如何合并针对不同实体的Expression<Func<T,bool>>表达式？

现有表达式 #

已有两个针对不同实体的过滤表达式：

public Expression<Func<Entity, bool>> EntityFilterForRead
{
    get
    {
        if (User.Roles.Contains(IdentityRole.ADMIN))
        {
            return _ => true;
        }

        return _ => false;
    }
}
    
public Expression<Func<AnotherEntity, bool>> AnotherEntityFilterForRead
{
    get
    {
        if (/* 自定义判断逻辑 */)
        {
            return _ => true;
        }

        return _ => false;
    }
}

需求 #

ThirdEntity包含Entity和AnotherEntity的关联属性（ThirdEntity.Entity、ThirdEntity.AnotherEntity），需要合并上述两个表达式，生成针对ThirdEntity的过滤表达式：

public Expression<Func<ThirdEntity, bool>> FilterForRead
{
    get
    {
        return // 合并两个表达式，满足 EntityFilterForRead && AnotherEntityFilterForRead
    }
}

遇到的异常 #

The LINQ expression ... could not be translated. Either rewrite the query in a form that can be translated, or switch to client evaluation explicitly by inserting a call to 'AsEnumerable', 'AsAsyncEnumerable', 'ToList', or 'ToListAsync'.

已尝试但无效的方案 #

• 直接引用类属性表达式用于LINQ查询的方案
• 合并同类型Expression<Func<T, bool>>的通用方案

特殊情况 #

涉及不同实体类型的表达式合并，常规方案无法直接适配。

可行解决方案 #

核心思路是通过表达式树参数替换，将针对Entity/AnotherEntity的表达式，转换为基于ThirdEntity关联属性的表达式，再合并逻辑。

1. 实现参数替换的表达式访问器 #

public class ParameterReplacer : ExpressionVisitor
{
    private readonly ParameterExpression _oldParameter;
    private readonly Expression _newExpression;

    public ParameterReplacer(ParameterExpression oldParameter, Expression newExpression)
    {
        _oldParameter = oldParameter;
        _newExpression = newExpression;
    }

    protected override Expression VisitParameter(ParameterExpression node)
    {
        // 替换目标参数为新的属性表达式
        return node == _oldParameter ? _newExpression : base.VisitParameter(node);
    }
}

2. 合并表达式的实现 #

public Expression<Func<ThirdEntity, bool>> FilterForRead
{
    get
    {
        // 获取原过滤表达式
        var entityFilter = EntityFilterForRead;
        var anotherEntityFilter = AnotherEntityFilterForRead;

        // 创建ThirdEntity类型的参数
        var thirdEntityParam = Expression.Parameter(typeof(ThirdEntity), "third");

        // 生成ThirdEntity关联属性的表达式
        var entityPropertyExpr = Expression.Property(thirdEntityParam, nameof(ThirdEntity.Entity));
        var anotherEntityPropertyExpr = Expression.Property(thirdEntityParam, nameof(ThirdEntity.AnotherEntity));

        // 替换原表达式的参数为关联属性表达式
        var transformedEntityFilter = new ParameterReplacer(entityFilter.Parameters[0], entityPropertyExpr)
            .Visit(entityFilter.Body) as Expression;

        var transformedAnotherEntityFilter = new ParameterReplacer(anotherEntityFilter.Parameters[0], anotherEntityPropertyExpr)
            .Visit(anotherEntityFilter.Body) as Expression;

        // 合并两个表达式为AND逻辑
        var combinedBody = Expression.AndAlso(transformedEntityFilter, transformedAnotherEntityFilter);

        // 生成最终的Lambda表达式
        return Expression.Lambda<Func<ThirdEntity, bool>>(combinedBody, thirdEntityParam);
    }
}

原理说明 #

通过ExpressionVisitor修改原表达式的参数节点，将原本指向Entity/AnotherEntity的参数，替换为ThirdEntity对应的关联属性。转换后的表达式树完全基于ThirdEntity，EF Core可以正常解析并转换为SQL语句，避免客户端求值的异常。

内容的提问来源于stack exchange，提问作者Amentos