DxListBox<TData, TValue>.ValuesChanging Event

Fires when the List Box’s selected values are being changed. Use this event to validate/cancel the newly selected values.

Namespace: DevExpress.Blazor

Assembly: DevExpress.Blazor.v25.2.dll

NuGet Package: DevExpress.Blazor

Declaration

[Parameter]
public Action<ParameterValueChangingEventArgs<IEnumerable<TValue>>> ValuesChanging { get; set; }

Event Data

The ValuesChanging event's data class is ParameterValueChangingEventArgs<IEnumerable<TValue>>. The following properties provide information specific to this event:

Property	Description
NewValue	Gets or sets the new value being assigned to the parameter.
OldValue	Gets the current parameter value.

Remarks

The List Box editor raises the ValuesChanging event before the ValuesChanged event. Both events fire in the following cases:

• A user changes item selection.
• The Values property is modified in the BeginUpdate() / EndUpdate() code block.

You can use the ValuesChanging event to validate/cancel the newly selected values. The following code prevents selection of restricted items. If a user tries to select an item that is marked as restricted, that item is excluded from the selection.

<DxListBox Data="@Products"
           @bind-Values="@SelectedProducts"
           ValuesChanging="@OnValuesChanging"
           TextFieldName="@nameof(Product.Name)"
           SelectionMode="ListBoxSelectionMode.Multiple"
           ShowCheckboxes="true">
</DxListBox>

<div class="mt-2">
    <span>Selected products: @(string.Join(", ", SelectedProducts?.Select(p => p.Name) ?? Array.Empty<string>()))</span>
</div>

@code {
    IEnumerable<Product> SelectedProducts { get; set; }
    IEnumerable<Product> Products { get; set; }

    protected override void OnInitialized() {
        Products = new List<Product> {
            new Product { Name = "Regular Item 1", IsRestricted = false },
            new Product { Name = "Premium Item", IsRestricted = true },
            new Product { Name = "Regular Item 2", IsRestricted = false }
        };
    }

    private void OnValuesChanging(ParameterValueChangingEventArgs<IEnumerable<Product>> e) {
        var oldValues = e.OldValue ?? Array.Empty<Product>();
        var newValues = e.NewValue ?? Array.Empty<Product>();

        // Exclude restricted items from selection
        var restrictedItems = newValues.Where(p => p.IsRestricted).ToList();
        if (restrictedItems.Any()) {
            e.NewValue = newValues.Except(restrictedItems).ToList();
        }
    }

    public class Product {
        public string Name { get; set; }
        public bool IsRestricted { get; set; }
    }
}