
This is how the .NET TryParse methods are implemented.

c#
        // Determines whether a String represents true or false.
        //
        public static bool TryParse([NotNullWhen(true)] string? value, out bool result) =>
            TryParse(value.AsSpan(), out result);

c#
        // Determines whether a String represents true or false.
        //
        public static bool TryParse([NotNullWhen(true)] string? value, out bool result) =>
            TryParse(value.AsSpan(), out result);

It says: If this method returns true this parameter won't be null.

Thinker•11/21/23, 12:18 PM

oh, read the question wrong, sorry. yeah this is correct

Nelson SousaOP•11/23/23, 4:40 PM

That was it, thank you!

How to let compiler know that List is not null?

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