XamarinForms Exception

Question

I created a search bar with database connection. When i run it. It get an exception. i use a database that was created before.

Model class entries

public class entries
{
    public entries()
    {

    }

    public entries(string word)
    {
        this.word = word;
    }
    public entries(string word, string wordtype, string definition)
    {
        this.word = word;
        this.wordtype = wordtype;
        this.definition = definition;
    }   
    public string word
    { get; set; }

    public string wordtype
    { get; set; }

    public string definition
    { get; set; }

    public List<string> GetWord { get; set; }
}

Class DatabaseManager:

public class DatabaseManager
{
    SQLiteConnection dbConnection;
    public DatabaseManager()
    {
        dbConnection = DependencyService.Get<ISQLite>().GetConnection();
    }



    public List<string> GetWord()
    {
        return dbConnection.Query<string>("Select word From [entries]").ToList();
    }


}

MainPage.xaml.cs:

    DatabaseManager dbManager = new DatabaseManager();
 private void MySearchBar_SearchButtonPressed(object sender, EventArgs e)
        {

        }

        private void MySearchBar_TextChanged(object sender, TextChangedEventArgs e)
        {
            var keyword = MySearchBar.Text;
            if(keyword.Length >= 1) { 
                var suggestions = dbManager.GetWord().Where(c => c.ToLower().Contains(keyword.ToLower()));

                SuggestionListView.ItemsSource = suggestions;
                SuggestionListView.IsVisible = true;
            }
            else
            {
                SuggestionListView.IsVisible = false;
            }
        }

This is Exception:

System.MissingMethodException: Constructor on type 'System.String' not found.

Please Help. Thank you so much.

'I'm new in ...' usually means 'I'm too lazy to use google'. Please do not start your posts like this. — Anton Belousov
Thanks Jason!! This is the line that throws the exception: return dbConnection.Query<string>("Select word From [entries]").ToList(); — TanVuBoyGib
@TanVuBoyGib You would need to provide a class that has just one string property and then you can use Linq projection to transform those query results into a string list — SushiHangover

MilanG MilanG · Accepted Answer · 2018-06-04T06:42:21

You might have created an Entity for the “entries” table, say for example it is,

[Table (“entries”)]
public class Entries
{
    …

    [Column (“word”)]
    public string Word;

    …
}

then change the line

dbConnection.Query<string>("Select word From [entries]").ToList();

to

dbConnection.Table<Entries>().Select(x => x.Word).ToList();

This will remove the requirement of creating one additional class as Sushi said in above comment.

Moreover, following line will throw a NullReferenceException at c.ToLower(), if any of the word contains Null into the table.

dbManager.GetWord().Where(c => c.ToLower().Contains(keyword.ToLower()));

So to get rid of this, select results which has not null values like,

dbConnection.Table<Entries>().Where(x => x.Word != null).Select(x => x.Word).ToList();

XamarinForms Exception

1 Answers