如何将遗留数据库搜索逻辑中的重复代码重构为简洁方法？

对接遗留数据库时，搜索逻辑只能基于表中Level-1、Level-2这类行名称匹配，现有代码冗余严重，希望优化成更简洁的实现：

if (criteria.Levels.Contains("Level-1", StringComparer.CurrentCultureIgnoreCase))
{
    var filteredResult = allCachedItems
        .Where(x => !string.IsNullOrEmpty(x.Level1));

    resultsSet.AddRange(filteredResult.Except(resultsSet));
}

if (criteria.Levels.Contains("Level-2", StringComparer.CurrentCultureIgnoreCase))
{
    var filteredResult = allCachedItems
        .Where(x => !string.IsNullOrEmpty(x.Level2));

    resultsSet.AddRange(filteredResult.Except(resultsSet));
}

if (criteria.Levels.Contains("Level-3", StringComparer.CurrentCultureIgnoreCase))
{
    var filteredResult = allCachedItems
        .Where(x => !string.IsNullOrEmpty(x.Level3));

    resultsSet.AddRange(filteredResult.Except(resultsSet));
}

需求说明：数据库表包含多个Level{X}列，适用记录会填充文本，不适用则为空；搜索表单传入Level-{x}格式的参数，需优化过滤逻辑，比如创建接收字符串和委托的通用方法，或其他简洁方案。

方案1：封装通用过滤方法 #

写一个通用方法统一处理过滤、去重逻辑，避免重复代码：

// 替换YourItemType为实际的模型类型
private void AddLevelResults(IEnumerable<YourItemType> allCachedItems, HashSet<YourItemType> resultsSet, string levelParam, Func<YourItemType, string> levelPropertySelector)
{
    if (criteria.Levels.Contains(levelParam, StringComparer.CurrentCultureIgnoreCase))
    {
        var filtered = allCachedItems.Where(x => !string.IsNullOrEmpty(levelPropertySelector(x)));
        resultsSet.UnionWith(filtered); // 用HashSet的UnionWith替代AddRange+Except，性能更优
    }
}

调用时直接传入对应参数即可：

var resultsSet = new HashSet<YourItemType>();
AddLevelResults(allCachedItems, resultsSet, "Level-1", x => x.Level1);
AddLevelResults(allCachedItems, resultsSet, "Level-2", x => x.Level2);
AddLevelResults(allCachedItems, resultsSet, "Level-3", x => x.Level3);

方案2：映射字典批量处理 #

如果后续要新增Level-4、Level-5这类级别，只需在字典中添加映射，无需修改核心逻辑：

// 建立级别参数与属性选择器的映射（忽略大小写）
var levelMappings = new Dictionary<string, Func<YourItemType, string>>(StringComparer.CurrentCultureIgnoreCase)
{
    ["Level-1"] = x => x.Level1,
    ["Level-2"] = x => x.Level2,
    ["Level-3"] = x => x.Level3
};

var resultsSet = new HashSet<YourItemType>();
foreach (var levelParam in criteria.Levels)
{
    if (levelMappings.TryGetValue(levelParam, out var selector))
    {
        var filtered = allCachedItems.Where(x => !string.IsNullOrEmpty(selector(x)));
        resultsSet.UnionWith(filtered);
    }
}

方案3：LINQ组合查询（一次性生成结果） #

通过组合条件，用LINQ一次性筛选出所有符合要求的记录：

var validConditions = new List<Func<YourItemType, bool>>();

if (criteria.Levels.Contains("Level-1", StringComparer.CurrentCultureIgnoreCase))
    validConditions.Add(x => !string.IsNullOrEmpty(x.Level1));
if (criteria.Levels.Contains("Level-2", StringComparer.CurrentCultureIgnoreCase))
    validConditions.Add(x => !string.IsNullOrEmpty(x.Level2));
if (criteria.Levels.Contains("Level-3", StringComparer.CurrentCultureIgnoreCase))
    validConditions.Add(x => !string.IsNullOrEmpty(x.Level3));

// 只要满足任一条件即可保留，Distinct自动去重
var resultsSet = allCachedItems.Where(item => validConditions.Any(cond => cond(item))).Distinct().ToList();

内容的提问来源于stack exchange，提问作者Username_null