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WPF Combobox will not send Selection Back to View Model [duplicate]

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DataGrid DataGridTemplateColumn ComboBox
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ComboBox SelectedItem property updating all other comboboxes in my Datagrid row
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I have a WPF application that has a Combobox that is supposed to be bound to a boolean value. The UI should have a combobox with just two choices to choose from. Here is what I have so far. The XAML:
<DataGridTemplateColumn Header="Instance" Width="Auto">
<DataGridTemplateColumn.CellTemplate>
<DataTemplate DataType="{x:Type dataModel:ParameterSetting}">
<Grid Visibility="{Binding IsPlaceholder, Converter={StaticResource BoolToVisInvert}}">
<ComboBox SelectedIndex="{Binding Instance, Mode=TwoWay, Converter={StaticResource ConvBoolToInstance}}">
<ComboBoxItem>Instance</ComboBoxItem>
<ComboBoxItem>Type</ComboBoxItem>
</ComboBox>
</Grid>
</DataTemplate>
</DataGridTemplateColumn.CellTemplate>
</DataGridTemplateColumn>
The referenced converter:
[ValueConversion(typeof(bool), typeof(int))]
public class ConvBoolToInstance : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
if (DesignerProperties.GetIsInDesignMode(new DependencyObject())) return 0;
if (!(value is bool e))
throw new ArgumentException(#"Value was not a boolean and could not be converted", nameof(value));
return e ? 0 : 1;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
if (DesignerProperties.GetIsInDesignMode(new DependencyObject())) return true;
if (value == null || !(value is int i))
throw new ArgumentException(#"Value was not an integer and could not be converted", nameof(value));
return i == 0;
}
}
And the property that it's trying to bind to:
/// <summary>
/// If the parameter should be added as instance
/// </summary>
public bool Instance
{
get => _instance;
set
{
if (_instance == value) return;
_instance = value;
OnPropertyChanged(nameof(Instance));
}
}
When I debug this I get the correct choices in the combobox. I can change the default of the field and the combobox displays the correct item, plus I can set a breakpoint on the getter for the property and it's hit so I know that is correctly bound and going to the property I want it to. However, when I change the combobox value it changes the UI but doesn't push back to the property value on the object. I have set break points in both the property setter and the converter and neither are hit. From what I can tell it's just not pushing it back which is what it should do with a two way binding...
What am I missing?
EDIT
The converter is thanks to this answer but I have also tried biding to it as a SelectedValue with boolean combobox items but nothing seems to make it bind back...
EDIT
I should also mention this is inside a DataGrid. Updated question to show more of the XAML

Ok finally figured this out and a bit of a face palm, but posting the answer to help anyone in my same position. The answer was to use UpdateSourceTrigger=PropertyChanged. I believe it defaults to LostFocus which is fine but in this case it was REALLY delayed. Like I selected something in the ComboBox, then clicked into another cell to edit it, then clickout out of THAT cell which apparently finally took focus because THEN it committed. Not sure why it would be so delayed like that but once I switched to PropertyChanged it worked like a charm.
Here is my final binding (after figuring this out I was also able to just use normal binding without the converter):
SelectedItem="{Binding Instance, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"

WPF invokes value converter with empty string

I have a ComboBox bound to a list of values, for the sake of this example, a simple array of ints. The control has a custom ItemTemplate assigned to format its elements, which, in turn, uses an IValueConverter for some of its operation.
There's a particular case where this fails: If the ComboBox has an element selected, and the list of items changes, the converter's Convert is invoked with an empty string, which is most definitely not one of the values bound to my control. (Note that I'm not talking about the ConvertBack method.)
My questions would be:
Why is the IValueConverter being invoked with value being an empty string ("") when no string is ever bound to the control?
What are some non-hacky solutions to the problem? (I could just place if (value is "") return null; in the converter, and it seems to make the error go away, but I feel like it's treating the symptoms, not the cause.)
The problem can be reproduced with a simple WPF project (dotnet new wpf) containing only these 3 files (the problem is present on both the Framework and Core versions of .NET):
MainWindow.xaml:
<Window x:Class="test.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:d="http://schemas.microsoft.com/expression/blend/2008"
xmlns:mc="http://schemas.openxmlformats.org/markup-compatibility/2006"
xmlns:local="clr-namespace:test"
mc:Ignorable="d"
Title="MainWindow" Height="450" Width="800">
<Grid>
<ComboBox x:Name="Selector" VerticalAlignment="Center" HorizontalAlignment="Center" Width="100">
<ComboBox.ItemTemplate>
<DataTemplate>
<TextBlock Text="{Binding Converter={x:Static local:Converter.Instance}}" />
</DataTemplate>
</ComboBox.ItemTemplate>
</ComboBox>
<Button HorizontalAlignment="Right" VerticalAlignment="Bottom" Content="Replace" Click="Button_Click" />
</Grid>
</Window>
MainWindow.xaml.cs:
using System;
using System.Windows;
namespace test
{
public partial class MainWindow : Window
{
public MainWindow()
{
InitializeComponent();
Selector.ItemsSource = new int[] { 1, 2, 3 };
}
private void Button_Click(object sender, RoutedEventArgs e)
{
Selector.ItemsSource = new int[] { -1, -2, -3 };
}
[STAThread]
public static void Main(String[] args)
{
new MainWindow().ShowDialog();
}
}
}
And finally Converter.cs:
using System;
using System.Diagnostics;
using System.Globalization;
using System.Windows.Data;
namespace test
{
class Converter : IValueConverter
{
public static Converter Instance { get; } = new Converter();
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
Debug.Assert(value is int);
return (2 * (int) value).ToString();
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotSupportedException();
}
}
}
These 3 files form a minimal example that exhibits this behavior. If the "replace" button is clicked without selecting anything from the ComboBox first, the program runs fine. If, however, any value is selected in the ComboBox before clicking the button, the assertion in the converter fails, because upon clicking the button, the converter is passed a value of "".

I Tried your code when a currently selected item is not in the newly set item source the selected item should be changed right? So they change the selected item to "". That is the selected item you see in the initial state of the app running.
For a fact, if you keep the selected item in the new item source then it won't execute convert with value "".
Consider your initial item source is
Selector.ItemsSource = new int[] { 1, 2, 3 };
Now you select the second element which is displayed as 4 in your combo box and click replace to run this.
Selector.ItemsSource = new int[]{ -1, 2, -3 };
Then it won't execute the converter with value "".
Here the problem is the selected item is not present in the new item source so the Combobox is selecting its default value and your Combobox will behave like the exact way you started your application.

Use namedSize in onplatform using XAML

I'm trying to use an specific size to the font of an label for each platform using exclusively XAML. This code works fine:
<Label x:Name="DescricaoLabel"
Grid.Row="1"
Grid.Column="0"
HorizontalTextAlignment="Start"
Text="{Binding Descricao}"
TextColor="#426d76">
<Label.Font>
<OnPlatform x:TypeArguments="Font"
Android="14"
iOS="Micro" />
</Label.Font>
</Label>
However, the tag Label.Font is marked as obsolete. I tried this:
<Label.FontSize>
<OnPlatform x:TypeArguments="x:Double"
Android="14"
iOS="Micro" />
</Label.FontSize>
But using the tag Label.FontSize I was unable to compile because of the use of the NamedSize 'Micro'.
What is the best way to use double and also NamedSize within XAML and without make use of an obsolete tag?

The type needs to be of the same type. You cannot have both named size and double together in the default one.
The NamedSize works via an inbuilt static converter. You can write your own converter, maybe taking both as string and then converting. Or use a converter with parameter and specify the value and type.
The default one doesn't handle the scenario you are trying to achieve.
public class StringToSizeConverter : IValueConverter
{
public object Convert(object value, Type targetType, object parameter, CultureInfo culture)
{
double size;
if(double.TryParse(value.ToString, out size))
{
//this is a double
return size;
}
// its a named size, so convert the named size to enum
NamedSize namedSize;
if (Enum.TryParse(value.ToString, true, out namedSize))
{
return Device.GetNamedSize(NamedSize.Default, typeof(Label));
}
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture)
{
throw new NotImplementedException();
}
}
You can read more about Converters below :
Bindings and Collections
How do I use IValueConverters?
Another alternative approach is to define a named label style in your app.cs or app.xaml.cs with the font size set and using that style in your XAML. In that case you need not use OnPlatform in XAML, it will be in your CS file.

Add a single row containing text taking up entire area of WPF Datagrid

I have a WPF datagrid and I would like to display a message saying "No Records Found" in the middle of the Datagrid.
One way I can see of doing this is to add a single datarow that spans all the columns and whos height is the height of the datagrid, then centre the text in the row.
The main thing Im struggling with is programatically adding the row to teh datagrid.
Is this possible or is there an easier way to do this?

Put the 'No Records Found' message in a TextBlock instead, and show/hide the TextBlock depending on whether the DataGrid has data.
<Grid>
<DataGrid x:Name="_dataGrid" />
<TextBlock
HorizontalAlignment="Center"
VerticalAlignment="Center"
Visibility="{Binding ElementName=_dataGrid, Path=HasItems, Converter={StaticResource BooleanToInverseVisibilityConverter}, Mode=OneWay}">
No Records Found
</TextBlock>
</Grid>
EDIT: Source for BooleanToInverseVisibilityConverter (I have a bunch of handy converters like this always available to me in my WPF projects)
public class BooleanToInverseVisibilityConverter : IValueConverter {
public object Convert(object value, Type targetType, object parameter, CultureInfo culture) {
return (bool) value ? Visibility.Collapsed : Visibility.Visible;
}
public object ConvertBack(object value, Type targetType, object parameter, CultureInfo culture) {
return null;
}
}

You can add items to the datagrid programatically like this:
myDataGrid.Items.Add(new MyDataGridItem());
If you have set the ItemsSource of the datagrid to some collection or something, you need to modify that collection instead. You can do that like this:
myCollection.Add(new MyDataGridItem());
myDataGrid.ItemsSource = null;
myDataGrid.ItemsSource = myCollection;

GridView CheckBox binding

How to change the value of a text block inside a grid view in WPF based on the check box selection . Grid view in WPF is populated from a sql table which has ID and Value as columns.Value here is YES or NO.I am using linq to sql .
I have a check box associated to each ID in the grid view.when a user selects some rows ,i have to save the changes back to the database.
So based on the selection i have to change the value field in the row in this fashion:
If the text in the "Value" field of the grid view is "YES" then i have to change it to "NO"
If the text in the "Value" field of the grid view is "NO" then i have to change it to "YES"
I am able to populate data into the gridview ,but i am not sure whether my questions in the above scenario will fit in WPF and c#.Need some guidance.

The best way to do this is to bind both the Text block and the checkbox to the same backend field in the data model and then to use code converters.
Here is a simple example.
Say you have the following simple view model with one bool property:
class SimpleViewModel: INotifyPropertyChanged
{
private bool _checked;
// The property to bind to
public bool Checked
{
get { return _checked; }
set { _checked = value; OnPropertyChanged("Checked"); }
}
public event PropertyChangedEventHandler PropertyChanged;
public void OnPropertyChanged(string propertyName)
{
if (PropertyChanged != null)
PropertyChanged(this, new PropertyChangedEventArgs(propertyName));
}
}
Here is also a simple page with a text block and a text field that both bind to the same backend field.
<Window x:Class="WpfApplication1.MainWindow"
xmlns="http://schemas.microsoft.com/winfx/2006/xaml/presentation"
xmlns:x="http://schemas.microsoft.com/winfx/2006/xaml"
xmlns:local="clr-namespace:WpfApplication1"
Title="MainWindow" Height="350" Width="525">
<Window.Resources>
<local:SimpleViewModel x:Key="simpleViewModel" />
<local:BoolToStringConverter x:Key="boolToStringConverter" />
</Window.Resources>
<Grid DataContext="{StaticResource simpleViewModel}">
<StackPanel>
<TextBlock Text="{Binding Checked, Converter={StaticResource boolToStringConverter}}" />
<CheckBox Content="something" IsChecked="{Binding Checked}" />
</StackPanel>
</Grid>
</Window>
Now notice that the text block binding statement contains a converter statement.
Text="{Binding Checked, Converter={StaticResource boolToStringConverter}}"
The converter here is very simple. It checks the value if it's true and returns Yes, otherwise returns NO.
public class BoolToStringConverter:IValueConverter
{
public object Convert(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
if (value == null) return null;
if ((bool)value == true)
return "YES";
else
return "NO";
}
public object ConvertBack(object value, Type targetType, object parameter, System.Globalization.CultureInfo culture)
{
// this scenario is not needed since the text block is read only
throw new NotImplementedException();
}
}

You need to do events. Click on the control and click the lightning bold and do it in the code behind in c#. The keyword is events. OnChanged, Onclicked, onrowchange, etc. is inside that properties box for that control and you change the value in the code.

Use two way binding to transfer changes from UI into database. Bind the checkboxes column to the Value field from SQL table. You will need a convertor for binding to transform from Yes/No into bool.
http://msdn.microsoft.com/en-us/magazine/cc163299.aspx#S3
http://msdn.microsoft.com/en-us/library/ms752347.aspx#data_conversion