Object reference not set to an instance of an object - Select statement Sqlite Xamarin iOS - ios

Question

I am fetching the values from sqlite database in my iOS application. I have written a Select statement like stringquery = "Select * from tablename" and executing it using
database.Query < table > (stringquery);
and assigning the values to the properties in a class. The class has properties with the same name that the columns has in the table in sqlite database.
as the above statement executes i am getting error Object reference not set to an instance of an object
Please suggest the solution this issue.
Thanks

Answer 1

It sounds like you are initializing the SQLite Database incorrectly. I've added code below that shows how to implement a SQLite Database in Xamarin.Forms.
This Xamarin.Forms app, contains a fully implemented SQLite Database:
https://github.com/brminnick/InvestmentDataSampleApp
ISQLite.cs
Create this file in the Xamarin.Forms PCL. It allows us to access the iOS and Android File Systems to create our Database Connection
using SQLite;
namespace SampleApp
{
public interface ISQLite
{
SQLiteAsyncConnection GetConnection();
}
}
SQLite_Android.cs
Create this file in the Android project. It returns the Android file path for our SQLite Database Connection.
using System.IO;
using SampleApp.Droid;
using SQLite;
using Xamarin.Forms;
[assembly: Dependency(typeof(SQLite_Android))]
namespace SampleApp.Droid
{
public class SQLite_Android : ISQLite
{
#region ISQLite implementation
public SQLiteAsyncConnection GetConnection()
{
var sqliteFilename = "DatabaseFileName.db3";
string documentsPath = System.Environment.GetFolderPath(System.Environment.SpecialFolder.Personal); // Documents folder
var path = Path.Combine(documentsPath, sqliteFilename);
var conn = new SQLiteAsyncConnection(path, SQLiteOpenFlags.ReadWrite | SQLiteOpenFlags.Create | SQLiteOpenFlags.SharedCache);
// Return the database connection
return conn;
}
#endregion
}
}
SQLite_iOS.cs
Create this file in the iOS project. It returns the iOS file path for our SQLite Database Connection.
using System;
using System.IO;
using SQLite;
using Xamarin.Forms;
using SampleApp.iOS;
[assembly: Dependency(typeof(SQLite_iOS))]
namespace SampleApp.iOS
{
public class SQLite_iOS : ISQLite
{
#region ISQLite implementation
public SQLiteAsyncConnection GetConnection()
{
var sqliteFilename = "DatabaseFileName.db3";
string documentsPath = Environment.GetFolderPath(Environment.SpecialFolder.Personal); // Documents folder
string libraryPath = Path.Combine(documentsPath, "..", "Library"); // Library folder
var path = Path.Combine(libraryPath, sqliteFilename);
var conn = new SQLiteAsyncConnection(path, SQLiteOpenFlags.ReadWrite | SQLiteOpenFlags.Create | SQLiteOpenFlags.SharedCache);
// Return the database connection
return conn;
}
#endregion
}
}
SampleModelDatabase.cs
Create this file in your Xamarin.Forms PCL
using System.Linq;
using System.Threading.Tasks;
using System.Collections.Generic;
using SQLite;
using Xamarin.Forms;
namespace SampleApp
{
public static class SampleModelDatabase
{
#region Constant Fields
static readonly SQLiteAsyncConnection _database = DependencyService.Get<ISQLite>().GetConnection();
#endregion
#region Fields
static bool _isInitialized;
#endregion
#region Methods
public static async Task<IList<SampleModel>> GetAllItemsAsync()
{
if (!_isInitialized)
await Initialize();
return await _database.Table<SampleModel>().ToListAsync();
}
public static async Task<int> SaveItemAsync(SampleModel model)
{
if (!_isInitialized)
await Initialize();
return await _database.InsertOrReplaceAsync(model);
}
public static async Task<int> DeleteItemAsync(SampleModel model)
{
if (!_isInitialized)
await Initialize();
return await _database.DeleteAsync(model);
}
public static async Task<int> GetNumberOfRowsAsync()
{
if (!_isInitialized)
await Initialize();
return await _database.Table<SampleModel>().CountAsync();
}
static async Task Initialize()
{
await _database.CreateTableAsync<SampleModel>();
_isInitialized = true;
}
#endregion
}
}

Answer 2

I encountered the same problem as you. I found out that the point is database connection, a process that takes time, hasn't completely finished at the moment I started my query. so the solution will be something like:
await Connectdb();
// now do your query
var treatment = _database.Query<ClassName>("SELECT * FROM [TableName] WHERE ...");
where the Connectdb() is a function that does:
DependencyService.Get<IDatabaseConnection>().DbConnection();