WPF change ComboBox user selection and focus programmatically with MVVM

Question

In my MVVM app I have a textbox and a combobox.

User write a numeric value in the textbox and the combobox dropdown (upon textbox input) and user select a level from the combobox (he cannot open the combobox by himself).

I want to check both inputs and change the combobox accordingly.

For example if user set the textbox to 1200.5 mV I would change the textbox to 1.0 and the combobox to V.

Question 1:

How can I change the combobox SelectedValue programmatically so the user could see the new value?

Question 2:

How can I dropdown the combobox but still keep the focus on the textbox (even if my mouse cursor is on the combobox dropdown)? It seems that I lose focus on the textbox after the first selection from the combobox .

Thanks.

XAML:

<Grid >

    <StackPanel Orientation="Horizontal">

        <TextBox Text="{Binding Path=Name, UpdateSourceTrigger=PropertyChanged, Mode=TwoWay}" Height="25" Width="100" 


                 />


        <ComboBox 
            IsDropDownOpen="{Binding IsDropDownOpen, Mode=TwoWay, UpdateSourceTrigger=PropertyChanged}" 
            ItemsSource="{Binding OffsetValues}"
            SelectedValue="{Binding NodeCategory, Mode=TwoWay}" 

            Height="25" Width="100" IsHitTestVisible="False" Background="AliceBlue">
            <ComboBox.Resources>
                <sys:Double x:Key="{x:Static SystemParameters.VerticalScrollBarWidthKey}">0</sys:Double>
            </ComboBox.Resources> 
        </ComboBox>
    </StackPanel>


</Grid>

ViewModel:

class ViewModel : ViewModelBase
{


    private IList<string> offsetValues =  new List<string>() { "mV", "V" };
    public IList<string> OffsetValues
    {
        get
        {
            return offsetValues;
        }
        set
        {
            offsetValues = value;
        }
    }




    private bool isDropDownOpen;

    public bool IsDropDownOpen
    {
        get { return isDropDownOpen; }
        set
        {
            isDropDownOpen = value;
            OnPropertyChanged();
        }
    }


    private string _name;
    public string Name
    {
        get { return _name; }
        set
        {

            _name = value;
            OnPropertyChanged( "Name" );

            if( _name != "" )
            {
                isDropDownOpen = true;
                OnPropertyChanged( "IsDropDownOpen" );
            }

        }
    }




    private string _NodeCategory;
    public string NodeCategory
    {
        get
        {
            return _NodeCategory;
        }
        set
        {
            if( Convert.ToDouble( _name ) > 1000 )
            {
                _name = "1.0";
                OnPropertyChanged( "Name" );




                _NodeCategory = OffsetValues[1];



                OnPropertyChanged( "NodeCategory" );


            }
            else
            {


                _NodeCategory = value;
                OnPropertyChanged( "NodeCategory" );

            }



        }
    }





}


public class ViewModelBase : INotifyPropertyChanged
{
    protected virtual void OnPropertyChanged( [CallerMemberName]string propertyName = null )
    {
        PropertyChanged.Invoke( this, new PropertyChangedEventArgs( propertyName ) );
    }

    public event PropertyChangedEventHandler PropertyChanged;


}

Answer 1

In textbox add 2 way binding to a property in viewmodel

In the setter of the property(means textbox value updated) change value of nodecategory and offsetvalues...raise property change for both

This should work

I am typing from mobile so can't add example.

Hope this helps