部分 Class 实例初始化空引用异常

Question

我有一个参考代码如下的服务：

[System.Xml.Serialization.SoapTypeAttribute(Namespace="urn:customer")]
public partial class Receipt : object, System.ComponentModel.INotifyPropertyChanged {

    private int counternoField;

    private double activekwhField;


    /// <remarks/>
    [System.Xml.Serialization.SoapElementAttribute("counter-no")]
    public int counterno {
        get {
            return this.counternoField;
        }
        set {
            this.counternoField = value;
            this.RaisePropertyChanged("counterno");
        }
    }

    /// <remarks/>
    [System.Xml.Serialization.SoapElementAttribute("active-km")]
    public double activekm {
        get {
            return this.activekm Field;
        }
        set {
            this.activekmField = value;
            this.RaisePropertyChanged("activekm");
        }
    }

    public event System.ComponentModel.PropertyChangedEventHandler PropertyChanged;

    protected void RaisePropertyChanged(string propertyName) {
        System.ComponentModel.PropertyChangedEventHandler propertyChanged = this.PropertyChanged;
        if ((propertyChanged != null)) {
            propertyChanged(this, new System.ComponentModel.PropertyChangedEventArgs(propertyName));
        }
    }
}
[System.Xml.Serialization.SoapTypeAttribute(Namespace="urn:customer")]
public partial class ArrayOfReceipt : object, System.ComponentModel.INotifyPropertyChanged {

    private Receipt[] itemField;

    /// <remarks/>
    public Receipt[] item {
        get {
            return this.itemField;
        }
        set {
            this.itemField = value;
            this.RaisePropertyChanged("item");
        }
    }

    public event System.ComponentModel.PropertyChangedEventHandler PropertyChanged;

    protected void RaisePropertyChanged(string propertyName) {
        System.ComponentModel.PropertyChangedEventHandler propertyChanged = this.PropertyChanged;
        if ((propertyChanged != null)) {
            propertyChanged(this, new System.ComponentModel.PropertyChangedEventArgs(propertyName));
        }
    }
}

而且，当我想创建 "ArrayOfReceipt" 的实例或设置一个值或访问它时，我总是遇到同样的问题：System.NullReferenceException。这是我尝试创建时的代码：

var prev_Cons = new myService.Receipt();
prev_Cons.counterno = 1;
prev_Cons.activekm = 3265;
myService.ArrayOfReceipt prev_ConsArr = new myService.ArrayOfReceipt();
prev_ConsArr.item.SetValue(prev_Cons, 0);

不幸的是，prev_ConsArr.item 总是空的，我无法初始化它。请告诉我一种初始化并为此对象设置一些值的方法。提前致谢。

Answer 1

您的代码编写方式，您可以像这样初始化prev_ConsArr.item：

prev_ConsArr.item = new Receipt[3];

这将创建一个可以容纳三个 Receipt 对象的新 ArrayOfReceipt。您还可以为 ArrayOfReceipt class 创建一个构造函数来初始化 item。这些方法中的任何一种都将消除您的 NullReferenceException

查看您使用 ArrayOfReceipt class 的方式，您可能需要考虑将 item 的类型更改为 List<Receipt>。这样可以更轻松地更改您存储的收据 classes 的数量。

根据您尝试执行的操作，您可能还想在 ArrayOfReceipts 中创建一个 AddReceipt 方法并将您的 PropertyChanged 事件移至该方法。现在，PropertyChanged 事件只会在您的 ArrayOfReceipts class 覆盖其数组 Receipts.

时触发

部分 Class 实例初始化空引用异常

Partial Class Instance Initialization Null Reference Exception

c#

constructor

partial-classes

nullreferenceexception

custom-type