c# - 如何在控件实例化后立即发送自定义事件消息？

Question

创建此自定义控件并在客户端中测试它时，发送 ValueChanged() 事件时出现空异常错误:

自定义控件来源:

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Drawing;
using System.Data;
using System.Text;
using System.Windows.Forms;

namespace customevent
{
    [DefaultEvent("ValueChanged")]
    public partial class UserControl1 : UserControl
    {
        private int m_value;

        public delegate void ValueChangedHandler();
        [Category("Action")]
        [Description("Value changed.")]
        public event ValueChangedHandler ValueChanged;

        public int Value
        {
            get { return m_value; }
            set { 
                m_value = value;
                ValueChanged();
            }
        }

        public UserControl1()
        {
            InitializeComponent();
        }
        public UserControl1(int iValue)
        {
            this.Value = iValue;
            InitializeComponent();
        }

    }
}

然后以测试形式:

    private void Form1_Load(object sender, EventArgs e)
    {
       userControl11.Value = 100;
    }

    private void userControl11_ValueChanged()
    {
        MessageBox.Show(userControl11.Value.ToString());
    }

或者代替 form_load，在构造函数中执行此操作:

    private void InitializeComponent()
    {
        this.userControl11 = new customevent.UserControl1(100);

Answer 1

您应该将事件处理声明如下:

public event EventHandler ValueChanged;

protected virtual void OnValueChanged(object sender, EventArgs e)
{
    if (ValueChanged != null)
    {
       ValueChanged(sender, e);
    }
}

public int Value
{
    get { return m_value; }
    set { 
        if (m_value == value) return;
        m_value = value;
        OnValueChanged(this, EventArgs.Empty);
    }
}

PS:有一个接口(interface) INotifyPropertyChanged，您应该使用它来遵守标准 .NET 数据绑定(bind)规则。