[Parameter]
public RenderFragment<DropDownBoxTemplateContext> DropDownFooterTemplate { get; set; }

@inject NwindDataService NwindDataService

<DxDropDownBox @bind-Value="Value"
                QueryDisplayText="QueryText"
                CssClass="ddBox">
    <DropDownHeaderTemplate>
        <span class="oi oi-person" />
        Select Employees:
    </DropDownHeaderTemplate>
    <DropDownBodyTemplate>
        <DxListBox @ref="listBox" 
                    Data="@ListBoxData" TData="Employee" TValue="Employee"
                    Values="@(GetListBoxValues(context.DropDownBox))"
                    TextFieldName="@nameof(Employee.Text)"
                    SelectionMode="ListBoxSelectionMode.Multiple"
                    ShowCheckboxes="true"
                    CssClass="templateListbox"/>
    </DropDownBodyTemplate>
    <DropDownFooterTemplate>
        <DxButton Text="OK" CssClass="btn-margin"
                  Click="@(() => SetDropDownBoxValue(context.DropDownBox))" />
        <DxButton Text="Cancel" RenderStyle="ButtonRenderStyle.Secondary"
                  Click="@(() => context.DropDownBox.HideDropDown())" />
    </DropDownFooterTemplate>
</DxDropDownBox>

@code {
    DxListBox<Employee, Employee> listBox;
    IEnumerable<Employee> ListBoxData { get; set; }
    object Value { get; set; }

    string QueryText(DropDownBoxQueryDisplayTextContext arg) {
        var names = (arg.Value as IEnumerable<Employee>)?.Select(x => x.LastName);
        return names != null ? string.Join(",", names) : string.Empty;
    }

    IEnumerable<Employee> GetListBoxValues(IDropDownBox dropDownBox) {
        return dropDownBox.Value as IEnumerable<Employee>;
    }

    void SetDropDownBoxValue(IDropDownBox dropDownBox) {
        dropDownBox.BeginUpdate();
        dropDownBox.Value = listBox.Values;
        dropDownBox.HideDropDown();
        dropDownBox.EndUpdate();
    }

    protected override async Task OnInitializedAsync() {
        ListBoxData = await NwindDataService.GetEmployeesAsync();
        Value = ListBoxData.Take(2);
    }
}

.templateListbox {
    --dxbl-list-box-border-width: 0px;
    width: 100%
}
.ddBox {
    max-width: 480px;
    width: 100%;
}
.btn-margin {
    margin-right: 8px;
}