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Read Text From TextBox in DataGrid MVVM (wpf databinding)

I have a datagrid of rows which contain data read from a web server and values I want to write into a webserver. I write the values in getting the user to input a number into the appropriate column and click an adjacent text box;
<DataGrid x:Name="datagridDERControl" HorizontalAlignment="Center" VerticalAlignment="Center" Background="#FF322D2D" Height="382" Margin="10,78,10,10" Width="972" ItemsSource="{Binding Path=NFDataSource, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}">
<DataGrid.Columns>
<DataGridTemplateColumn Width="100" Header="Write Set Point">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<TextBox Width="100" Text="{Binding Path=WriteSetPoint, Mode=OneWayToSource, UpdateSourceTrigger=PropertyChanged}"></TextBox>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
<DataGridTemplateColumn Width="100" Header="Global Trip">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate>
<Button Name="buttonGlobalTrip" Width="100" Click="buttonGlobalTrip_Click"></Button>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
</DataGrid
How do I extract the specific textbox string per row to use in my view model.

It's always difficult to answer a question where the relevant details have been omitted by the question author. However, I shall try!
You have data bound (presumably) a collection property named NFDataSource to your DataGrid.ItemsSource property. That is the collection that represents the data in your DataGrid, so to 'extract' a specific value, you need to look into your data items in your collection.
One handy property in the DataGrid class is the SelectedItem property. this enables you to data bind an object (of the same type as those in your NFDataSource collection) to this property, which accesses the data object behind the row that is currently selected in the UI:
<DataGrid ItemsSource="{Binding NFDataSource}" SelectedItem="{Binding SelectedItem}" />
Now you can utilise your SelectedItem property to access the values from the selected row in the DataGrid:
string someValue = SelectedItem.SomeProperty;

As you tagged this with MVVM and databinding, I'll assume you're using these and have just got muddled.
"I have a datagrid of rows which contain data read from a web server
and values I want to write into a webserver."
So your viewmodel has a property, which is a collection of a custom class, that represents data fetched from a webservers.
"I write the values in getting the user to input a number into the
appropriate column and click an adjacent text box"
So this VM property is two-way bound to a datagrid, so that each item in the collection represents 'one row', and the properties on those items represent your 'columns'. The user can make changes to the UI displayed values, and because of the two way databinding the VM property is also updated.
"How do I extract the specific textbox string per row to use in my
view model."
Why do you need the specific textbox string, if it is databound to a property (or rather to a property on a class contained in a collection) in your VM anyway? If you've set up your VM this way, and are using databinding, you rarely need to worry about UI specific things such as which row in a datagrid is clicked.
As Sheridan points out though, you can also bind to properties on the datagrid such as SelectedItem so that you can perform additional operations beyond just reading/writing data. SelectedItem for your datagrid will be of the type that populates your VM collection, so will have the appropriate properties.
For example, if your VM collection is an IQueryable<Person> and that is bound to the ItemsSource of the datagrid then the SelectedItem will be of type Person. You could then have a VM property called SelectedPerson which is bound to that SelectedItem, and access things like SelectedPerson.Name etc.

WPF Dynamic Ordering

I have a Model with one of it property being Order with is a type of int. The model is put inside an ObservableCollection say for example ModelList is bound to a listbox.
Using this code
<CollectionViewSource Source="{StaticResource ModelList}" x:Key="SortedItems">
<CollectionViewSource.SortDescriptions>
<scm:SortDescription PropertyName="Order"/>
</CollectionViewSource.SortDescriptions>
</CollectionViewSource>
Then this list is Bound to a listbox using
<ListView ItemsSource="{Binding Source={StaticResource SortedItems}}" />
Works Fines. But I want to change the order of the list by its Order property, that is when i change the Order via code I want the listbox to reflect the change.
How can I achieve this.

In order to resort a CollectionViewSource, you need to update the SortDescriptions property
To access your CollectionViewSource from code, declare it in your viewmodel and bind to it (instead of declaring it in XAML). For items in the visual tree, you can use x:Name, though this does not seem to work from a Resources tag, at least via initial testing

Xml Binding ComboBox different SelectedValue and Itemssource DataContext

I have read many posts on this subject but I can't wrap my head around it. I have a ComboBox of which the ItemSource is binded to an XmlDataProvider. It looks like this:
<ComboBox ItemsSource="{Binding XPath=List/ListItem}">
I need the SelectedValue to be binded to an XPath property of a selected item of a ListBox. So I have two different datacontexts. The ListBox is binded to the same XmlDataProvider. I tried this:
<ComboBox ItemsSource="{Binding XPath=List/ListItems}" SelectedValue="{Binding ElementName="MyListBox" Path=SelectedItem.MyXPathProperty}">
But it didn't work. In the other posts people referenced to a normal property but I need to reference to an XPath property.
How can I bind the SelectedValue to an XPath property of a different element?
<ListItems>
<MyNode MyXPathProperty="SomeValue">
<MyNode MyXPathProperty="SomeValue">
<MyNode MyXPathProperty="SomeValue">
</ListItems>

Data Binding Label Content to Array

I am somewhat new to WPF and Data Binding, it seems very powerful. I'm wondering if there's a way to have a set of labels, and have there Content property all binded to a different index in array of strings. So then as the array is updated, the labels automatically change too.
The xaml syntax is still a bit foreign to me and I haven't been able to get it to work.

If this is a dynamic set of labels, then you may be better off using an ItemsControl, and changing its ItemTemplate to display a label for each item in the collection that it is bound to (a collection of strings in your case).
Something like:
<ItemsControl ItemsSource="{Binding MyLabelStrings}">
<ItemsControl.ItemTemplate>
<DataTemplate>
<Label Content="{Binding}" ... />
</DataTemplate>
</ItemsControl.ItemTemplate>
</ItemsControl>
As Bojin mentions, if you wish your UI to update if strings are added/removed from the collection, then use an ObservableCollection for the MyLabelStrings property.

Editable WPF ListBox

I have a ObservableCollection that's bound to a ListBox in WPF. I want the ListBox to be editable, and for the editing changes to be saved to the collection. Since WPF doesnt provide an editable listbox, I've tried creating my own by changing the ListBox.ItemTemplate.
<ListBox.ItemTemplate>
<DataTemplate>
<TextBox Name="EditableText" Text="{TemplateBinding Content}"/>
</DataTemplate>
</ListBox.ItemTemplate>
Changing the ItemTemplate gives me editable boxes, but any changes to the textboxes dont get saved to the ObservableCollection. Is there a way to have an editable ListBox with two way binding?

You cannot do it this way.
To achieve that kind of trick, you would need your items to be "holder classes" that expose a property you can bind your textbox to.
To understand it, imagine the following pseudo sequence of calls:
class ListBox
{
Bind(Items)
{
foreach(var item in Items)
{
DataTemplate Template = LoadTemplateForItem(item.GetType()); // this is where your template get loaded
Template.Bind(item); //this is where your template gets bound
}
}
}
Your template (the DataTemplate with the listbox) is loaded and the item (which I assume is a string in your case) gets passed in.
At this point, it only knows the string, and cannot influence anything upwards. A two-way binding cannot influence the collection because the template does not know in which context it is being used, so it cannot reach back to the original collection and modify its contents.
For that matter, this is the same thing for the TextBox. If it is not given a conainer and a property name, it has nowhere to "store back" the changes.
This basically the same as passing a string into a function call. The function cannot change which string was passed in (ignoring tricks such as by-reference argument passing).
To get back to your case, you need to build a collection of objects which expose a property containing the value that needs to be edited:
public class MyDataItem
{
string Data { get; set;}
}
Then you can bind your ListBox to a collection of those items and modifiy your datatemplate:
<ListBox.ItemTemplate>
<DataTemplate>
<TextBox Name="EditableText" Text="{Binding Data, Mode=TwoWay}"/>
</DataTemplate>
</ListBox.ItemTemplate>

Bind to a model property -- i.e. a property of the data object -- rather than to a view property such as Content. For example:
// model class
public class Widget : INotifyPropertyChanged
{
public string Description { ... }
}
<!-- view -->
<DataTemplate>
<TextBox Text="{Binding Description}" />
</DataTemplate>
Note this will not work if your ItemsSource is ObservableCollection (because there's no property to bind to).