Debug.WriteLine(value.ToString("F"))

The standard formatting characters are:

C - Currency
D - Decimal
E - Scientific notation (exponential)
F - Fixed-point
G - General
N - Number
P - Percent
R - Round-trip (the result can be parsed to the original value)
X - Hexadecimal

This example program lets you enter values and enter or pick format strings to see the results. When you change the value or format string, the ShowValue routine displays the result. The subroutine treats the value as an integer if it has no fractional part so you can test the D and X, which only work for integers. It uses the leftmost character displayed by the cboFormat ComboBox (for example, this might be "D - Decimal").
              

Private Sub ShowValue()
    Try
        ' Get the value.
        Dim value As Double = Double.Parse(txtValue.Text)
        Dim ivalue As Integer = CInt(value)

        ' See if it's an integer.
        If value = ivalue Then
            ' It's an integer. Treat it as one so D and X
            ' work.
            txtResult.Text = _
                ivalue.ToString(cboFormat.Text.Substring(0, _
                1))
        Else
            txtResult.Text = _
                value.ToString(cboFormat.Text.Substring(0, _
                1))
        End If
    Catch ex As Exception
        txtResult.Text = ""
    End Try
End Sub