I have used an approach similar to the one suggested by Captain. I have created one property in my ViewModel which represents multiple items (i.e. all selected items). In the property get- and set-accessors I have used the following methods to determine/set the shared values of/for all items. This approach does not use any reflection, but uses delegates in the form of lambda expressions which makes it pretty fast.
As an overview, this is my basic design:
public class MyMultiSelectionViewModel
{
private List<MyItemType> m_selectedItems = new List<MyItemType>();
public void UpdateSelectedItems(IList<MyItemType> selectedItems)
{
m_selectedItems = selectedItems;
this.OnPropertyChanged(() => this.MyProperty1);
this.OnPropertyChanged(() => this.MyProperty2);
// and so on for all relevant properties
}
// properties using SharedValueHelper (see example below)
}
The properties look like this:
public string Name
{
get
{
return SharedValueHelper.GetSharedValue<MyItemType, string>(m_selectedItems, (item) => item.Name, String.Empty);
}
set
{
SharedValueHelper.SetSharedValue<MyItemType, string>(m_selectedItems, (item, newValue) => item.Name = newValue, value);
this.OnPropertyChanged(() => this.Name);
}
}
And the code for the SharedValueHelper class looks like this:
/// <summary>
/// This static class provides some methods which can be used to
/// retrieve a <i>shared value</i> for a list of items. Shared value
/// means a value which represents a common property value for all
/// items. If all items have the same property value, this value is
/// the shared value. If they do not, a specified <i>non-shared value</i>
/// is used.
/// </summary>
public static class SharedValueHelper
{
#region Methods
#region GetSharedValue<TItem, TProperty>(IList<TItem> items, Func<TItem, TProperty> getPropertyDelegate, TProperty nonSharedValue)
/// <summary>
/// Gets a value for a certain property which represents a
/// <i>shared</i> value for all <typeparamref name="TItem"/>
/// instances in <paramref name="items"/>.<br/>
/// This means, if all wrapped <typeparamref name="TItem"/> instances
/// have the same value for the specific property, this value will
/// be returned. If the values differ, <paramref name="nonSharedValue"/>
/// will be returned.
/// </summary>
/// <typeparam name="TItem">The type of the items for which a shared
/// property value is requested.</typeparam>
/// <typeparam name="TProperty">The type of the property for which
/// a shared value is requested.</typeparam>
/// <param name="items">The collection of <typeparamref name="TItem"/>
/// instances for which a shared value is requested.</param>
/// <param name="getPropertyDelegate">A delegate which returns the
/// property value for the requested property. This is used, so that
/// reflection can be avoided for performance reasons. The easiest way
/// is to provide a lambda expression like this:<br/>
/// <code>(item) => item.MyProperty</code><br/>
/// This expression will simply return the value of the
/// <c>MyProperty</c> property of the passed item.</param>
/// <param name="nonSharedValue">The value which should be returned if
/// the values are not equal for all items.</param>
/// <returns>If all <typeparamref name="TItem"/> instances have
/// the same value for the specific property, this value will
/// be returned. If the values differ, <paramref name="nonSharedValue"/>
/// will be returned.</returns>
public static TProperty GetSharedValue<TItem, TProperty>(IList<TItem> items, Func<TItem, TProperty> getPropertyDelegate, TProperty nonSharedValue)
{
if (items == null || items.Count == 0)
return nonSharedValue;
TProperty sharedValue = getPropertyDelegate(items[0]);
for (int i = 1; i < items.Count; i++)
{
TItem currentItem = items[i];
TProperty currentValue = getPropertyDelegate(currentItem);
if (!sharedValue.Equals(currentValue))
return nonSharedValue;
}
return sharedValue;
}
#endregion
#region SetSharedValue<TItem, TProperty>(IList<TItem> a_items, Action<TItem, TProperty> a_setPropertyDelegate, TProperty a_newValue)
/// <summary>
/// Sets the same value for all <typeparamref name="TItem"/>
/// instances in <paramref name="a_items"/>.
/// </summary>
/// <typeparam name="TItem">The type of the items for which a shared
/// property value is requested.</typeparam>
/// <typeparam name="TProperty">The type of the property for which
/// a shared value is requested.</typeparam>
/// <param name="items">The collection of <typeparamref name="TItem"/>
/// instances for which a shared value should be set.</param>
/// <param name="setPropertyDelegate">A delegate which sets the
/// property value for the requested property. This is used, so that
/// reflection can be avoided for performance reasons. The easiest way
/// is to provide a lambda expression like this:<br/>
/// <code>(item, newValue) => item.MyProperty = newValue</code><br/>
/// This expression will simply set the value of the
/// <c>MyProperty</c> property of the passed item to <c>newValue</c>.</param>
/// <param name="newValue">The new value for the property.</param>
public static void SetSharedValue<TItem, TProperty>(IList<TItem> items, Action<TItem, TProperty> setPropertyDelegate, TProperty newValue)
{
if (items == null || items.Count == 0)
return;
foreach (TItem item in items)
{
try
{
setPropertyDelegate(item, newValue);
}
catch (Exception ex)
{
// log/error message here
}
}
}
#endregion
#endregion
}