"Conversion from string "" to type 'Double' is not valid." In VB

vb.net

Solution

One of your `TextBox` items has not been filled in.

As such, when you use `CDbl`, such as `P = CDbl(txtAmount.Text)`, if the `TextBox` is empty, it will cause this error.

A better option would be to use Double.TryParse instead of `CDbl`, as it will allow you to raise a proper message:

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

Problem

When I try to run the program to calculate payment and total intrest I get "Conversion from string "" to type 'Double' is not valid." What am I doing wrong? ``` 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 ```

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