使用Linq Xamarin时出现System.NullReferenceException

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I have a SQLite Table created from a class named Strategy , which has two properties String StrategyCode and String StrategyID . I am Binding the StrategyCode Column to a Spinner with this code.

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();
    }
}  

I have a textview right below this Spinner, Where, i want to show the StrategyID of the corresponding StrategyCode selected from the Spinner. This is How I am doing it.

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();
}    

But i am getting a System.NullReferenceException - (Object Reference not Set to Instance of an Object) and the StrategyID is not getting set onto the Textview. But, When i HardCode the StrategyCode value For Example:

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

Then the TextView Shows the ID of the Hardcoded Strategy. What is Possibly Going Wrong Here? I tried this

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

                    StrategyID.Text = userselection.ToString();

But Still i am not able to get the StrategyID on the TextView.

Answer 1

Let's have a look at this line:

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

What if userselection is null ?

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