"Conversion from string " " to type 'Double' is not valid"

Question

I am doing a project in microsoft visual studio 2012 and i am trying to write an application to determine the module average.

The script is the following:

 Private Sub Button2_Click(sender As Object, e As EventArgs) Handles confirm.Click
        Dim sum As Double
        sum = CDbl(test.Text) * 50% + CDbl(project.Text) * 30% + CDbl(quiz.Text) * 30%
        Dim modulemark As Double
        modulemark = CDbl(CAmarks.Text) * 50% + CDbl(exam.Text) * 50%

        Dim Grade As String
        If sum < 40 Then
            Grade = "F"
        ElseIf sum >= 40 And modulemark < 65 And modulemark >= 40 Then
            Grade = "C"
        ElseIf sum >= 40 And modulemark < 75 And modulemark >= 65 Then
            Grade = "B"
        Else
            Grade = "A"
        End If

The script is intended to calculate the marks and give a grade after clicking a button named "Confirm".

However, when i tried to run the coding it said:

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

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

can someone see what is wrong? i am new to Visual studio and i appreciate your help.

P.S. edited recent script.

P.S. Thank you for the user "Tim" for the script but for some unknown reason on the line "Double.TryParse(caMarks.Text, caMarks)" there's a blue squiggly that directs to caMarks that says "'Text' is not a member of 'double'". This is literally pulling my hair off! please help!

It looks like the names of the objects conflicted.

Solution

As Idle_Mind said in their answer, Double.TryParse is the way to go. This provides a safe way to attempt to convert a value to a double. If the conversion succeeds, the method returns true, and the resulting double is returned in the out parameter. If the conversion fails, false is returned and the default value of double (which is 0) is returned.

A simple example:

Dim result As Double
Dim score As String = "75"

If Double.TryParse(score, result) Then
   ' result will be a double with the value of 75
Else
   ' The conversion attempt failed, and result will have a value of 0
End If

So to apply that to your method (with no validation, though Idle_Mind's answer gives a good approach):

Private Sub Button2_Click(sender As Object, e As EventArgs) Handles confirm.Click

    Dim sum As Double
    Dim modulemark As Double
    Dim testScore As Double
    Dim projectScore As Double
    Dim quizScore As Double
    Dim marks As Double
    Dim examScore As Double

    Double.TryParse(test.Text, testScore)
    Double.TryParse(project.Text, projectScore)
    Double.TryParse(quiz.Text, quizScore)
    Double.TryParse(CAmarks.Text, marks)
    Double.TryParse(exam.Text, examScore)

    sum = (testScore * .5) + (projectScore * .3) + (quizScore * .3)
    modulemark = (marks * .5) + (examScore * .5)

    Dim Grade As String

    If sum < 40 Then
        Grade = "F"
    ElseIf sum >= 40 And modulemark < 65 And modulemark >= 40 Then
        Grade = "C"
    ElseIf sum >= 40 And modulemark < 75 And modulemark >= 65 Then
        Grade = "B"
    Else
        Grade = "A"
    End If
End Sub

Explanation of the above code.

First, 6 Double variables are declared - this necessary because Double.TryParse takes an out parameter as the second argument, and that must be declared before its used. You could use one variable (and reuse it), but for simplicity I chose one for each score.

Once the scores have been parsed (successfully or not) the cumulative, weighted totals are determined. Note that parentheses were used when applying the weight modifier, to ensure operator precedence doesn't give you a result other than expected.

Hopefully this clarifies things for you.

Comments

[Idle_Mind]

You need to move from using all of those CDbl() calls to proper Double.TryParse() calls. Look it up...

[Idle_Mind]

Additonally, it is unclear if your use of "50%" is correct. You most likely meant that to be ".5". The "%" symbol causes that 50 to be treated as an integer, which it already is. See Type Characters (Visual Basic).

[gheys damn]

Tried. It said: "Overload resolution failed because no accessible tryparse accepts this number of arguments." The script is: "modulemark = Double.TryParse(CAmarks.Text) * 50% + Double.TryParse(exam.Text) * 50%". the dim is already set as double.

[gheys damn]

Now, on the line "Double.TryParse(CAMarks.Text, CAMarks)" has an error that says:"'Text' is not a member of 'Double'", and only on that line and on CAmarks without any apparent reason. without that there are no problems. once i tried getting that line in, it appears.

[Tim]

Forgot VB.NET isn't case-sensitive. Change the double caMarks to marks and try again. The reason you got the error was because VB.NET saw CAMarks (The textbox) and caMarks (the double) as the same name.