<select name="@Html.NameFor(model => Model.SelectedRoles)" id="@Html.IdFor(model => Model.SelectedRoles)" class="form-control selectpicker" data-live-search="true" autocomplete="off" data-size="8" multiple>
@foreach (var role in Model.RolesSelectListItems)
{
    <option data-subtext="@role.Description" value="@role.Id">@role.Code</option>
}
</select>

@Html.DropDownListFor(x => Model.SelectedRoles, new SelectList(Model.RolesSelectListItems, nameof(RoleViewModel.Id), nameof(RoleViewModel.Code)),
            new { @class = "form-control selectpicker", @data_live_search = "true", @autocomplete = "off", @data_size = "8", @multiple = "" })

public IEnumerable<RoleViewModel> RolesSelectListItems { get; set; }

[Display(Name = "Requested roles")]
[Required(ErrorMessage = "Select at least one role")]
public IEnumerable<int> SelectedRoles { get; set; }