Nullable Collection Types in C#

内容目录

When the collection is null, the code is as follows:

            List<int>? _a = null;
            List<int>? _b = null;

            var a = _a?.Any() == false;
            var b = _a?.Any() == true;
            var c = _b?.Any() == false;
            var d = _b?.Any() == true;

When the collection is null, both ?.Any() == false and ?.Any() == true yield false.

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When the collection is not null and has 0 elements, the code is as follows:

            List<int>? _a = new List<int>();
            List<int>? _b = new List<int>();

            var a = _a?.Any() == false;
            var b = _a?.Any() == true;
            var c = _b?.Any() == false;
            var d = _b?.Any() == true;

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The conclusion is that the intent is expressed correctly.

If you want to check whether both collections are not null and have elements:

            List<int>? _a = null;
            List<int>? _b = new List<int>();

            var result = _a?.Any() == true && _b?.Any() == true;

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When one has a value that is not null and has elements:

            List<int>? _a = null;
            List<int>? _b = new List<int>()
            {
                1,2
            };

            var result = _a?.Any() == true || _b?.Any() == true;

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Determining if both are either null or have no elements can be tricky and prone to bugs, as shown in the following code:

            List<int>? _a = null;
            List<int>? _b = new List<int>();

            var result = !(_a?.Any() == true && _b?.Any() == true);
            _b = new List<int>() { 1,2,3,4};
            result = !(_a?.Any() == true && _b?.Any() == true);

It's best to write an extension method and avoid trying to resolve everything in one line:

        internal static bool IsNullOrEmpty<T>(this IEnumerable<T>? source)
        {
            if (source == null) return true;
            if (source.Count() == 0) return true;
            return false;
        }

Although syntactic sugar is convenient, it is important to adhere to standards to avoid writing dark code.

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