如何解決NullReferenceException WPF文本框

我有這種情況下數據已被加載到組合框和文本框，但當應用程序第一次被加載和組合框=菸草使用？並嘗試將文本框設置爲空，我得到「NullReferenceException - 對象引用未設置爲對象的實例」。不知道如何解決這個問題，並能夠在選擇「使用菸草？」時保持清除文本框。還有「菸草使用？」是組合框的默認值。如何解決NullReferenceException WPF文本框

組合框cbTobacco.Text

的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; 
    } 
}

來源

2013-10-07 Robert

哪條線

if(cbTobacco != null)

線是給你調試的錯誤？這將告訴我們在初始化之前你正在訪問哪個對象是null。 http://stackoverflow.com/a/4660186/2145211 – Harrison

cbTobaccoCode.Text = strTobaccoCode;是我得到例外的地方。只有在應用第一次加載時纔會發生。 – Robert

確定該應用程序在第一種情況下進入，並seted strTobaccoCode的價值？放一個斷點並檢查。

使用「默認」選項，以使你的代碼更安全一點：

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; 
    } 
}

來源

2013-10-07 14:51:54 rkawano

仍然會收到「NullReferenceException - 對象引用未設置爲對象的實例」。在應用程序第一次加載時cbTobaccoCode.Text = strTobaccoCode – Robert

它是Windows窗體還是WebForms？ – rkawano

您應該設置一個默認的情況下也是如此。另外，你爲什麼直接訪問cbTobacco？你應該從那裏投發件人（cbTobacco如果你已經正確連接好您的活動），以文本框和訪問值：

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 ?? ""; 
    } 
}

來源

2013-10-07 14:53:33

textBox.Text如何設置cbTobaccoCode.Text？發件人是cbTobacco.Text。 – Robert

嘗試添加這個（驗證如果文本框爲空，然後驗證該文本空）前

if (cbTobacco.Text != null)

來源

2013-10-07 17:17:04 Alexandre

如何解決NullReferenceException WPF文本框

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