如何单独更新进度条(同时下载)

您要做的是使用其他DownloadFileAsync方法：http://msdn.microsoft.com/en-us/library/ms144197.aspx

第三个参数是 userToken，它作为DownloadProgressChangedEventArgs的一部分传递（它位于 UserState 属性中）。

因此，当您进行DownloadFileAsync调用时，传入一个唯一令牌（整数或其他内容），然后可以将其与需要更新的进度栏相关联。

    //(Snip)
    //in download_button_Click, pass the row you are updating to the event.
    for (int z = 1; z < CheckedCount; z++)
    {             
        _MultipleWebClients = new WebClient();
        _MultipleWebClients.DownloadFileCompleted += new AsyncCompletedEventHandler(_DownloadFileCompleted);
        _MultipleWebClients.DownloadProgressChanged += new System.Net.DownloadProgressChangedEventHandler(_DownloadProgressChanged);
        _MultipleWebClients.DownloadFileAsync(new Uri(_downloadUrlList[z].ToString()), @"F:'test" + z + ".mp4", dataGridView1.Rows[z]);     
    }
}
private void _DownloadProgressChanged(object sender, DownloadProgressChangedEventArgs e)
{
    var rowToUpdate = (DataGridViewRow)e.UserState;
    RowToUpdate["ProgressBar"].Value = e.ProgressPercentage;
    RowToUpdate["TextProgress"].Value = e.ProgressPercentage;
    RowToUpdate["BytesToRecive"].Value = ((e.TotalBytesToReceive / 1024) / 1024).ToString();
    double dn = (double)e.BytesReceived / 1024.0 / (DateTime.Now - start).TotalSeconds;
    RowToUpdate["Speed"].Value = (dn.ToString("n") + " KB/s) " + e.ProgressPercentage);
}

听起来你需要一个进度条来进行多部分进度：

public partial class ProgressBarEx : ProgressBar
{
    private readonly Dictionary<Guid, double> _partsProgress =
        new Dictionary<Guid, double>();
    private readonly Dictionary<Guid, double> _partsSizes =
        new Dictionary<Guid, double>();
    private double _value;
    private double _maximum;
    public ProgressBarEx()
    {
        this.InitializeComponent();
    }
    public int Parts
    {
        get { return this._partsSizes.Count; }
    }
    public new int Minimum { get; private set; }
    public new double Maximum
    {
        get { return this._maximum; }
        private set
        {
            this._maximum = value;
            base.Maximum = (int)value;
        }
    }
    public new double Value
    {
        get { return this._value; }
        private set
        {
            this._value = value;
            base.Value = (int)value;
        }
    }
    [Obsolete("Not useable in ProgressBarEx.")]
    public new int Step
    {
        get { return 0; }
    }
    public Guid AddPart(double size)
    {
        if (size <= 0)
        {
            throw new ArgumentException("size");
        }
        var partId = Guid.NewGuid();
        this.Maximum += size;
        this._partsSizes.Add(partId, size);
        this._partsProgress.Add(partId, 0);
        return partId;
    }
    public bool RemovePart(Guid partId)
    {
        double size;
        if (!this._partsSizes.TryGetValue(partId, out size))
        {
            return false;
        }
        this.Maximum -= size;
        this._partsSizes.Remove(partId);
        this.Value -= this._partsProgress[partId];
        this._partsProgress.Remove(partId);
        return true;
    }
    public bool ContainsPart(Guid partId)
    {
        return this._partsSizes.ContainsKey(partId);
    }
    public double GetProgress(Guid partId)
    {
        return this._partsProgress[partId];
    }
    public void SetProgress(Guid partId, double progress)
    {
        if (progress < 0 || this._partsSizes[partId] < progress)
        {
            throw new ArgumentOutOfRangeException("progress");
        }
        this.Value += progress - this._partsProgress[partId];
        this._partsProgress[partId] = progress;
    }
    public void AddProgress(Guid partId, double progress)
    {
        this.SetProgress(partId, progress + this._partsProgress[partId]);
    }
    [Obsolete("Not useable in ProgressBarEx.")]
    public new void PerformStep()
    {
    }
}

用法示例：

public Form1()
{
    InitializeComponent();
    var pbe = new ProgressBarEx {Location = new Point(100, 100)};
    this.Controls.Add(pbe);
    for (var i = 0; i < 4; i++)
    {
        var size = i * 10 + 30;
        var partId = pbe.AddPart(size);
        var pb = new ProgressBar
                     {
                         Maximum = size,
                         Location = new Point(100, i * 30 + 130)
                     };
        this.Controls.Add(pb);
        var timer = new Timer {Interval = 1000 + i * 100};
        timer.Tick += (sender, args) =>
                          {
                              pb.Value += 5;
                              pbe.AddProgress(partId, 5);
                              if (pb.Value == pb.Maximum)
                              {
                                  timer.Stop();
                              }
                          };
        timer.Start();
    }
}