当我尝试运行该程序以计算付款和总利息时，我会从字符串"转换为"“转换为’double’无效”。
我究竟做错了什么？
Dim P As Double
Dim R As Double
Dim N As Double
Dim Payment As Double
Dim totalInterest As Double
Private Sub btnAnalyze_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAnalyze.Click
P = CDbl(txtAmount.Text)
N = CDbl(txtDuration.Text)
R = CDbl(txtInterestRate.Text)
Payment = (P * R) / (1 - (1 + R) ^ (-N))
totalInterest = (N * Payment) - P
Payment = CDbl(txtPayment.Text)
totalInterest = CDbl(txtInterest.Text)
If P < 0 Then
MessageBox.Show("Please enter in loan amount")
End If
If R <= 0 Then
MessageBox.Show("Please enter in loan amount")
End If
If N <= 0 Then
MessageBox.Show("Please enter in loan amount")
End If
End Sub
End Class
答案
你的其中一位TextBox项目尚未填写。
因此，当您使用CDbl， 例如P = CDbl(txtAmount.Text)，如果是TextBox是空的，会导致此错误。
一个更好的选择是使用双重特里普尔斯代替CDbl，因为这将使您能够发出适当的信息：
Private Sub btnAnalyze_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles btnAnalyze.Click
If Not Double.TryParse(txtAmount.Text, P) Then
MessageBox.Show("Please correct the loan amount")
Exit Sub
End If
' Do the same for all other CDbl checks