How do I manage the wrong data type being sent to a class property? I'm using a property Grid, and I have bound the values to an object (my own, so I can modify the code).
So, for example, I have an object property of type Integer. This is bound to a property Grid value. The user type "Tosh", and an error is displayed telling me the value is incompatible.
I can't see a way to catch the exception, as it occurs as the value is passed to the object property.
This is the extended version of the control from CodeProject.
Property code:
Code:
Property sizeX() As Single
Get
Return sSize.X
End Get
Set(ByVal value As Single)
If value > sMax.X Then
Throw New System.Exception("Value (" & value.ToString & " is too large. Maximum is " & sMax.X.ToString)
Else
If value <= 0 Then
Throw New System.Exception("Value must be > 0")
Else
sSize.X = value
End If
End If
End Set
End Property
Re: Dealing with wrong data types in class properties
Implement your property as Object and use a class derived from TypeConverter ...
vb.net Code:
Private m_SizeX As Single
<TypeConverter(GetType(SizeXConverter))> _
Public Property SizeX() As Object
Get
Return m_SizeX
End Get
Set(ByVal value As Object)
m_SizeX = CType(value, Single)
End Set
End Property
Public Class SizeXConverter
Inherits TypeConverter
Public Overrides Function CanConvertFrom(ByVal context As System.ComponentModel.ITypeDescriptorContext, ByVal sourceType As System.Type) As Boolean
If sourceType.Equals(GetType(String)) Then
Return True
Else
Return MyBase.CanConvertFrom(context, sourceType)
End If
End Function
Public Overrides Function ConvertFrom(ByVal context As System.ComponentModel.ITypeDescriptorContext, ByVal culture As System.Globalization.CultureInfo, ByVal value As Object) As Object
Try
Dim c As Single = CSng(value), s As String = String.Empty
Select Case True
Case c > Single.MaxValue
s = "Number entered is too high!"
Case c < Single.MinValue
s = "Number entered is too low!"
End Select
If s <> String.Empty Then Throw New InvalidCastException(s)
Return c
Catch ex As InvalidCastException
MsgBox(ex.ToString)
Return CSng(0)
End Try
End Function
End Class
This code works relatively well, except that the error message shows at least twice, but I can't figure out why. If anyone can, please post...
Re: Dealing with wrong data types in class properties
Declaring your property as type Object when it is supposed to represent a Single is simply not a practical option. You'd get no type safety at design time then.
The reason that you can't catch an exception is because there is no exception. The value never gets as far as your property. Run your app and enter and invalid value and watch the Output window. There's no mention of a first chance exception because no exception is ever thrown. The PropertyGrid validates the data before it ever even tries to set your property.
If you wanted to intercept this situation then it would have to be via an event of the PropertyGrid itself, but as far as I can see there's no event raised when invalid data is entered.
Given that the PropertyGrid notifies the user of the error and allows them to retry or cancel, why exactly do you want do this anyway? Are you looking to provide a more idiot-friendly error message? If you want to describe a valid value to the user you can apply the Description attribute to your property, whose value will then be displayed at the bottom of the grid when that property is selected.
Re: Dealing with wrong data types in class properties
Thanks for the info so far. I have a description, so that side is covered. My issue is that the error looks quite daunting to a non-technical person. It makes perfect sense to me as the developer, but it doesn't provide a solution to an end-user, only a cause. Additionally, I would like to be able to revert the value back to the original if it's invalid.
I have a solution where I don't bind the values, and handle it all manually. It's fine, but very long-winded when the option is there to "take care of itself".
Re: Dealing with wrong data types in class properties
If the user presses the Cancel button on that dialogue the value displayed in the grid will revert to the current property value, while pressing OK will leave the pending value displayed and selected.
It probably would have been smart for them to have provided an event that was raised before that dialogue was shown, allowing you to provide a custom message and perhaps some other things. I've scoured the documentation for the PropertyGrid and also had a good look at it in Reflector and I can't see anything that would allow you to change that message or display your own dialogue.
Re: Dealing with wrong data types in class properties
BigMeUp, I haven't, but for 2 reasons. Firstly it is quite complex to understand at my level of VB, and I like to code in a way that I can maintain the code afterwards. I'm sure that at some point in the future it will make far more sense for me to look at inmore detail.
Secondly, I didn't because...
Declaring your property as type Object when it is supposed to represent a Single is simply not a practical option. You'd get no type safety at design time then.
Re: Dealing with wrong data types in class properties
I fully understand and appreciate your first point. The second point though is rigidity gone mad! There are good reasons to adhere to type-safety rules, but a good programmer should understand the risks and know when to stick to the rules, and when he can safely break them.
Looking closely over what I posted, you will find I catch all (design time) InvalidCastExceptions - therefore, the issue of type safety at design time simply doesn't arise. WE ARE EXPECTING CASTING ERRORS, THAT IS WHY WE ARE WRITING THIS CODE!!!!
Please free your mind from unnecessry constraints. Thinking outside the box will ultimately reward you with elegant solutions.
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