防爆pression是一个值，因此不能作为赋值目标 [英] Expression Is a value and therefore cannot be the target of an assignment

问题描述

我跑进使用结构今天抓住我措手不及的问题，我希望有人能提供一些线索就可以了我。

I ran into an issue using a struct today that caught me off guard, and I was hoping someone could shed some light on it for me.

我有这样定义一个结构：

I have a struct defined like this:

public struct PaymentDetail
{
    public Decimal Amount{get;set;}
    public string CheckNumber{get;set;}
    public PaymentType PaymentType{get;set;}
}

我有一个包含此信息的类

I have a class that contains this info

public class Transaction
{
    public PaymentDetail Payment{get;}
}

我这，我想设置这样的基本属性presentation模型

I have a presentation model which in which I want to set the underlying properties like this

public class ViewModel
{
    public Decimal Amount
    {
        get{return _Transaction.PaymentDetail.Amount;}
        set
        {
             //This is the offending line of code
             _Transaction.PaymentDetail.Amount = value;
             RaisePropertyChanged("Amount");
        }
    }
}

什么是奇怪的是，我可以做这项工作，如果我在付款属性更改为公共​​领域是这样的：

What's wierd is I can make this work if I change the Payment property to a public field like this:

public class Transaction
{
    public PaymentDetail Payment;
}

有obviosuly东西我不理解导致此结构。这是一个坏主意？有没有更好的办法？我究竟做错了什么？

there's obviosuly something I don't understand about structs that are causing this. Is this a bad idea? Is there a better way? What am I doing wrong?

推荐答案

第一 - 没有可变的结构体（即一个结构，你可以建成后，通过制定者等改变价值观，）。即混乱这里的首要原因。

First - don't have mutable structs (i.e. a struct where you can change the values after construction, via setters etc). That is the primary cause of confusion here.

问题的关键是，当你调用一个属性（如付款）你（在您的本地栈区）的值的复制。一类，即基准（没有问题）的副本。对于一个结构它是结构本身的副本。任何更改该值将被丢弃，所以编译器已经从丢失数据拦你。

The point is; when you call a property (like Payment) you get a copy of the value (in your local stack area). For a class, that is a copy of the reference (no problem). For a struct it is a copy of the struct itself. Any changes to that value will be discarded, so the compiler has stopped you from losing data.

当它是一个公共领域，你是直接变异的原始值，这就是为什么它不介意。但变异结构确实不是一个好主意。

When it is a public field, you are mutating the original value directly, which is why it doesn't mind. But mutating structs really isn't a good idea.

请 PaymentDetail A类;这是正确的解决方案在这里...

Make PaymentDetail a class; that is the correct solution here...

在.NET中，结构不是无物的行为 - 他们是价值类型。比如像货币/值对，时间范围，等等可能使有效的结构体 - 而不是 PaymentDetail

In .NET, structs aren't "objects without behaviour" - they are "value-types". Things like "a currency/value pair", "a time range", etc might make valid structs - but not PaymentDetail.

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