如何将一种类型的表达式树转换为不同的表达式类型？

Question

如果我有两个几乎相同的类 Animal 和 AnimalViewModel 以及与视图模型相关的表达式树，我如何将其转换为 Animal？

public class Animal
{
   public string Species { get; set; }
   public string Name { get; set; }
   public string Sound { get; set; }
}
public class AnimalViewModel : ViewModelBase
{
   public string Species { get; set; }
   public string Name { get; set; }
   public string Sound { get; set; }
}

如何将 Expression> 转换为 Expression>？

public static Expression<Func<Animal,bool>> Translate (Expression<Func<AnimalViewModel,bool>> expression)
{
  // What goes here?  I assume I have to traverse the tree somehow.
}

Answer 1

这是一位完成这项工作的访客。

• 它创建参数的副本（因为我们需要创建一个新参数并用旧参数的所有引用替换新参数）
• 它遍历树的 .Body，替换参数，并将针对 old 类型的任何成员访问切换为 new 类型上的类似命名成员，
• 它使用我们发明的参数重新组装 lambda 早期

：

class TypeChangeVisitor : ExpressionVisitor
{
    private readonly Type from, to;
    private readonly Dictionary<Expression, Expression> substitutions;
    public TypeChangeVisitor(Type from, Type to, Dictionary<Expression, Expression> substitutions)
    {
        this.from = from;
        this.to = to;
        this.substitutions = substitutions;
    }
    public override Expression  Visit(Expression node)
    { // general substitutions (for example, parameter swaps)
        Expression found;
        if(substitutions != null && substitutions.TryGetValue(node, out found))
        {
            return found;
        }
        return base.Visit(node);
    }
    protected override Expression VisitMember(MemberExpression node)
    { // if we see x.Name on the old type, substitute for new type
        if (node.Member.DeclaringType == from)
        {
            return Expression.MakeMemberAccess(Visit(node.Expression),
                to.GetMember(node.Member.Name, node.Member.MemberType,
                BindingFlags.Instance | BindingFlags.Static | BindingFlags.Public | BindingFlags.NonPublic).Single());
        }
        return base.VisitMember(node);
    }
}
public class Program
{
    public static void Main()
    {
        Expression<Func<AnimalViewModel, bool>> predicate = x => x.Name == "abc";
        var switched = Translate<AnimalViewModel, Animal>(predicate);
    }
    public static Expression<Func<TTo, bool>> Translate<TFrom, TTo>(Expression<Func<TFrom, bool>> expression)
    {
        var param = Expression.Parameter(typeof(TTo), expression.Parameters[0].Name);
        var subst = new Dictionary<Expression, Expression> { { expression.Parameters[0], param } };
        var visitor = new TypeChangeVisitor(typeof(TFrom), typeof(TTo), subst);
        return Expression.Lambda<Func<TTo, bool>>(visitor.Visit(expression.Body), param);
    }
}

代码 如果你有x.Something.Name 你可能需要更加小心，但这应该会给你一个合理的方法。

Answer 2

您是否尝试过 Automapper 来处理类似的事情？