Simple Cast not working

I cannot figure out what I am doing wrong. This is a very simple cast:

vb Code:

'sng casting test
        Dim varFF As Single = CSng(tbCalcFF.Text) - CSng(tbGoalFF.Text)
        If varFF < 0 Then
            tbCalcFF.BackColor = Color.Gold
        Else
            tbCalcFF.BackColor = Color.LightGoldenrodYellow
        End If

I am getting this error: Conversion from string "" to type 'Single' is not valid.

Thats because either tbCalcFF.Text or tbGoalFF.Text is equal to "", you cant turn that into a valid single value.
What you should do is use Single.TryParse, its handy whenever you are unsure if the string can be converted or not.

VB Code:

Dim value1 As Single
        Dim value2 As Single
        If Single.TryParse(tbCalcFF.Text, value1) AndAlso Single.TryParse(tbGoalFF.Text, value2) THen
            Dim varFF As Single = value1 - value2
            If varFF < 0 Then
                tbCalcFF.BackColor = Color.Gold
            Else
                tbCalcFF.BackColor = Color.LightGoldenrodYellow
            End If 
        Else
            MessageBox.Show("Enter valid single values!")
        End If

Another thing you might try, along the same lines, would be to split that one If statement into two, because if the return from either one is false, you could simply put a 0 into that value on the assumption that an empty string is basically 0 (or some other default value if something else makes sense).

Come to think of it, that's kind of obvious, but sometimes we try to make nice neat, tight, code, and sacrifice useability.

Quote:

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Originally Posted by Shaggy Hiker

Another thing you might try, along the same lines, would be to split that one If statement into two, because if the return from either one is false, you could simply put a 0 into that value on the assumption that an empty string is basically 0 (or some other default value if something else makes sense).

Come to think of it, that's kind of obvious, but sometimes we try to make nice neat, tight, code, and sacrifice useability.

---

Yeah good thinking. Then I suppose this would work aswell.

VB Code:

Dim value1 As Single
        Dim value2 As Single
        Single.TryParse(tbCalcFF.Text, value1)
        Single.TryParse(tbGoalFF.Text, value2)
        Dim varFF As Single = value1 - value2
        If varFF < 0 Then
            tbCalcFF.BackColor = Color.Gold
        Else
            tbCalcFF.BackColor = Color.LightGoldenrodYellow
        End If

Because value1 and value2 will keep their default values of 0 if TryParse fails. However, I suppose that using an IF statement to check wether the textboxes are empty or not as you mentioned would be more efficient.