Converting Boolean and Date from Generic type problem

[NickThissen]

Hi,

I am creating a 'generic input dialog': InputDialog(Of T). The idea is that I can create new instances of this dialog for different values of T. For example, I can create an InputDialog(Of String) and it displays a textbox. I create an InputDialog(Of Boolean) and it displays two radiobuttons (I could use a Checkbox, irrelevant). I create an InputDialog(Of Date) and it shows a DateTimePicker.

I do this by simply checking the type of T at run-time. In this way it is not really generic, as the dialog still has to know which type T is (which is usually not the case), but the generic is in the fact that the T can be multiple types that require a TextBox. For example, InputDialog(Of String), (Of Integer), (Of Single), (Of Double), etc, all simply display a TextBox (and it is validated later), so I still want to use generics as much as possible.


Anyway, the result of the dialog is of course a property of type T. This property needs to return the value in the textbox, converted from string to T, OR the value in the DateTimePicker, converted from Date to T, OR the checked property of the 'Yes' radiobutton, converted from Boolean to T.

In order to convert from any object to T I googled and found this code:

Imports System.ComponentModel
 
Public Class GenericTypeConverter
 
    Public Shared Function FromObject(Of T)(ByVal value As Object) As T
        Dim tc As TypeConverter = TypeDescriptor.GetConverter(GetType(T))
        Return CType(tc.ConvertFrom(value), T)
    End Function
 
End Class

I have to use this, I cannot simply CType a string to T since a String cannot be converted to any type T.


So, my Result property looks like this at the moment:

Public ReadOnly Property Result() As T
        Get
            If GetType(T) Is GetType(Boolean) Then
                Return GenericTypeConverter.FromObject(Of T)(rbYes.Checked)
            ElseIf GetType(T) Is GetType(Date) Then
                Return GenericTypeConverter.FromObject(Of T)(CType(datePicker.Value, Date))
            Else
                Return GenericTypeConverter.FromString(Of T)(txtAnswer.Text.Trim)
            End If
        End Get
    End Property

This actually works for Strings, Integers, Doubles, Singles, etc. I pass it the String in the textbox and it converts it to 'T' (note: T is then a String, Integer or Double!) just fine.

So, it can convert a String to a String, Integer or Double without any problems (I am validating the text before using the Result property so it will always be a valid integer, double, etc).

However, it does not work for a Boolean, nor for a DateTime. When I try that, it says "BooleanConverter cannot convert from System.Boolean" or "DateTimeConverter cannot convert from DateTime".


I realize it is a bit of a strange thing, since the object already IS a Boolean or a Date(Time) so no conversion should be done at all, but this doesn't work in design-time because I need to return the objects as type T.
I know that T will be Boolean when the value to be converted is a Boolean, and that T will be DateTime when the value to be converted is a DateTime, but the designer does not accept a simple CType. As I said, it does not accept this:

Return CType(rbYes.Checked, T)

because 'Boolean cannot be converted to T'.


However, I did find a way to make it work, but it seems like a complete hack... I can assign the boolean to an Object and then convert that using CType:

Dim obj As Object
obj = rbYes.Checked
Return CType(obj, T)

This works, it's accepted during design-time, and it works during run-time, but it seems very wrong... Is there no better way to handle this??

Thanks!

[cicatrix]

A strange behaviour indeed.
Well, I can only suggest to implement your own type converter class:
http://msdn.microsoft.com/en-us/library/ayybcxe5.aspx

[Lightning]

You could also "hack" it like this:

return CType(DirectCast(rbYes.Checked,object),T)

[cicatrix]

There's some mysterious remark in BooleanConverter MSDN article:

This converter can only convert a Boolean object to and from a string.

[NickThissen]

Yeah I noticed that too. I guess it makes sense that a converter cannot convert a boolean to a boolean, cause why would you ever need that..? Well, I am trying to do it now, but I am convinced that there is a better way. The only reason I am trying it is because I cannot tell the designer that T is already a boolean, so that CType(bln, T) is legal. Telling the designer that T is a boolean of course goes against the entire idea of generics, where T is an unknown type during design-time...

[cicatrix]

BooleanConverter inherits from TypeConverter
Extending BooleanConverter (if it's not inheritable inherit from TypeConverter) and overriding CanConvertFrom, ConvertFrom and some other methods might turn the trick.

BUT. Instead, simply use that 'hack' of yours. Less coding with the same results. )))