如何让自定义Include/ThenInclude与EF Core原生实现行为一致？

要让自定义实现和EF Core原生行为一致，核心是让编译器能正确识别集合类型（List<B>/B[]）到IEnumerable<B>的隐式转换，并匹配对应重载。以下是具体修改方案：

1. 调整IIncludableQueryable的泛型设计 #

确保接口的第二个泛型参数支持集合类型的抽象表示，而非具体实现：

public interface IIncludableQueryable<TEntity, TProperty> : IQueryable<TEntity>
    where TEntity : class
{
}

2. 修正Include扩展方法的重载 #

为集合类型导航属性单独定义重载，让List<B>/B[]能隐式匹配到IEnumerable<B>版本，返回的IIncludableQueryable使用抽象集合类型作为泛型参数：

// 集合类型导航属性的Include重载
public static IIncludableQueryable<TEntity, IEnumerable<TProperty>> Include<TEntity, TProperty>(
    this IQueryable<TEntity> source,
    Expression<Func<TEntity, IEnumerable<TProperty>>> navigationPropertyPath)
    where TEntity : class
{
    // 自定义逻辑：记录导航属性、包装查询等
    return new Includer<TEntity, IEnumerable<TProperty>>(source, navigationPropertyPath);
}

// 非集合类型导航属性的Include重载
public static IIncludableQueryable<TEntity, TProperty> Include<TEntity, TProperty>(
    this IQueryable<TEntity> source,
    Expression<Func<TEntity, TProperty>> navigationPropertyPath)
    where TEntity : class
    where TProperty : class
{
    return new Includer<TEntity, TProperty>(source, navigationPropertyPath);
}

3. 补全ThenInclude的集合专属重载 #

明确针对IEnumerable<TPreviousElement>的ThenInclude重载，让编译器能正确推断集合元素类型：

// 集合导航属性的ThenInclude重载（链式包含集合）
public static IIncludableQueryable<TEntity, IEnumerable<TProperty>> ThenInclude<TEntity, TPreviousElement, TProperty>(
    this IIncludableQueryable<TEntity, IEnumerable<TPreviousElement>> source,
    Expression<Func<TPreviousElement, IEnumerable<TProperty>>> navigationPropertyPath)
    where TEntity : class
{
    return new Includer<TEntity, IEnumerable<TProperty>>(source, navigationPropertyPath);
}

// 集合导航属性的ThenInclude重载（链式包含单个属性）
public static IIncludableQueryable<TEntity, TProperty> ThenInclude<TEntity, TPreviousElement, TProperty>(
    this IIncludableQueryable<TEntity, IEnumerable<TPreviousElement>> source,
    Expression<Func<TPreviousElement, TProperty>> navigationPropertyPath)
    where TEntity : class
    where TProperty : class
{
    return new Includer<TEntity, TProperty>(source, navigationPropertyPath);
}

// 非集合导航属性的ThenInclude重载
public static IIncludableQueryable<TEntity, TProperty> ThenInclude<TEntity, TPreviousProperty, TProperty>(
    this IIncludableQueryable<TEntity, TPreviousProperty> source,
    Expression<Func<TPreviousProperty, TProperty>> navigationPropertyPath)
    where TEntity : class
    where TPreviousProperty : class
    where TProperty : class
{
    return new Includer<TEntity, TProperty>(source, navigationPropertyPath);
}

4. 实现Includer类的通用支持 #

确保Includer能兼容抽象集合类型的泛型参数：

public class Includer<TEntity, TProperty> : IIncludableQueryable<TEntity, TProperty>
    where TEntity : class
{
    private readonly IQueryable<TEntity> _source;

    public Expression Expression => _source.Expression;
    public Type ElementType => _source.ElementType;
    public IQueryProvider Provider => _source.Provider;

    public Includer(IQueryable<TEntity> source, LambdaExpression navigationPath)
    {
        _source = source;
        // 自定义逻辑：存储导航路径，用于后续中介处理
    }

    public IEnumerator<TEntity> GetEnumerator() => _source.GetEnumerator();
    IEnumerator IEnumerable.GetEnumerator() => GetEnumerator();
}

核心原理说明 #

之前的问题根源是：自定义Include返回的IIncludableQueryable使用了具体集合类型（如List<B>）作为泛型参数，导致编译器尝试匹配非集合版本的ThenInclude（要求前序属性是单个实体，而非集合）。

修改后，List<B>/B[]会隐式转换为IEnumerable<B>，匹配集合版Include，返回的IIncludableQueryable<TEntity, IEnumerable<B>>会触发集合版ThenInclude，编译器能正确推断出前序元素是B，从而匹配o => o.Cs的导航属性。

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