c# - 使用 LINQ GroupBy 获取忽略属性的唯一集合

Question

使用 Rules 集合，我试图创建另一个 Rules 集合，忽略 Site 属性并创建一个唯一列表。

public class Rule
{
    public int TestId { get; set; }
    public string File { get; set; }
    public string Site { get; set; }
    public string[] Columns { get; set; }
}

所以如果我的收藏有如下值:

var rules = new List<Rule>
{
    new Rule { TestId = 1, File = "Foo", Site = "SiteA", Columns = new string[] { "ColA", "ColB" }},
    new Rule { TestId = 1, File = "Foo", Site = "SiteB", Columns = new string[] { "ColA", "ColB" }}
};

我要的是最终结果

var uniqueRules = new List<Rule>
{
    new Rule { TestId = 1, File = "Foo", Site = null, Columns = new string[] { "ColA", "ColB" }}
};

尝试了如下所示的各种组合后，我仍然得到 2 个结果，如何才能达到预期的结果？

var uniqueRules = rules
    .GroupBy(r => new { r.TestId, r.File, r.Columns })
    .Select(g => g.Key)
    .Distinct()
    .ToList();

Answer 1

问题是 string[]没有覆盖 Equals和 GetHashCode ，这就是为什么只在 r.Columns 比较引用文献的原因.您需要提供自定义 IEqualityComparer<T> :

public class RuleComparer : IEqualityComparer<Rule>
{
    public bool Equals(Rule x, Rule y)
    {
        if (object.ReferenceEquals(x, y)) return true;
        if (x == null || y == null) return false;
        if(!(x.TestId == y.TestId && x.File == y.File)) return false;
        return x.Columns.SequenceEqual(y.Columns);
    }

    // from: https://stackoverflow.com/questions/263400/what-is-the-best-algorithm-for-an-overridden-system-object-gethashcode
    public int GetHashCode(Rule obj)
    {
        unchecked
        {
            int hash = 17;
            hash = hash * 23 + obj.TestId.GetHashCode();
            hash = hash * 23 + (obj.File?.GetHashCode() ?? 0);
            foreach(string s in obj.Columns)
                hash = hash * 23 + (s?.GetHashCode() ?? 0);
            return hash;
        }
    }
}

现在 LINQ 查询变得很简单:

List<Rule> uniqueRules = rules.Distinct(new RuleComparer()).ToList();