Question

我想将作为myButton命令参数的assignment_id传递给.cs文件来更新数据库中的值。我写了这段代码但是当我点击myButton时，它显示了一个消息框，方法和操作失败了。我该如何解决这个问题？

private void Command(Int32 parameter)
{
    p = parameter;
}

private void btnUpdate_Click(Object sender, RoutedEventArgs e)
{  
    try
    {
        SqlConnection con = new SqlConnection("server=(local); Integrated Security=true; Database=nrcs");
        con.Open();
        SqlCommand comm = new SqlCommand("UPDATE Assignments SET assignment_title='myassignment'  WHERE assignment_id=" + p + ";", con);
        comm.ExecuteNonQuery();
        con.Close();
    }
    catch (Exception ex)
    {
        MessageBox.Show(ex.Message.ToString());
    }
}

这里是Xaml代码：

<Button x:Name="myButton" Content="update" Command="{Binding Command}"     CommandParameter="{Binding assignment_id}" Click="btnUpdate_Click" ></Button>

Answer 1

似乎您可能正在尝试混合使用命令处理程序和事件处理程序。如果要使用命令，则应首先在其中定义命令的情况下创建一个视图模型。然后在xaml中将数据上下文设置为该视图模型。然后，您应该能够以所需的方式绑定命令。一种常见的方法是首先定义一个RelayCommand类。

一个简单的中继类可能看起来像这样：

public class RelayCommand<T> : ICommand
{
    #region Fields

    readonly Action<T> _execute = null;
    readonly Predicate<T> _canExecute = null;

    #endregion

    #region Constructors

    /// <summary>
    /// Initializes a new instance of <see cref="DelegateCommand{T}"/>.
    /// </summary>
    /// <param name="execute">Delegate to execute when Execute is called on the command.  This can be null to just hook up a CanExecute delegate.</param>
    /// <remarks><seealso cref="CanExecute"/> will always return true.</remarks>
    public RelayCommand(Action<T> execute)
        : this(execute, null)
    {
    }

    /// <summary>
    /// Creates a new command.
    /// </summary>
    /// <param name="execute">The execution logic.</param>
    /// <param name="canExecute">The execution status logic.</param>
    public RelayCommand(Action<T> execute, Predicate<T> canExecute)
    {
        if (execute == null)
            throw new ArgumentNullException("execute");

        _execute = execute;
        _canExecute = canExecute;
    }

    #endregion

    #region ICommand Members

    ///<summary>
    ///Defines the method that determines whether the command can execute in its current state.
    ///</summary>
    ///<param name="parameter">Data used by the command.  If the command does not require data to be passed, this object can be set to null.</param>
    ///<returns>
    ///true if this command can be executed; otherwise, false.
    ///</returns>
    public bool CanExecute(object parameter)
    {
        return _canExecute == null ? true : _canExecute((T)parameter);
    }

    ///<summary>
    ///Occurs when changes occur that affect whether or not the command should execute.
    ///</summary>
    public event EventHandler CanExecuteChanged
    {
        add { CommandManager.RequerySuggested += value; }
        remove { CommandManager.RequerySuggested -= value; }
    }

    ///<summary>
    ///Defines the method to be called when the command is invoked.
    ///</summary>
    ///<param name="parameter">Data used by the command. If the command does not require data to be passed, this object can be set to <see langword="null" />.</param>
    public void Execute(object parameter)
    {
        _execute((T)parameter);
    }

    #endregion
}

该示例来自此问题Why RelayCommand

一旦有了RelayCommand类，就可以在ViewModel中定义诸如以下的命令

 public class MainWindowViewModel
 {
       private ICommand _Command;
        public ICommand Command
        {
            get
            {
                if (_Command == null)
                    _Command = new RelayCommand<object>((x) => ExecuteCommand(x));
                return _Command;
            }
            set
            {
                _Command = value;
            }
        }

       private void ExecuteCommand(object parameter)
        {
            try
            {
                if (!string.IsNullOrEmpty(parameter.ToString()){
                  //Do sql call here. parameter.ToString() is a string representation of the parameter that was bound on the xaml

               }
            }
            catch (Exception ex)
            {
                MessageBox.Show(ex.Message.ToString());
            }
        }
    }

现在已经完成设置，只需将视图的数据上下文设置为视图模型即可。您可以根据需要在背后的代码或xaml中执行此操作。像

    public MainWindow()
        {
            InitializeComponent();
            DataContext = new MainWindowViewModel();
        }

现在您在xaml中对命令的绑定应如下所示：

<Button x:Name="myButton" Content="update" Command="{Binding Command}"     CommandParameter="{Binding assignment_id}"></Button>

但是，当我们在这里时，请更改您的sql命令以使用参数，而不是将其串联为这样的字符串。按现在的方式进行操作将使您可以进行sql注入。

编辑：应该注意，您绑定到CommandParameter的对象也应该存在于您的视图模型中。理想情况下，您将拥有一个包含Assignment_id属性的模型，然后在视图模型中实例化该模型的类型，然后将该模型的实例绑定到您的View。

Answer 2

我认为它是调用click事件（btnUpdate_Click）和命令的顺序。如果首先调用该事件，则变量＆＃39; p＆＃39;没有值，查询将失败。

我认为你应该使用事件或命令来执行查询，而不是两者。如果您打算继续使用MVVM模式（我推荐），请在另一个类中使用该命令作为您的viewmodel。

如何传递Button的命令参数

2 个答案: