如何修复NullReferenceException WPF文本框

Question

在将数据加载到combobox和textbox之后，我有这样的情况，但是当应用程序第一次加载时，组合框是=烟草使用吗？并尝试将textbox设置为空，我得到"NullReferenceException -对象引用未设置为对象的实例“。不知道如何解决这一问题，并使它能够在“烟草使用”时清除文本框？被选中。还有“烟草使用？”是组合框的默认设置。

comboBox cbTobacco.Text

textBox = cbTobaccoCode.Text

private void cbTobacco_SelectionChanged(object sender, SelectionChangedEventArgs e)
 {
   if (cbTobacco.Text != null)
   {
     switch (Convert.ToString(cbTobacco.Text))
      {
        case "Tobacco Use?": strTobaccoCode = ""; break;
        case "1 - Current every day smoker": strTobaccoCode = "449868002"; break;
        case "2 - Current some day smoker": strTobaccoCode = "428041000124106"; break;
        case "3 - Former smoker": strTobaccoCode = "8517006"; break;
        case "4 - Never smoker": strTobaccoCode = "266919005"; break;
        case "5 - Smoker, current status unknown": strTobaccoCode = "77176002"; break;
        case "6 - Unknown if ever smoked": strTobaccoCode = "266927001"; break;
        case "7 - Heavy tobacco smoker": strTobaccoCode = "428071000124103"; break;
        case "8 - Light tobacco smoker": strTobaccoCode = "428061000124105"; break;
      }
  cbTobaccoCode.Text = strTobaccoCode;
  }
}

Answer 1

您确定应用程序输入的是第一种情况，并设置了strTobaccoCode的值吗？放个断点检查一下。

使用“默认”选项可以使代码更加安全：

private void cbTobacco_SelectionChanged(object sender, SelectionChangedEventArgs e)
{
    if (cbTobacco.Text != null)
    {
        string strTobaccoCode;
        switch (Convert.ToString(cbTobacco.Text))
        {
            case "1 - Current every day smoker": strTobaccoCode = "449868002"; break;
            case "2 - Current some day smoker": strTobaccoCode = "428041000124106";     break;
            case "3 - Former smoker": strTobaccoCode = "8517006"; break;
            case "4 - Never smoker": strTobaccoCode = "266919005"; break;
            case "5 - Smoker, current status unknown": strTobaccoCode = "77176002"; break;
            case "6 - Unknown if ever smoked": strTobaccoCode = "266927001"; break;
            case "7 - Heavy tobacco smoker": strTobaccoCode = "428071000124103"; break;
            case "8 - Light tobacco smoker": strTobaccoCode = "428061000124105"; break;
            default: strTobaccoCode = ""; break;
        }
        cbTobaccoCode.Text = strTobaccoCode;
    }
}

Answer 2

您也应该设置默认的大小写。此外，您为什么要直接访问cbTobacco？您应该将发送方(如果您已经正确地连接了您的事件)转换到文本框中，并从那里访问值：

 private void cbTobacco_SelectionChanged(object sender, SelectionChangedEventArgs e)
 {
   var textBox = sender as TextBox;

   if (textBox != null && textBox.Text != null)
   {
     switch (Convert.ToString(textBox.Text))
      {
        case "Tobacco Use?": strTobaccoCode = ""; break;
        case "1 - Current every day smoker": strTobaccoCode = "449868002"; break;
        case "2 - Current some day smoker": strTobaccoCode = "428041000124106"; break;
        case "3 - Former smoker": strTobaccoCode = "8517006"; break;
        case "4 - Never smoker": strTobaccoCode = "266919005"; break;
        case "5 - Smoker, current status unknown": strTobaccoCode = "77176002"; break;
        case "6 - Unknown if ever smoked": strTobaccoCode = "266927001"; break;
        case "7 - Heavy tobacco smoker": strTobaccoCode = "428071000124103"; break;
        case "8 - Light tobacco smoker": strTobaccoCode = "428061000124105"; break;
        default: strTobaccoCode = ""; break;
      }

      textBox.Text = strTobaccoCode ?? "";
  }
}

Answer 3

尝试添加以下内容(验证文本框是否为空，然后验证文本是否为空)

    if(cbTobacco != null)

前面的行

if (cbTobacco.Text != null)