Using null-conditional bool? in if statement [duplicate]

问题

This question already has answers here:

Converting Nullable<bool> to bool (6 answers)

Compiler Error for Nullable Bool (5 answers)

cannot implicitly convert type 'bool?' to 'bool'. An explicit conversion exists (are you missing a cast?) (9 answers)

Closed 3 years ago.

Why this code works:

if (list?.Any() == true)

but this code doesn't:

if (list?.Any())

saying Error CS0266 Cannot implicitly convert type 'bool?' to 'bool'

So why is it not a language feature making such an implicit conversion in the if statement?

回答1:

An if statement will evaluate a Boolean expression.

bool someBoolean = true;

if (someBoolean)
{
    // Do stuff.
}

Because if statements evaluate Boolean expressions, what you are attempting to do is an implicit conversion from Nullable<bool>. to bool.

bool someBoolean;
IEnumerable<int> someList = null;

// Cannot implicity convert type 'bool?' to 'bool'.
someBoolean = someList?.Any();

Nullable<T> does provide a GetValueOrDefault method that could be used to avoid the true or false comparison. But I would argue that your original code is cleaner.

if ((list?.Any()).GetValueOrDefault())

An alternative that could appeal to you is creating your own extension method.

public static bool AnyOrDefault<T>(this IEnumerable<T> source, bool defaultValue)
{
    if (source == null)
        return defaultValue;

    return source.Any();
}

Usage

if (list.AnyOrDefault(false))

来源：https://stackoverflow.com/questions/45596941/using-null-conditional-bool-in-if-statement