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I am writing a C# WPF application and following the MVVM pattern.
My ViewModel has an object in it called NetworkMain
My View has a DataGrid which binds to an ObservableCollection called FlowControls which is inside NetworkMain

My DataGrid Itemsource is defined as follows:

<DataGrid ItemsSource="{Binding Path=NetworkMain.FlowControls}">

The above is working fine.

I want one of my columns in the DataGrid to be a ComboBox.
My column is defined as shown in the code at the bottom of this question.

The problem I have is I want the source of the ComboBox to use a collection called Pipes which is in NetworkMain and not FlowControls.

How can I do this?
I tried using Binding Path=NetworkMain.Pipes but I think it is looking for NetworkMain from within FlowControls which isn't correct.

Hope the above makes sense.

What I have tried:

<DataGridTemplateColumn Header="Downstream Pipe ID">
                SelectedItem="{Binding Path=SelectedDownstreamPipeID, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
                ItemsSource="{Binding Path=NetworkMain.Pipes}"
                DisplayMemberPath="Description" >


I have updated the XAML as follows and I now get the list I want in the ComboBox. What I can't get to work is the setting of the selected item.
I get the following error:
System.Windows.Data Error: 1 : Cannot create default converter to perform 'two-way' conversions between types 'Drain.Models.Network.Conduit' and 'System.String'. Consider using Converter property of Binding.

        x:Key="Conduits" Source="{Binding NetworkMain.Conduits}">

    Header="Downstream Pipe ID"  
    SelectedItemBinding="{Binding outgoingConduit, Mode=OneWayToSource}" 
    ItemsSource="{Binding Source={StaticResource Conduits}}" 

public ObservableCollection<Conduit> Conduits { get; set; } = new();

public Conduit outgoingConduit 

I tried adding the following convertor:

<conv:ConduitObjectConvertor x:Key="ConduitObjectConvertor" />

SelectedItemBinding="{Binding outgoingConduit, Mode=TwoWay, Converter={StaticResource ConduitObjectConvertor}}"

I copied and pasted a convertor from an example online and I thought I adapted it correctly but I still get errors. The convertor is here:

public class ConduitObjectConvertor : TypeConverter
        public override bool CanConvertFrom(ITypeDescriptorContext context,
        Type sourceType)
            return sourceType == typeof(Conduit);

        public override object ConvertFrom(ITypeDescriptorContext context,
            System.Globalization.CultureInfo culture, object value)
            return value as Conduit;

        public override bool CanConvertTo(ITypeDescriptorContext context,
            Type destinationType)
            return destinationType == typeof(Conduit);

        public override object ConvertTo(ITypeDescriptorContext context,
            System.Globalization.CultureInfo culture, object value, Type destinationType)
            return value == null ? null : value as Conduit;

The error I get is:

InvalidCastException: Unable to cast object of type 'Drain.Helper.Convertors.ConduitObjectConvertor' to type 'System.Windows.Data.IValueConverter'.
Updated 14-Jun-21 22:44pm
Gerry Schmitz 14-Jun-21 13:30pm
Probably should be considering a DataGridComboBoxColumn first.

1 solution

How about naming an element further up the hierarchy:
<UserControl x:Name="RootElement" ...
Then bind to the DataContext.NetworkMain.Pipes property on that element:
    SelectedItem="{Binding Path=SelectedDownstreamPipeID, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}"
    ItemsSource="{Binding ElementName=RootElement, Path=DataContext.NetworkMain.Pipes}"
Richard!i! 15-Jun-21 4:50am
I was pulling my hair with this in an parallel to you providing an answer. See edits to the question. I alsmost got it to work, but I saw from your response I was missing the UpdateSourceTrigger=PropertyChanged} on the selected item. I'll keep the list resource as I have it since it's working. Thanks for that final nugget of info to get it working.

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