如何修复空引用异常WPF文本框

本文关键字：WPF 文本 异常 引用 何修复 | 更新日期: 2023-09-27 18:34:10

我有这种情况，它在将数据加载到组合框和文本框后起作用，但是当应用程序首次加载并且组合框是 = 烟草使用？ 并尝试将文本框设置为空，我得到"空引用异常 - 对象引用未设置为对象的实例"。 不确定如何解决此问题，并在选择"烟草使用？ "被选中时，不知道如何解决这个问题并能够保持它清除文本框。此外，"烟草使用？"是组合框上的默认值。

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;
  }
}

如何修复空引用异常WPF文本框

您确定应用程序在第一个案例中输入并设置了 strTobaccoCode 的值吗？ 放置断点并检查。

使用"默认"选项使您的代码更安全：

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;
    }
}

您还应该设置默认大小写。另外，你为什么直接访问cbTobacco？您应该将发送者（cbTobacco 如果您已正确连接事件）强制转换为文本框并从那里访问值：

 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 ?? "";
  }
}

尝试添加此内容（验证文本框是否为空，然后验证文本是否为空）

    if(cbTobacco != null)

行之前 

if (cbTobacco.Text != null)
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