ICollectionView筛选器属性-空引用异常 [英] ICollectionView Filter property - Null reference exception

问题描述

我试图在WPF数据绑定中使用ICollectionView对象的filter属性.下面的代码-

 ICollectionView视图= CollectionViewSource.GetDefaultView(GridProductInformation.DataContext);
 int  SupplierID = GetComboBoxSelectedItem();
// 谓词< object> pred = new Predicate< object>(MatchSupplier); 
谓词< object> pred = 委托(对象 obj)
{
    NorthwindProduct _product = 新 NorthwindProduct();
    _product = obj  as  NorthwindProduct;
     int  _supplierID = Convert.ToInt16(_product.SupplierID);
    如果(_supplierID ==供应商ID)
        返回 真；
    其他
        返回 假；
};
view.Filter(pred);

代码在最后一行抛出运行时异常-空引用异常.对象引用未设置为对象的实例.

知道出什么问题了吗?

解决方案

@anshudutta看来，这是一个强制转换问题.尝试在

_product = obj as NorthwindProduct;

处设置断点，我认为这始终为空.

评估谓词中包含的

obj 

内容.

view.Filter = pred;

I am trying to use the filter property of ICollectionView object in WPF databinding. Code below -

ICollectionView view = CollectionViewSource.GetDefaultView(GridProductInformation.DataContext);
int supplierID = GetComboBoxSelectedItem();
//Predicate<object> pred = new Predicate<object>(MatchSupplier);
Predicate<object> pred = delegate(object obj)
{
    NorthwindProduct _product = new NorthwindProduct();
    _product = obj as NorthwindProduct;
    int _supplierID = Convert.ToInt16(_product.SupplierID);
    if (_supplierID == supplierID)
        return true;
    else
        return false;
};
view.Filter(pred);

The code throwsa run time exception in the last line - Null reference exception. Object reference not set to instance of an object.

Any idea what is going wrong?

解决方案

@anshudutta their is a casting problem itseems. Try setting a breakpoint at

_product = obj as NorthwindProduct;

I think this is always null.

Evaluate, what

obj 

which is coming inside your predicate.

Apologies if you''ve found the answer already, but this if probably what you need ...

view.Filter = pred;

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