为复合 DTO 编写表达式树

Question

假设我有以下 3 个 DTO

public class Mailing
{
    public long Id { get; set; }
    //...

    public long IdSender  { get; set; }
    public Sender Sender { get; set; }

    public long IdTemplate { get; set; }
    public Template Template { get; set; }
}
public class Sender
{
    public long Id { get; set; }
    public string Email { get; set; }
    //...
}
public class Template
{
    public long Id { get; set; }
    public string Name { get; set; }
    //...
}

我有 3 个表达式树来管理 DAO 到 DTO 的转换：

private static readonly Expression<Func<DaoMailing, Mailing>> ToMailingShort =
        input => new Mailing
                        {
                            Id = input.Id,
                            IdSender = input.IdSender,
                            IdTemplate = input.IdTemplate,
                            // ...
                        };

private static readonly Expression<Func<DaoTemplate, Template>> ToTemplate =
        input => new Template
                        {
                            Id = input.Id,
                            Name = input.Name,
                            // ...
                        };


private static readonly Expression<Func<DaoSender, Sender>> ToSender =
        input => new Sender
                        {
                            Id = input.Id,
                            Email = input.Email,
                            // ...
                        };

如何从上面的 3 个中构建给定的表达式？

private static readonly Expression<Func<DaoMailing, DaoTemplate, DaoSender, MailingFull>> ToMailingFull =
        (input, template, sender) => new Mailing
                        {
                            Id = input.Id,
                            IdSender = input.IdSender,
                            IdTemplate = input.IdTemplate,
                            // ...
                            Template = new Template
                            {
                                Id = template.Id,
                                Name = template.Name,
                                // ...
                            },
                            new Sender
                            {
                                Id = sender.Id,
                                Email = sender.Emai;,
                                // ...
                            }
                        };

显然，目标是避免重写复合转换中的每个单独转换

Answer 1

简短的回答是使用AutoMapper，或者将编译后的表达式转换成函数。在 C# 中编写函数很容易，编写表达式要难得多。

较长的答案是这是可能的，但并不容易。您的代码使用 'friendly' 表达式语法，但要真正混合和匹配表达式，您需要使用不友好的版本，该版本更加丑陋且难以维护：

    private static readonly Expression<Func<DaoMailing, DaoTemplate, DaoSender, Mailing>> ToMailingFull =
        (Expression<Func<DaoMailing, DaoTemplate, DaoSender, Mailing>>)Expression.Lambda(
            Expression.MemberInit(
                Expression.New(typeof(Mailing).GetConstructor(Type.EmptyTypes)),
                (ToMailingShort.Body as MemberInitExpression).Bindings
                    .Concat(new List<MemberBinding>{
                        Expression.MemberBind(typeof(Mailing).GetProperty("Sender"), (ToSender.Body as MemberInitExpression).Bindings),
                        Expression.MemberBind(typeof(Mailing).GetProperty("Template"), (ToTemplate.Body as MemberInitExpression).Bindings)
                    })
            ),
            ToMailingShort.Parameters[0],
            ToTemplate.Parameters[0],
            ToSender.Parameters[0]
        );

为复合 DTO 编写表达式树

Composing expression trees for composite DTO

c#

composition

expression-trees