如何在LINQ中根据条件分组以消除重复代码？

你可以通过两种简洁方式实现条件分组，避免重复编写Select转换逻辑：

方法一：提前定义分组键选择器 #

先根据isRecipe的值确定分组键的选择逻辑，再复用后续统一的转换代码：

var allergensList = _context.RecipeAllergens
    .Where(x => x.ParentId == Id && x.AllergenId != null)
    .ToList();

// 根据isRecipe选择对应的分组键
Func<RecipeAllergen, object> groupKeySelector = isRecipe 
    ? (Func<RecipeAllergen, object>)(x => new { x.AllergenName, x.AllergenIcon, x.AllergenMaycontains })
    : x => new { x.AllergenName, x.AllergenIcon };

// 统一执行分组和转换逻辑
var allergens = allergensList
    .GroupBy(groupKeySelector)
    .Select(a =>
    {
        var v = a.OrderBy(x => x.AllergenMaycontains).First();
        return new AllergenInfo
        {
            AllergenName = v.AllergenName,
            AllergenIcon = v.AllergenIcon,
            AllergenMayContain = v.AllergenMaycontains ?? false
        };
    })
    .ToList();

方法二：在分组键中嵌入条件判断 #

直接在匿名类型的分组键里，根据isRecipe动态决定AllergenMaycontains字段是否参与分组（通过设为null消除其分组影响）：

var allergensList = _context.RecipeAllergens
    .Where(x => x.ParentId == Id && x.AllergenId != null)
    .ToList();

var allergens = allergensList
    .GroupBy(x => new { 
        x.AllergenName, 
        x.AllergenIcon, 
        // isRecipe为false时统一设为null，消除该字段对分组的影响
        AllergenMaycontains = isRecipe ? x.AllergenMaycontains : (bool?)null 
    })
    .Select(a =>
    {
        var v = a.OrderBy(x => x.AllergenMaycontains).First();
        return new AllergenInfo
        {
            AllergenName = v.AllergenName,
            AllergenIcon = v.AllergenIcon,
            AllergenMayContain = v.AllergenMaycontains ?? false
        };
    })
    .ToList();

两种方法都将重复的Select逻辑完全复用，仅在分组键部分做条件区分，大幅减少代码冗余。

内容的提问来源于stack exchange，提问作者Karim Ali