c# 可空值类型

//语法糖
int? count=0;

//完整写法
Nullable<int> count = 0;

Nullable类型定义

public partial struct Nullable<T> where T : struct
    {
        private readonly bool hasValue; // Do not rename (binary serialization)
        internal T value; // Do not rename (binary serialization) or make readonly (can be mutated in ToString, etc.)

        [NonVersionable]
        public Nullable(T value)
        {
            this.value = value;
            hasValue = true;
        }

        public readonly bool HasValue
        {
            [NonVersionable]
            get => hasValue;
        }

        public readonly T Value
        {
            get
            {
                if (!hasValue)
                {
                    ThrowHelper.ThrowInvalidOperationException_InvalidOperation_NoValue();
                }
                return value;
            }
        }

        [NonVersionable]
        public readonly T GetValueOrDefault() => value;

        [NonVersionable]
        public readonly T GetValueOrDefault(T defaultValue) =>
            hasValue ? value : defaultValue;

        public override bool Equals(object? other)
        {
            if (!hasValue) return other == null;
            if (other == null) return false;
            return value.Equals(other);
        }

        public override int GetHashCode() => hasValue ? value.GetHashCode() : 0;

        public override string? ToString() => hasValue ? value.ToString() : "";

        [NonVersionable]
        public static implicit operator Nullable<T>(T value) =>
            new Nullable<T>(value);

        [NonVersionable]
        public static explicit operator T(Nullable<T> value) => value!.Value;
    }