Question

所以在下面的代码中，我应该能够连接到数据库，然后将该数据库输出到数据网格以供用户查看。然而实际发生的是，一旦代码命中Load方法，它就会抛出一个空错误。我不知道为什么，如果你们能解释为什么我没有正确地提取数据，我将不胜感激...... 有问题的一行是：

List<Users> _users = UserRepository.GetUsers();

我的代码是这样的：

DBDisplay.xaml.cs

public partial class DBDisplay : Window
{
    public DBDisplay()
    {
        InitializeComponent();
        var viewModel = new DbDisplayViewModel();
        viewModel.UserRepository = new UserRepository();
        this.DataContext = viewModel;
    }
}

DBDisplayViewModel.cs

public class DbDisplayViewModel
{
    private ObservableCollection<Users> _users;

    public IUserRepository UserRepository{ get; set; }

    public DbDisplayViewModel()
    {
        Load();
    }



    public ObservableCollection<Users> Users
    {
        get
        {
            if (_users == null)
            {
                _users = new ObservableCollection<Users>();
            }

            return _users;
        }
        set
        {
            if (value != null)
            {
                _users = value;
            }
        }
    }

    private void Load()
    {
        List<Users> _users = UserRepository.GetUsers(); // THIS IS THE BROKEN LINE THAT THROWS A NULL ERROR
        Users = new ObservableCollection<Users>(_users);
    }
}

UserRepository.cs

    public class UserRepository : IUserRepository
{
    private string connectionStr;

    public System.Data.SqlClient.SqlConnection connectionToDB;

    private System.Data.SqlClient.SqlDataAdapter dataAdapter;

    public void openConnection()
    {
        // create the connection to the database as an instance of System.Data.SqlClient.SqlConnection
        connectionToDB = new System.Data.SqlClient.SqlConnection(connectionStr);

        //open the connection
        connectionToDB.Open();
    }


    // Get the data set generated by the sqlStatement
    public System.Data.DataSet getDataSet(string sqlStatement)
    {
       dataAdapter = new System.Data.SqlClient.SqlDataAdapter(sqlStatement, connectionToDB);

        // create the dataset
        var dataSet = new System.Data.DataSet();

        dataAdapter.Fill(dataSet);

        //return the dataSet
        return dataSet;
    }
    public List<Users> GetUsers()
    {
        connectionToDB = new System.Data.SqlClient.SqlConnection(connectionStr);
        connectionToDB.Open();
        List<Users> userlist = new List<Users>();
        DataSet ds = getDataSet("Select UserID, FirstName, LastName from UserData");
        Users users;

        foreach (DataRow row in ds.Tables[0].Rows)
        {
            users = new Users
            {
                UserID = row["UserID"].ToString(),
                FirstName = row["FirstName"].ToString(),
                LastName = row["LastName"].ToString()
            };


            userlist.Add(users);
        }

        return userlist;

    }


}

Users.cs只有一些属性 IUserRepository有一个用户存储库的get set方法

Answer 1

您正在从构造函数中调用Load。那时，UserRepository仍为null（您在构造函数之后的调用中设置它）。

将存储库提供给构造函数并从那里进行设置。

public DbDisplayViewModel(IUserRepository repository) { this.UserRepository = repository; this.Load(); }

必读：What is a NullReferenceException, and how do I fix it?

调用数据库以填充datagrid行时出现空异常

1 个答案: