Microsoft Silverlight

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964 points

http://www.mostlydevelopers.com/blog/post/2009/04/23/Set-an-Enum-Dependency-Property-with-XAML.aspx

Enum as DepedencyProperty

04-23-2009 10:54 AM |

HI,

can somebody tell me how to register a dependency property of type enum? My property is accessable in xaml, but not how you would expect from an enum (drop down, etc). And the runtime throws a weird exception which doesn't make much sense...

Thanks in advance...

wjchrist...

Member

104 points

51 Posts

Re: Enum as DepedencyProperty

04-23-2009 12:17 PM |

You will need to create a TypeConverter for your property so that it will know how to convert from a string to your enumerated type. I wrote a quick demo for you on my blog. The link is below:

Bill - Senior .NET Developer
A Mostly Developers Blogger

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Re: Enum as DepedencyProperty

04-23-2009 12:28 PM |

Thank you very much, i will try that tomorrow! i'm just curious, your solution dictates to have a particular converter for every enum i will work with. How much work would it be to implement a generic converter? If i look at all the out-of-the-box controls which actually use enums for their properties i doubt the MS guys have written a converter for every particular enum they're using.

wjchrist...

Member

104 points

51 Posts

Re: Enum as DepedencyProperty

04-23-2009 12:41 PM |

I'm sure you could create a generic enum converter. M$ may have one out there that I'm not aware of. I've not done a lot of searching on it. Please post if you find one.

Bill - Senior .NET Developer
A Mostly Developers Blogger

Please remember to click “Mark as Answer” on the post that helps you.

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964 points

http://www.kindblad.com/2008/08/18/how-to-convert-from-string-to-enum-using-c-and-generics/

Re: Enum as DepedencyProperty

04-23-2009 12:52 PM |

I've found this article on the net, maybe it can be used to extend your implementation...

wjchrist...

Member

104 points

51 Posts

Re: Enum as DepedencyProperty

04-23-2009 3:58 PM |

Below is a generic EnumTypeConverter class i wrote.

Here's a property using it:

    <TypeConverter(GetType(EnumTypeConverter(Of Vehicle.VehicleType)))> _
    Public Property Type() As VehicleType
        Get
            Return DirectCast(GetValue(Vehicle.TypeProperty), VehicleType)
        End Get
        Set(ByVal value As VehicleType)
            SetValue(Vehicle.TypeProperty, value)
        End Set
    End Property

Here is the generic EnumTypeConverter:

Imports System.ComponentModel

Public Class EnumTypeConverter(Of T)
    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 CanConvertTo(ByVal context As System.ComponentModel.ITypeDescriptorContext, ByVal destinationType As System.Type) As Boolean
        If destinationType.Equals(GetType(String)) Then
            Return True
        Else
            Return MyBase.CanConvertTo(context, destinationType)
        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
        If TypeOf value Is String Then
            Try
                Return CType([Enum].Parse(GetType(T), value, True), T)
            Catch
                Throw New InvalidCastException(value)
            End Try
        Else
            Return MyBase.ConvertFrom(context, culture, value)
        End If
    End Function

    Public Overrides Function ConvertTo(ByVal context As System.ComponentModel.ITypeDescriptorContext, ByVal culture As System.Globalization.CultureInfo, ByVal value As Object, ByVal destinationType As System.Type) As Object
        If destinationType.Equals(GetType(String)) Then
            Return value.ToString()
        Else
            Return MyBase.ConvertTo(context, culture, value, destinationType)
        End If
    End Function
End Class

Bill - Senior .NET Developer
A Mostly Developers Blogger

Please remember to click “Mark as Answer” on the post that helps you.

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964 points

Re: Enum as DepedencyProperty

04-23-2009 4:09 PM |

I'll post the c# version tomorrow! Thanks for your effort! Yes

dmw

Member

106 points

Re: Enum as DepedencyProperty

04-23-2009 10:06 PM |

DJanjicek:
i doubt the MS guys have written a converter for every particular enum they're using

Your are quite correct; they have not. The XAML parser can automatically parse strings to named enum values, and if you examine an enum-based property in Blend, you will see that it automatically displays a drop-down for the property. You therefore do not need to use a TypeConverter, and doing so will achieve little for a simple enum. I'm not denying that you might have had problems, but a TypeConverter is not the solution to them.

Regards

Dave

dmw

Member

106 points

Re: Enum as DepedencyProperty

04-23-2009 10:11 PM |

A further thought has crossed my mind. A common mistake to make when registering DPs is to leave in the default PropertyMetadata(0) in the DP.Register method. This will result in a TypeInitializationException at runtime, and potentially strange problems at design-time. Make sure that you change this to a proper value of the enum, as the PropertyMetadata constructor takes a parameter of type object. Thus boxing will occur, and unboxing an Int32 to an enum will fail during type initialization.

Regards

Dave

Participant

964 points

Re: Enum as DepedencyProperty

04-24-2009 2:33 AM |

dmw:
A further thought has crossed my mind. A common mistake to make when registering DPs is to leave in the default PropertyMetadata(0) in the DP.Register method. This will result in a TypeInitializationException at runtime, and potentially strange problems at design-time. Make sure that you change this to a proper value of the enum, as the PropertyMetadata constructor takes a parameter of type object. Thus boxing will occur, and unboxing an Int32 to an enum will fail during type initialization.

I don't understand this one... Could you provide an example?

DMW

Member

106 points

Re: Enum as DepedencyProperty

04-24-2009 5:06 AM |

In the call to DependencyProperty.Register, make sure that the last parameter reads something like this new PropertyMetadata( Foo.A ) assuming that you have an enum called Foo, with a named value A.

Regards

Dave

Participant

964 points

Re: Enum as DepedencyProperty

04-24-2009 11:53 AM |

Okay, so i have to create for every enum value a property?

dmw

Member

106 points

Re: Enum as DepedencyProperty

04-24-2009 12:30 PM |

No, absolutely not. You can expose a single DP, whose type is the type of the enum. Then you will be able to set the value of that property to any of the values of the enum.

Regards

Dave

Participant

964 points