連接到數據庫時出現空引用異常

Question

當我通過DataBaseService類實例調用UserExists方法時，它立即無法連接到數據庫。 盡管我為SqlConnection提供了連接字符串，但實例為null。

using System;
using System.Collections.Generic;
using System.Data;
using System.Data.SqlClient;
using System.Windows.Forms;

namespace Somename
{
    public class DataBaseService
    {
        // Setting up the Database Services
        private SqlConnection myConnection;
        private const string ConnectionString = "Data Source=Test-LAPTOP\\MSSQLSERVER1;Initial Catalog=myDatabaseTest;Integrated Security=True";

        private static readonly DataBaseService _instance;

        static DataBaseService()
        {
            _instance = new DataBaseService();
            Initilize();
        }

        public static DataBaseService Instance;
        private static void Initilize()
        {
            try
            {
                Instance.myConnection = new SqlConnection(ConnectionString);
                Instance.myConnection.Open();
            }
            catch (Exception ex)
            {
                MessageBox.Show("Error while connecting to DB: " + ex.Message);
                return;
            }
        }

        public bool UserExists(string username)
        {
            var myQuery = "Select * from UserProfile where userId ="+username+"";
            SqlCommand sqlCmd = new SqlCommand(myQuery, Instance.myConnection);
            sqlCmd.CommandType = CommandType.Text;
            //sqlCmd.CommandText = "Select * from Customers";
            SqlDataAdapter sqlDataAdap = new SqlDataAdapter(sqlCmd);

            DataTable dtRecord = new DataTable();
            sqlDataAdap.Fill(dtRecord);

            if (dtRecord == null)
            {
                return false;

            }  
            else 
            {
                return true;
            }
        }
    }
}

Answer 1

您的代碼中有兩個DataBaseService實例， _instance和Instance 。 您可以在靜態構造函數中初始化_instance ，但不要初始化Instance 。

您可能想在當前正在使用Instance任何地方使用_instance ，並完全擺脫Instance ：

...

private static readonly DataBaseService _instance;

static DataBaseService()
{
    _instance = new DataBaseService();
    Initilize();
}

private static void Initilize()
{
    try
    {
        _instance.myConnection = new SqlConnection(ConnectionString);
        _instance.myConnection.Open();
    }
    catch (Exception ex)
    {
        MessageBox.Show("Error while connecting to DB: " + ex.Message);
        return;
    }
}

...

Answer 2

您可能要遵循以下模式：

private static DataBaseService _instance;
// public property
public static DataBaseService Instance
{
    get
    {
        return _instance ?? (_instance = new DataBaseService());
    }
}