LINQ对象引用未设置 [英] LINQ Object Referance not set

问题描述

搜索从数据网格中选择的项目(在Guid上)时，出现对象引用未设置为对象实例"错误.我检查了该项目是否确实返回了Guid(通过将其写入页面上的标签)，但是在我的linq查询(我认为)中，我进行了不正确的比较.

I am getting an "Object Reference not set to an instance of an object" error when searching for an item (on Guid) that is selected from a datagrid. I have checked that the item does return the Guid correctly (by writing it to a label on the page), however in my linq query (i assume) i am comparing incorrectly.

ctx是domaindatasource，我知道我要删除的元素存在.

ctx is the domaindatasource, I know the element im trying to remove exists.

      private void medItemRemove_Click(object sender, RoutedEventArgs e)
    {

            MedicineInventory M = (MedicineInventory)medicineInventoryDataGrid.SelectedItem;
            Guid Mid = M.MedicineInventoryId;
            MedicineInventory toRemove = new MedicineInventory();
            toRemove = (from a in ctx.MedicineInventories where (a.MedicineInventoryId == Mid) select a).Single();
            ctx.MedicineInventories.Remove(toRemove);
            ctx.SubmitChanges();

        }

推荐答案

按如下所示重写代码:

private void medItemRemove_Click(object sender, RoutedEventArgs e)
{

    MedicineInventory M = (MedicineInventory)medicineInventoryDataGrid.SelectedItem;
    Guid Mid = M.MedicineInventoryId;
    MedicineInventory toRemove = (from a in ctx.MedicineInventories where (a != null && a.MedicineInventoryId == Mid) select a).SingleOrDefault();
    if (toRemove != null){
       ctx.MedicineInventories.Remove(toRemove);
       ctx.SubmitChanges();
    }
    else { .... } // code if toRemove is null

 }

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