Question

已解决

我有一个从类创建的名为 Strategy 的SQLite表，它具有两个属性String StrategyCode和String StrategyID。我正在使用此代码将StrategyCode列绑定到微调器。

private void BindList()
{
    try
    {
        var list = connection.Query<Strategy>("SELECT StrategyCode FROM Strategy");
        BindStrategy = list.ConvertAll(x => Convert.ToString(x));
        var StratAdpater = new ArrayAdapter<System.String>(this, Resource.Layout.spinner_item, BindStrategy);
        StratAdpater.SetDropDownViewResource(Resource.Layout.spinner_item);
        spinner2.Adapter = StratAdpater;       
    }
    catch(Exception e)
    {
        Toast.MakeText(this, e.ToString(), ToastLength.Long).Show();
    }
}

在此微调框的正下方有一个 textview ，我想在其中显示从微调框选择的相应StrategyCode的StrategyID。这就是我的做法。

try
 {
   var selectedstrat = spinner2.SelectedItem.ToString();
   var userselection = (from a in connection.Table<Strategy>()
                                     where a.StrategyCode == selectedstrat
                                     select a);            
    strategyID.Text = userselection.FirstOrDefault().StrategyID;
 }
catch (Exception e)
{
    Toast.MakeText(this, e.ToString(), ToastLength.Long).Show();
}

但是我正在收到System.NullReferenceException-（对象引用未设置为对象的实例），并且StrategyID尚未设置为Textview。但是，当我使用HardCode的StrategyCode值时，例如：

var userselection = (from a in connection.Table<Strategy>()
                                         where a.StrategyCode == "Strategy1"
                                         select a);

然后，TextView显示硬编码策略的ID。这里可能出什么问题了？我尝试过

 var userselection = (from a in connection.Table<Strategy>()
                                         where a.StrategyCode == selectedstrat
                                         select a.StrageyID).FirstOrDefault();

                    StrategyID.Text = userselection.ToString();

但是我仍然无法在TextView上获取StrategyID。

Answer 1

让我们看看这一行：

var userselection = (from a in connection.Table<Strategy>() where a.StrategyCode == selectedstrat select a.StrageyID).FirstOrDefault();

如果userselection是null怎么办？

StrategyID.Text = userselection.ToString(); ^^^^^^^^^^^^^ <- null => throws

使用Linq Xamarin时出现System.NullReferenceException

1 个答案: