对象输入事件“字符串到十进制无效”

Question

我有VB程序，它连接了2类，收入和费用的小数。

出于某种原因，incomeTextBox.Enter和expensesTextBox.Enter有怪异的行为，他说：

Please enter numeric value for expenses

当我请使用TAB键或鼠标选择其他的InputBox

即使输入整数之后（这则IsNumeric = TRUE） 。

我也得到错误：

An unhandled exception of type 'System.InvalidCastException' occurred in Microsoft.VisualBasic.dll

Additional information: Conversion from string "" to type 'Decimal' is not valid.

此外，这是VB，不VB.Net，但没有VB标签？

Option Explicit On 
Public Class MainForm 

Public decexpenses, decincome As Decimal 
Public dectotalexpenses As Decimal = 0 
Public dectotalincome As Decimal = 0 

Private Sub MainForm_Load(sender As Object, e As EventArgs) Handles MyBase.Load 
    MsgBox("Use Enter key to enter values.") 
End Sub 

Private Sub expensesTextBox_Enter(sender As Object, e As EventArgs) Handles expensesTextBox.Enter 
    Dim expenses = expensesTextBox.Text 
    If IsNumeric(expenses) = False Then 
     MsgBox("Please enter numeric value for expenses.") 
    End If 
    expensesTextBox.Text = "" 
    decexpenses = CDec(expenses) 
    dectotalexpenses = decexpenses + dectotalexpenses 
End Sub 

Private Sub incomeTextBox_Enter(sender As Object, e As EventArgs) Handles incomeTextBox.Enter 
     Dim income = incomeTextBox.Text 
     If IsNumeric(income) = False Then 
     MsgBox("Please enter numeric value for income.") 
    ElseIf IsNumeric(income) = True Then 
     decincome = CDec(income) 
     dectotalincome = decincome + dectotalexpenses 
    End If 
End Sub 
End Class 

Answer 1

的Enter事件引发，如果TextBox获得焦点的第一次，大概文本为空，在这一点上它引发你的异常。

在你的代码中，你也使用If...Else netiher，如果值不是数字，也不会返回，所以CDec(expenses)无论如何都会被执行导致异常。此外，而不是IsNumeric使用Decimal.TryParse：

Private Sub expensesTextBox_Enter(sender As Object, e As EventArgs) Handles expensesTextBox.Enter 
    Dim value As String = expensesTextBox.Text 
    Dim decincome As Decimal 
    If Not Decimal.TryParse(value, decincome) Then 
     MsgBox("Please enter numeric value for expenses.") 
     Return 
    End If 

    dectotalexpenses = decincome + dectotalexpenses 
End Sub 

你也应该使用不同的事件像TextBox.TextChanged。

Answer 2

我会不会使用回车的引发事件我会使用Text_Changed并使用KeyDown输入。一旦文本输入，然后他们点击输入文本框中的例程将运行示例如下：

Private Sub expensesTextBox_TextChanged(ByVal sender As System.Object, ByVal e As System.Windows.Forms.KeyEventArgs) Handles expensesTextBox.KeyDown 
    Dim dectotalexpenses As Decimal = 0 
    Dim dectotalincome As Decimal = 0 

    If e.KeyCode = Keys.Enter Then 

     Dim value As String = TextBox1.Text 
     Dim decincome As Decimal 
     If Not Decimal.TryParse(value, decincome) Then 
      MsgBox("Please enter numeric value for expenses.") 
      Return 
     End If 
    End If 
End Sub