[Visual Basic]

My code gives me an error called

System.InvalidCastException: 'Conversion from string "" to type 'Double' is not valid.'

I guess it happens when the user don't fill out every single boxes(miles, yards, fett, inches).

Please modify it to get output even though the user fills out only one box.(i.e just miles box).

No exception messages please!

Public Class Form1

'Build a button event Private Sub btnConvert_Click(sender As Object, e As EventArgs) Handles btnConvert.Click 'Declares variables for US customary system Dim inches, feet, yards, miles As Double 'Declares variables for metric system Dim sumInInches, centimeters, meters, kilometers As Double

'Gets input from a user inches = CDbl(txtInches.Text) feet = CDbl(txtFeet.Text) yards = CDbl(txtYards.Text) miles = CDbl(txtMiles.Text)

'Get sumIninches: Convert other lengths into inches and sum them all up sumInInches = 63360 * miles + 36 * yards + 12 * feet + inches

'Converts lengths from US customary system into metric system meters = sumInInches / 39.37 kilometers = Int(meters / 1000) meters = meters Mod 1000 centimeters = (meters * 100) Mod 100

'Displays output lstOutput.Items.Clear() lstOutput.Items.Add("In Metric System:") lstOutput.Items.Add(kilometers.ToString() & "km") lstOutput.Items.Add(Int(meters).ToString() & "m") lstOutput.Items.Add(centimeters.ToString() & "cm")

End Sub End Class

Conversion Miles Convert to Metric Yards: Feet OBJECT PROPERTY Text Text SETTING Conversion blMiles txt Miles blYards txtYards blFeet txtFeet lblInches btnConvert stOutput Miles Text Yards Text Text Text Convert to Metric