后台工作人员没有停止（使用Maps API Web Services）

Question

这是我的代码使用bwpickup是后台工作者的名字，代码是后台工作者的开始

if (bwPickUp.IsBusy)
{
    bwPickUp.CancelAsync();
}

if (!bwPickUp.IsBusy)
{                
    if (txtPickUpPostCode.Text.Trim() != "")
    {
        txtPickUpAddress.Text = "";
    }
    bwPickUp.RunWorkerAsync();
}

在后台工作者代码的DoWork方法中：

if (bwPickUp.CancellationPending)
{
    e.Cancel = true;
    return;
}
if (txtPickUpPostCode.Text.Trim() != "")
{
    try
    {
        GoogleApi gapi = new GoogleApi();
        _PickUpAddress = gapi.FindAdrressUsingPostCode(txtPickUpPostCode.Text);
    }
    catch (Exception)
    {
    }
}

请帮我解决如何阻止后台工作人员的问题