TwoWay binding on CommandParameter works on objects, but not on string 1

[kristofdielis]

Hi everyone,

I have several controls on a WPF user control, most of these are comboboxes. Behind each control, there is a button, that will open up a new window, which will return a user selected value.

Consider, one of these comboboxes contains a list of sites:

These sites are defined like this:

public ObservableCollection<Site> Sites { get; private set; }

And the combobox like this:

<telerik:RadComboBox Name="cbxSite" 
    ItemsSource="{Binding Hierarchy.Sites}" 
    SelectedValue="{Binding Hierarchy.SiteId, Mode=TwoWay}"
    DisplayMemberPath="Name" 
    SelectedValuePath="site_id"
/>

This depends on a ViewModel which contains the Hierarchy property (object), which contains both the Sites and SiteId. SiteId is used only so you have a binding point to work with (which is also the Combobox's SelectedValue.

The button behind the Sites combobox is defined like this:

<tlrk:RadButton Name="btnSiteIdConstructValue">
    <i:Interaction.Triggers>
        <i:EventTrigger EventName="Click">
            <cmd:EventToCommand PassEventArgsToCommand="False"
                Command="{Binding ConstructValueCommand}"
                CommandParameter="{Binding Hierarchy.SiteId}"/>
        </i:EventTrigger>
    </i:Interaction.Triggers>
    <Image Source="someImage.png" />
</tlrk:RadButton>

Now, Hierarchy.SiteId is a string. It is passed as a parameter to the receiving method (through RelayCommand), which handles it as an object.

And here's the problem:
Even though the parameter has TwoWay binding, any changes to the value, are not returned. But, if I pass the entire object, say a TextBox, and set the .text value, than everything is alright.

But, preferably, I'd rather not have to do it like this, because I can expect any sort of object, and I'd have to build a fairly elaborate type check, to know what I am supposed to be doing with it. And, in case of the comboboxes, it's not exactly easy, because I have multiple items to deal with. Sure, I could use the SelectedItem property, but then I'd have to type check multiple types on top of the ComboBox type check.

The above is well, a possible solution, but getting the string binding to properly work, would be far more elegant.

Any suggestions?

 

[jmeckley]

this may have to do with the object type. strings are immutable, where POCO objects, by default, are not. so if a string is changed, technically a new instance is created a returned, but the original value remains the same.

with a custom POCO object you can change the value of the referenced properties, therefore that instanced changed. it now references a new string.

rather than bind a specific type to the drop down you could convert your custom types to a common UI object. I believe there is even an Linq Extension for this

new[]{}.ToLooup(key selector, value selector);

you may also want to take a look at Caliburn.Micro as a framework for MVVM/MVP/MVC etc. The framework is built on the notion of convention over configuration so a lot of the boiler plate, repetitive code is handled for you automatically.


Jason Meckley
Senior Programmer

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[kristofdielis]

Hi Jason,

Thanks for the response, to be honest, I didn't really expect one, so you get a star at least for trying. I'll check it out.