vb.net - "Conversion from string ""to type ' 双' is not valid."在VB

Question

当我尝试运行程序来计算付款和总利息时，我得到“从字符串转换为类型 'Double' 无效。”

我做错了什么？

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

Answer 1

您的 TextBox 项之一尚未填写。

因此，当你使用CDbl时，如P = CDbl(txtAmount.Text)，如果TextBox为空，则会导致这个错误。

更好的选择是使用 Double.TryParse而不是 CDbl，因为它可以让您发出正确的消息:

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