Setting object property by reflection   

Hi
I would like to set a property of an object (like Web UI control)
based on the property name to given value (as string.)

I have:


Dim obj_FinalProperty As System.Reflection.PropertyInfo =
obj_ref.GetType().GetProperty(PropertyName)

Dim obj_MethodInfo As MethodInfo =
obj_FinalProperty.GetSetMethod(True)
obj_MethodInfo.Invoke(obj_ref, Value)


obj_ref is the control reference. Now let's say I have a button and I
want to set its "Height" to "20" (provided as string). Height property
is System.Web.UI.WebControls.Unit, and I am getting exception when it
comes to Invoke call:

Object of type 'System.String' cannot be converted to type
'System.Web.UI.WebControls.Unit'. I don't know why this happens,
because when I write directly - button.Height = "20" everything is
fine. How can I overcome this problem ?

Many Thanks
Bartek

date: Sun, 24 Aug 2008 01:27:00 -0700 (PDT)   author:   Omikron

Re: Setting object property by reflection   

Omikron wrote:
> Hi
> I would like to set a property of an object (like Web UI control)
> based on the property name to given value (as string.)
> 
> I have:
> 
> 
> Dim obj_FinalProperty As System.Reflection.PropertyInfo =
> obj_ref.GetType().GetProperty(PropertyName)
> 
> Dim obj_MethodInfo As MethodInfo =
> obj_FinalProperty.GetSetMethod(True)
> obj_MethodInfo.Invoke(obj_ref, Value)
> 
There's no need to call .GetSetMethod(); you can call .SetValue() directly.

> obj_ref is the control reference. Now let's say I have a button and I
> want to set its "Height" to "20" (provided as string). Height property
> is System.Web.UI.WebControls.Unit, and I am getting exception when it
> comes to Invoke call:
> 
> Object of type 'System.String' cannot be converted to type
> 'System.Web.UI.WebControls.Unit'. I don't know why this happens,
> because when I write directly - button.Height = "20" everything is
> fine. How can I overcome this problem ?
> 
Visual Basic supports some convenient implicit conversions that reflected 
properties don't. You need to supply a value of the exact same type as the 
property, not merely one that's convertible to it. You can do this by 
calling Convert.ChangeType() and supplying the property's type.

-- 
J.

date: Sun, 24 Aug 2008 13:19:28 +0200   author:   Jeroen Mostert

Re: Setting object property by reflection   

> Visual Basic supports some convenient implicit conversions that reflected
> properties don't. You need to supply a value of the exact same type as the
> property, not merely one that's convertible to it. You can do this by
> calling Convert.ChangeType() and supplying the property's type.
>
> --
> J.

I tried this before as well. I had:

Dim obj_FinalProperty As System.Reflection.PropertyInfo =
obj_Control.GetType().GetProperty(PropertyName)

            If Not obj_FinalProperty Is Nothing AndAlso
obj_FinalProperty.CanWrite = True Then
                Try
                    Dim ValueType As Type =
System.Type.GetType(obj_FinalProperty.PropertyType.AssemblyQualifiedName.ToString(),
False, True)

                    If Not ValueType Is Nothing Then
                        obj_FinalProperty.SetValue(obj_Control,
System.Convert.ChangeType(Value, ValueType), Nothing)
                    End If

Still Convert.ChangeType returns exception about invalid conversion.
So I understand there is no way to apply string value to Unit type by
the reflection ?

date: Sun, 24 Aug 2008 05:38:02 -0700 (PDT)   author:   Omikron

Re: Setting object property by reflection   

Ok, I think I find a solution. I need to check if the type implements
Parse method, if so I can invoke it.

date: Sun, 24 Aug 2008 05:44:51 -0700 (PDT)   author:   Omikron

Re: Setting object property by reflection   

Omikron wrote:
>> Visual Basic supports some convenient implicit conversions that reflected
>> properties don't. You need to supply a value of the exact same type as the
>> property, not merely one that's convertible to it. You can do this by
>> calling Convert.ChangeType() and supplying the property's type.
>>
> 
> I tried this before as well. I had:
> 
> Dim obj_FinalProperty As System.Reflection.PropertyInfo =
> obj_Control.GetType().GetProperty(PropertyName)
> 
>             If Not obj_FinalProperty Is Nothing AndAlso
> obj_FinalProperty.CanWrite = True Then
>                 Try
>                     Dim ValueType As Type =
> System.Type.GetType(obj_FinalProperty.PropertyType.AssemblyQualifiedName.ToString(),
> False, True)
> 
What on earth is this supposed to achieve? Just stop at .PropertyType.

>                     If Not ValueType Is Nothing Then
>                         obj_FinalProperty.SetValue(obj_Control,
> System.Convert.ChangeType(Value, ValueType), Nothing)
>                     End If
> 
ValueType cannot possibly be Nothing. The property must have a type and it 
must be accessible, otherwise you wouldn't have been able to get it at all.

> Still Convert.ChangeType returns exception about invalid conversion.

Yes, I should have dug further. Convert only operates on system types. You 
want TypeConverter here.

> So I understand there is no way to apply string value to Unit type by
> the reflection ?

Yes, there is. Here's a full snippet:

       Dim PropertyInfo As PropertyInfo = 
obj_Control.GetType().GetProperty(PropertyName)
     If PropertyInfo Is Nothing Then Throw New 
InvalidOperationException(String.Format("Property '{0}' does not exist.", 
PropertyName))
     If Not PropertyInfo.CanWrite Then Throw New 
InvalidOperationException(String.Format("Property '{0}' is not writable.", 
PropertyName))
     PropertyInfo.SetValue(obj_Control, 
TypeDescriptor.GetConverter(PropertyInfo.PropertyType).ConvertFromString(Value), 
Nothing)

-- 
J.

date: Sun, 24 Aug 2008 18:21:40 +0200   author:   Jeroen Mostert