c# – WebClient.DownloadDataAsync冻结了我的UI

using (WebClient client = new WebClient())
{
    client.DownloadDataCompleted += new DownloadDataCompletedEventHandler(client_DownloadDataCompleted);
    client.DownloadDataAsync("http://example.com/version.txt");
}

当我启动表单时,UI不会出现,直到引发client_DownloadDataCompleted.
client_DownloadDataCompleted方法为空,因此没有问题.

我做错了什么？
如何在不冻结UI的情况下做到这一点？

谢谢你的时间.
最好的祝福.

完整代码：

Program.cs中

using System;
using System.Windows.Forms;

namespace Lala
{
    static class Program
    {
        /// <summary>
        /// The main entry point for the application.
        /// </summary>
        [STAThread]
        static void Main()
        {
            Application.EnableVisualStyles();
            Application.SetCompatibleTextRenderingDefault(false);
            Application.Run(new Form1());
        }
    }
}

Form1.cs的

using System;
using System.Net;
using System.Windows.Forms;

namespace Lala
{
    public partial class Form1 : Form
    {
        WebClient client = new WebClient();

        public Form1()
        {
            client.DownloadDataCompleted += new DownloadDataCompletedEventHandler(client_DownloadDataCompleted);
            client.DownloadDataAsync(new Uri("http://www.google.com"));
            InitializeComponent();
        }

        void client_DownloadDataCompleted(object sender,DownloadDataCompletedEventArgs e)
        {
            textBox1.Text += "A";
        }
    }

    partial class Form1
    {
        /// <summary>
        /// Required designer variable.
        /// </summary>
        private System.ComponentModel.IContainer components = null;

        /// <summary>
        /// Clean up any resources being used.
        /// </summary>
        /// <param name="disposing">true if managed resources should be disposed; otherwise,false.</param>
        protected override void Dispose(bool disposing)
        {
            if (disposing && (components != null))
            {
                components.Dispose();
            }
            base.Dispose(disposing);
        }

        #region Windows Form Designer generated code

        /// <summary>
        /// Required method for Designer support - do not modify
        /// the contents of this method with the code editor.
        /// </summary>
        private void InitializeComponent()
        {
            this.button1 = new System.Windows.Forms.Button();
            this.textBox1 = new System.Windows.Forms.TextBox();
            this.SuspendLayout();
            // 
            // button1
            // 
            this.button1.Location = new System.Drawing.Point(12,12);
            this.button1.Name = "button1";
            this.button1.Size = new System.Drawing.Size(75,23);
            this.button1.TabIndex = 0;
            this.button1.Text = "button1";
            this.button1.UseVisualStyleBackColor = true;
            // 
            // textBox1
            // 
            this.textBox1.Location = new System.Drawing.Point(12,41);
            this.textBox1.Multiline = true;
            this.textBox1.Name = "textBox1";
            this.textBox1.Size = new System.Drawing.Size(468,213);
            this.textBox1.TabIndex = 1;
            // 
            // Form1
            // 
            this.AutoScaleDimensions = new System.Drawing.SizeF(6F,13F);
            this.AutoScaleMode = System.Windows.Forms.AutoScaleMode.Font;
            this.ClientSize = new System.Drawing.Size(492,266);
            this.Controls.Add(this.textBox1);
            this.Controls.Add(this.button1);
            this.Name = "Form1";
            this.Text = "Form1";
            this.ResumeLayout(false);
            this.PerformLayout();

        }

        #endregion

        private System.Windows.Forms.Button button1;
        private System.Windows.Forms.TextBox textBox1;
    }
}