VB調試錯誤

Question

當我去調試我的項目在Visual Basic中，我得到它說的錯誤消息：

An unhandled exception of type 'System.InvalidOperationException' occurred in Name Database.exe

Additional information: An error occurred creating the form. See Exception.InnerException for details. The error is: Object reference not set to an instance of an object.

這裏是我的代碼：

Public Class Form1 
Dim member As String = TextBoxName.Text 
Private Sub ButtonAdd_Click(sender As Object, e As EventArgs) Handles ButtonAdd.Click 
    ListBoxName.Items.Add(member & " " & TextBoxDate.Text) 
    TextBoxDate.Text = "" 
    TextBoxName.Text = "" 
End Sub 

Private Sub ButtonRemove_Click(sender As Object, e As EventArgs) Handles ButtonRemove.Click 
    ListBoxName.Items.Remove(ListBoxName.SelectedItem) 
End Sub 

Private Sub ButtonCheck_Click(sender As Object, e As EventArgs) Handles ButtonCheck.Click 
    If ListBoxName.Items.Contains(member & " " & Date.Today.ToString) Then 
     MsgBox(ListBoxName.Items.Contains(Date.Today.ToString) & "membership needs to be renewed") 
    End If 
End Sub 
End Class 

Answer 1

Dim member As String = TextBoxName.Text 

這不能工作，TextBoxName變量在執行此分配時尚未初始化。您必須使用如下構造函數：

Dim member As String 

Public Sub New() 
    InitializeComponent() 
    member = TextBoxName.Text 
End Sub 

它是設置TextBoxName控件引用的InitializeComponent（）。請注意，它仍然不太可能工作，Text屬性不會被用戶設置。所以會員將包含一個空字符串。除非你給設計師一個價值。這表明這更可能是正確的：

Dim member As String = String.Empty