Avalonia MenuItem命令绑定异常求助：控件灰显且Command为null

• 环境：Windows 10 21H2、VS2022，已安装Avalonia Toolkit和Avalonia for VS2022插件
• 异常现象：
  1. 绑定Command="{Binding MainWindow.OnOpenCommand}"的MenuItem始终呈灰显状态
  2. 通过MenuItem myMenu = this.FindControl<MenuItem>("FileOpen");能正常找到控件，但该控件的Command属性为null

用户提供的代码如下：

XAML代码 #

<Window xmlns="https://github.com/avaloniaui"
        xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
        xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
        xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
        mc:Ignorable="d" d:DesignWidth="800" d:DesignHeight="450"
        x:Class="AvaloniaApplication1.MainWindow"
        Title="AvaloniaApplication1">
        <DockPanel VerticalAlignment="Top">
          <Menu DockPanel.Dock="Top">
            <MenuItem Header="_File">
              <MenuItem Name="FileOpen" IsEnabled="True" Header="_Open..." Command="{Binding OpenCommand}" CommandParameter="{Binding ElementName=MainWindow}"/><Separator/>
              <MenuItem Header="_Exit"/>
            </MenuItem>
            <MenuItem Header="_Edit" >
              <MenuItem Header="Copy"/>
              <MenuItem Header="Paste"/>
            </MenuItem>
          </Menu>
        </DockPanel>
</Window>

C#代码 #

using Avalonia.Controls;
using System.Windows.Input;
using System.Reactive;
using System;

namespace AvaloniaApplication1
{
    public partial class MainWindow : Window
    {
        public MainWindow()
        {
            InitializeComponent();
            // yep, this works. we find the control
            MenuItem myMenu = this.FindControl<MenuItem>("FileOpen");
            // how to wire this up in code ????
            // myMenu.Command = openCommand;
        }
        //
        public bool CanExitCommand()
        {
            return true;
        }
        public void OnExitCommand() 
        {
            int i = 0;
        }
        //
        public bool CanOpenCommand()
        {
            return true;
        }
        public void OnOpenCommand()
        {
            int i = 0;
        }
    }
}

你的核心问题是没遵循Avalonia的命令绑定规则：直接绑定方法名无效，且未设置DataContext让绑定找到目标属性。下面提供两种可行的修复方式：

方式一：用ReactiveCommand（Avalonia官方推荐） #

1. 先确保项目安装Avalonia.ReactiveUI NuGet包（右键项目→管理NuGet程序包搜索安装）
2. 修改C#代码，定义命令属性并设置DataContext：

using Avalonia.Controls;
using ReactiveUI;
using System;

namespace AvaloniaApplication1
{
    public partial class MainWindow : Window
    {
        // 声明命令属性
        public ReactiveCommand<object, Unit> OpenCommand { get; }
        public ReactiveCommand<object, Unit> ExitCommand { get; }

        public MainWindow()
        {
            InitializeComponent();
            
            // 初始化命令，绑定执行逻辑和可执行判断
            OpenCommand = ReactiveCommand.Create<object>(param =>
            {
                // 原OnOpenCommand的逻辑
                int i = 0;
            }, canExecute: Observable.Return(true));

            ExitCommand = ReactiveCommand.Create(() =>
            {
                // 原OnExitCommand的逻辑
                int i = 0;
            }, canExecute: Observable.Return(true));

            // 将窗口自身设为DataContext，让XAML绑定能找到命令
            DataContext = this;
        }
    }
}

3. 调整XAML的Command绑定，直接指向属性名：

<MenuItem Name="FileOpen" Header="_Open..." Command="{Binding OpenCommand}"/>
<MenuItem Header="_Exit" Command="{Binding ExitCommand}"/>

方式二：手动实现ICommand接口 #

如果不想依赖ReactiveUI，可自定义命令类：

1. 创建基础RelayCommand类：

using System;
using System.Windows.Input;

public class RelayCommand : ICommand
{
    private readonly Action<object> _execute;
    private readonly Func<object, bool> _canExecute;

    public event EventHandler CanExecuteChanged;

    public RelayCommand(Action<object> execute, Func<object, bool> canExecute = null)
    {
        _execute = execute ?? throw new ArgumentNullException(nameof(execute));
        _canExecute = canExecute;
    }

    public bool CanExecute(object parameter)
    {
        return _canExecute == null || _canExecute(parameter);
    }

    public void Execute(object parameter)
    {
        _execute(parameter);
    }

    public void RaiseCanExecuteChanged()
    {
        CanExecuteChanged?.Invoke(this, EventArgs.Empty);
    }
}

2. 修改MainWindow代码，定义并初始化命令：

using Avalonia.Controls;
using System.Windows.Input;

namespace AvaloniaApplication1
{
    public partial class MainWindow : Window
    {
        public ICommand OpenCommand { get; }
        public ICommand ExitCommand { get; }

        public MainWindow()
        {
            InitializeComponent();
            OpenCommand = new RelayCommand(OnOpenCommand, CanOpenCommand);
            ExitCommand = new RelayCommand(OnExitCommand, CanExitCommand);
            DataContext = this;
        }

        public bool CanExitCommand(object parameter)
        {
            return true;
        }
        public void OnExitCommand(object parameter) 
        {
            int i = 0;
        }

        public bool CanOpenCommand(object parameter)
        {
            return true;
        }
        public void OnOpenCommand(object parameter)
        {
            int i = 0;
        }
    }
}

3. XAML的Command绑定同方式一，直接绑定OpenCommand即可。

1. 未设置DataContext：XAML的Binding默认读取当前控件的DataContext，你之前没赋值，导致绑定找不到目标属性，Command自然为null
2. 错误的绑定语法：Avalonia不支持直接绑定方法名（比如MainWindow.OnOpenCommand是无效写法），必须绑定实现ICommand接口的属性
3. 代码与XAML不匹配：XAML里写了Command="{Binding OpenCommand}"，但代码中根本没有OpenCommand这个属性，绑定必然失败

内容的提问来源于stack exchange，提问作者J Evans