c# - 字典或列表的 GetValueOrDefault 或空合并

Question

我正在尝试从列表中获取一些值。但我想确保如果 key 不存在，它将返回默认值 0而不是抛出异常。

var businessDays = days.Where(x => x.Year == year).ToDictionary(x => x.Month, x => x.Qty);

var vmBusinessDays = new BusinessDay()
{
    Jan = businessDays[1],
    Feb = businessDays[2],
    Mar = businessDays[3],
    Apr = businessDays[4],
    May = businessDays[5]
    [..]
};

如何使用类似 Nullable<T>.GetValueOrDefault 的东西？或 null-coalescing不会污染太多代码？

var vmBusinessDays = new BusinessDay()
{
    Jan = businessDays[1].GetValueOrDefault(),
    Feb = businessDays[1]?? 0
}

我知道 Dictionary<TKey, TValue>.TryGetValue , 但它会复制用于为每个属性设置输出值的代码行。

Answer 1

您可以定义自己的扩展方法:

public static class DictionaryExtensions
{
    public static TValue GetValueOrDefault<TKey, TValue>(
        this IDictionary<TKey, TValue> dict, TKey key)
    {
        TValue val;
        if (dict.TryGetValue(key, out val))
            return val;
        return default(TValue);
    }
}

然后你可以像这样使用它:

var vmBusinessDays = new BusinessDay()
{
    Jan = businessDays.GetValueOrDefault(1),
    Feb = businessDays.GetValueOrDefault(2)
}