通过调用datalayer获取数据到ViewModel

Question

我正在尝试将存储的数据从 SQLite 数据库中获取到一个简单的文本框中，但我总是得到 null。当数据层被调用时，数据首先正确加载，但在某些时候它再次变为空，我无法弄清楚它在哪里或为什么出错。

这是我的视图模型

public class FirstReadViewModel : NotifyUIBase
{    
    private string _scriptNotes;
    public string ScriptNotes    //binded in the View
    {
        get { return _scriptNotes; }
        set { _scriptNotes = value; RaisePropertyChanged("ScriptNotes"); }
    }

    public FirstReadViewModel()
    {
        var dbFunctions = new DataLayer();
        dbFunctions.GetFirstReadNotes();
    } 
}

数据层

public void GetFirstReadNotes()
{
    String dbConnectionString = @"Data Source =DB.sqlite";

    SQLiteConnection cnn = new SQLiteConnection(dbConnectionString);
    cnn.Open();

    SQLiteCommand cmd = new SQLiteCommand(cnn);
    cmd.CommandText = "SELECT* FROM projects WHERE projectID = 1";

    SQLiteDataReader reader = cmd.ExecuteReader();
    while (reader.Read())
    {
         string ScriptNotes = reader["scriptnotes"].ToString();
    }
    reader.Close();
    cnn.Close();
    }
}

Answer 1

你的数据层方法应该如下所示return只需要先注意：

public string GetFirstReadNotes()
{
    string scriptNotes = string.Empty;
    String dbConnectionString = @"Data Source =DB.sqlite";

    SQLiteConnection cnn = new SQLiteConnection(dbConnectionString);
    cnn.Open();

    SQLiteCommand cmd = new SQLiteCommand(cnn);
    cmd.CommandText = "SELECT* FROM projects WHERE projectID = 1";

    SQLiteDataReader reader = cmd.ExecuteReader();
    if(reader.Read())
    {
         scriptNotes = reader["scriptnotes"].ToString();
    }
    reader.Close();
    cnn.Close();
    }
    return scriptNotes;
}

和FirstReadViewModel如下：

public FirstReadViewModel()
{
        var dbFunctions = new DataLayer();
        this.ScriptNotes = dbFunctions.GetFirstReadNotes();
}

Answer 2

你应该使用接口将你的 DataLayer 注入你的 FirstReadViewModel 并稍微更改你的代码

创建接口

interface IFirstReadViewModelDAL
{
    string GetFirstReadNotes();
}

使用它，也利用using函数

   internal class DataLayer: IFirstReadViewModelDAL
    {
        public string GetFirstReadNotes()
        {
            string ScriptNotes;
            String dbConnectionString = @"Data Source =DB.sqlite";

            using(SQLiteConnection cnn = new SQLiteConnection(dbConnectionString))
            {
                cnn.Open();

                SQLiteCommand cmd = new SQLiteCommand(cnn);
                cmd.CommandText = "SELECT* FROM projects WHERE projectID = 1";

                using (SQLiteDataReader reader = cmd.ExecuteReader())
                {
                    while (reader.Read())
                    {
                        ScriptNotes = reader["scriptnotes"].ToString();
                    }
                }
            }

            return ScriptNotes;
        }
    }

使用接口

注入你的DataLayer

    private IFirstReadViewModelDAL dbFunctions;

    private string _scriptNotes;
    public string ScriptNotes    //binded in the View
    {
        get { return _scriptNotes; }
        set { _scriptNotes = value; RaisePropertyChanged("ScriptNotes"); }
    }

    public FirstReadViewModel(IFirstReadViewModelDAL injectDAL)
    {
        dbFunctions = injectDAL;
    }

    private void LoadData();
    {
        // use your dbFunctions
    }

要注入它，您需要一个单独的 class，比如

public class Controller
{
    public static FirstReadViewModel getNewFirstReadViewModel()
    {
        var dal = new DataLayer();
        var vm = new FirstReadViewModel(dal);

        return vm;
    }
}

通过调用datalayer获取数据到ViewModel

Getting data into ViewModel by calling datalayer

c#

wpf

binding

mvvm