如何从另一个DataContext绑定ComboBox SelectedItem？

Question

I have an edit usercontrol , that is used to edit Workout details. On that UC there is a combobox that has its datacontext set to another viewmodel of Exercise Equipment. How can i get after opening uc to selected item gets to the property of Workout.

Here is the code:

<ComboBox Name="ExeEquComboBox" 
        ItemsSource="{Binding AllExerciseEquipment}" 
        DisplayMemberPath="Name" 
        SelectedValue="{Binding ExerciseEquipment, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type Grid}}, Mode=TwoWay}">
    <ComboBox.DataContext>
        <ExerciseEquipmentViewModel/>
    </ComboBox.DataContext>
</ComboBox>

EDIT: posting more details as requested:

Workout properties are: Name, Description IdExerciseEquipment (foreign key to ExerciseEquipment table).

I have 2 ViewModels: WorkoutViewModel ExerciseEquipmentViewModel

When i open the UC i pass the Workout object to it and set the datacontext of the usercontrol to that object.

On that UC there is a combobox that has its datacontext set to the ExerciseEquipmentViewModel. I get the itemsource from that viewmodel.

What i need is to properly bind Workout.IdExerciseEquipment to the Combobox itemsource. When opening UserControl, Combobox.SelectedItem should be the item with Workout.IdExerciseEquipment, but i dont know how to achieve that.

Thanks.

Answer 1

There are two ways you can do this:

Ancestor Binding like you have done. Just correct the binidng as below:

SelectedValue="{Binding DataContext.ExerciseEquipment, RelativeSource={RelativeSource FindAncestor, AncestorType={x:Type UserControl}}, Mode=TwoWay}"

Element Binding , as below:

ComboBox Name="ExeEquComboBox" 
    ItemsSource="{Binding AllExerciseEquipment}" 
    DisplayMemberPath="Name" 
    SelectedValue="{Binding ElementName=UserControlName,Path=DataContext.ExerciseEquipment}">

Update: from the information you provided I'm able to understand that there is IdExerciseEquipment property in Workout object, i think this is some kind of ID for distinguish AllExerciseEquipment items. I think you should create your property as below in Workout class:

private string exerciseEquipment;

    public string ExerciseEquipment
    {
        get 
        {
            if (exerciseEquipment == null)
            {
                exerciseEquipment = AllExerciseEquipment.FirstOrDefault(x => x.ID == this.IdExerciseEquipment);
            }
            return exerciseEquipment;
        }
        set 
        { 
            exerciseEquipment = value; 
        }
    }

So by default when ExerciseEquipment is blank, we set a value to the item where ID is matched, by iterating AllExerciseEquipment which is your ItemSource of ComboBox .

Since I don't have your exact class definitions please change the code where needed.