Question

<ComboBox   TextSearch.TextPath="MemberFullName"  IsEditable="True" Height="23" VerticalAlignment="Center" Grid.Row="1" Grid.Column="1" Margin="5,0,0,0" ItemsSource="{Binding MemberCollection}" SelectedItem="{Binding SelectedSearchMember,Mode=TwoWay,ValidatesOnDataErrors=True}">
            <ComboBox.ItemTemplate>
              <DataTemplate>
                   <TextBlock Text="{Binding MemberFullName}" VerticalAlignment="Center"/>
               </DataTemplate>
            </ComboBox.ItemTemplate>
</ComboBox>

On Mouse Left Button Down it become Editable

Answer 1

If you use MVVM pattern, add to your ViewModel variable:

  private bool _isEditableComboBox = false;
        public bool IsEditableComboBox
        {
            get { return _isEditableComboBox; }
            set { _isEditableComboBox = value; RaisePropertyChanged(() => IsEditableComboBox); }
        }

Add to your project assembly: System.Windows.Interactivity

Add to your View namespace to this assembly:

xmlns:i="clr-namespace:System.Windows.Interactivity;assembly=System.Windows.Interactivity"

Change your ComboBox to this:

 <ComboBox TextSearch.TextPath="MemberFullName" IsEditable="{Binding IsEditableComboBox, UpdateSourceTrigger=PropertyChanged}" Height="23" VerticalAlignment="Center" Grid.Row="1" Grid.Column="1" Margin="5,0,0,0" ItemsSource="{Binding MemberCollection}" SelectedItem="{Binding SelectedSearchMember,Mode=TwoWay,ValidatesOnDataErrors=True}">
            <i:Interaction.Triggers>
                <i:EventTrigger EventName="PreviewMouseDown">
                    <i:InvokeCommandAction Command="{Binding TurnOnEditMode}" />
                </i:EventTrigger>
            </i:Interaction.Triggers>
            <ComboBox.ItemTemplate>
                <DataTemplate>
                    <TextBlock Text="{Binding MemberFullName}" VerticalAlignment="Center"/>
                </DataTemplate>
            </ComboBox.ItemTemplate>
        </ComboBox>

And you must add to your ViewModel this command:

public ICommand TurnOnEditMode { get; private set; }
        private void OnTurnoOnEditMode()
        {
            IsEditableComboBox = true;
        }