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using System.Data.SQLite;

[...]
MessageBox.Show("RUN");
new SQLiteConnection();
MessageBox.Show("INIT CONN");
[...]                    

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using System;
using System.Collections.Generic;
using System.Data.SQLite;
using System.IO;
using System.Linq;
using System.Text;
using System.Threading.Tasks;
using System.Windows.Forms;

namespace testdb.libs
{
    class Database {

        public static String sourceDirectory = Environment.GetFolderPath(Environment.SpecialFolder.ApplicationData)+"\\testdb\\";
        public const String sqliteFile = "testdb.sdb";

        public int lastQueryCount = 0;
        public String lastQuery = "";

        private SQLiteConnection connection;
        private static Database instance = null;

        private bool boolInstallTables = false;

        public Database() {
            this.install();
            try
            {
                MessageBox.Show(boolInstallTables.ToString());
                connection = new SQLiteConnection();
                MessageBox.Show("Connected");
                connection.ConnectionString = "Data Source=" + sourceDirectory + sqliteFile + ";Version=3;FailIfMissing=True";
                MessageBox.Show(connection.State.ToString());
                if (connection.State != System.Data.ConnectionState.Open) {
                    connection.Open();
                }
            }
            catch (SQLiteException e)
            {
                MessageBox.Show(e.Message);
            }
            if (this.boolInstallTables) {
                MessageBox.Show("IS TRUE");
                this.installTables();
            }
        }

        private String install() {
            if ( !Directory.Exists(sourceDirectory) ) {
                Directory.CreateDirectory(sourceDirectory);
            }
            if ( !File.Exists(sourceDirectory+sqliteFile) ) {
                SQLiteConnection.CreateFile(sourceDirectory + sqliteFile);
                this.boolInstallTables = true;
                MessageBox.Show("INST");
            }

            return sourceDirectory;
        }

        private void installTables() {
            MessageBox.Show("folgt");
        }
    }
}