@using Telerik.DataSource

This custom filter menu lets you choose more than one option to match against the data source

<TelerikGrid Data=@GridData FilterMode="@GridFilterMode.FilterMenu"
             Height="400px" Width="600px" Pageable="true">
    <GridColumns>
        <GridColumn Field="Id" Filterable="false" Width="80px" />

        <GridColumn Field="Size">
            <FilterMenuTemplate>
                @{
                    // we store a reference to the filter context to use in the business logic to show we can
                    // we could, alternatively pass it as an argument to the event handler in the lambda expression
                    // which can be useful if you want to use the same filter for several columns
                    // you could then pass more arguments to the business logic such as field name and so on
                    theFilterContext = context;
                }

                @foreach (var size in Sizes)
                {
                    <div>
                        <TelerikCheckBox Value="@(IsCheckboxInCurrentFilter(context.FilterDescriptor, size))"
                                         TValue="bool"
                                         ValueChanged="@((value) => UpdateCheckedSizes(value, size))"
                                         Id="@($"size_{size}")">
                        </TelerikCheckBox>
                        <label for="@($"size_{size}")">
                            @if (size == null) // part of handling nulls - show meaningful text for the end user
                            {
                                <text>Empty</text>
                            }
                            else 
                            {
                                @size
                            }
                        </label>
                    </div>
                }
            </FilterMenuTemplate>
        </GridColumn>

        <GridColumn Field="ProductName" Title="Product" Filterable="false" />
    </GridColumns>
</TelerikGrid>

@code {
    FilterMenuTemplateContext theFilterContext { get; set; }
    public List<string> CheckedSizes { get; set; } = new List<string>();

    public bool IsCheckboxInCurrentFilter(CompositeFilterDescriptor filterDescriptor, string size)
    {
        // get all current filter descriptors and evaluate whether to select the current checkbox
        // the default value for string filter descriptors is null so it would select the null checkbox always
        // so we will add a check to ensure it matches the desired operator - IsNull (see the UpdateCheckedSizes method below)
        if (size == null)
        {
            foreach (FilterDescriptor item in filterDescriptor.FilterDescriptors)
            {
                if(item.Operator == FilterOperator.IsNull)
                {
                    return true;
                }
            }
            return false;
        }
        return filterDescriptor.FilterDescriptors.Select(f => (f as FilterDescriptor).Value?.ToString()).ToList().Contains(size);
    }

    public void UpdateCheckedSizes(bool value, string itemValue)
    {
        // update the list of items we want to filter by
        var isSizeChecked = CheckedSizes.Contains(itemValue);
        if (value && !isSizeChecked)
        {
            CheckedSizes.Add(itemValue);
        }

        if (!value && isSizeChecked)
        {
            CheckedSizes.Remove(itemValue);
        }

        // prepare filter descriptor
        var filterDescriptor = theFilterContext.FilterDescriptor;

        filterDescriptor.FilterDescriptors.Clear();
        // use the OR logical operator so we include all possible values
        filterDescriptor.LogicalOperator = FilterCompositionLogicalOperator.Or;
        CheckedSizes.ForEach(s => {
            // instantiate a filter descriptor for the desired field, and with the desired operator and value
            FilterDescriptor fd = new FilterDescriptor("Size", FilterOperator.IsEqualTo, s);
            // set its type to the field type you filter (the Size field in this example)
            fd.MemberType = typeof(string);
            // handle null values - use a specific filter operator that the user cannot select on their own
            // in this custom filter template (the grid has it in a dropdown by default)
            if(s == null)
            {
                fd.Operator = FilterOperator.IsNull;
            }

            filterDescriptor.FilterDescriptors.Add(fd);
        });

        //ensure there is at least one blank filter to avoid null reference exceptions
        if (!filterDescriptor.FilterDescriptors.Any())
        {
            filterDescriptor.FilterDescriptors.Add(new FilterDescriptor());
        }
    }

    // sample grid data

    public List<SampleData> GridData { get; set; }

    protected override void OnInitialized()
    {
        GridData = Enumerable.Range(1, 70).Select(x => new SampleData
        {
            Id = x,
            Size = Sizes[x % Sizes.Length],
            ProductName = $"Product {x}"
        }).ToList();
        base.OnInitialized();
    }

    public class SampleData
    {
        public int Id { get; set; }
        public string Size { get; set; }
        public string ProductName { get; set; }
    }

    public string[] Sizes = new string[] { "XS", "S", "M", "L", "XL", null };
}

if (index > 0)
{
   <TelerikButton ButtonType="ButtonType.Button" OnClick="@(() => step -= 1)">Previous</TelerikButton>
}
if (index < lastStep)
{
   <TelerikButton ButtonType="ButtonType.Button" OnClick="@(() => step += 1)" Primary="true">Next</TelerikButton>
}

@foreach (var chapters in leaseContractDefaultTemplateChapters.OrderBy(s => s.DisplayOrder))
                                        {
                                            <MudContainer Fixed="true">
                                                <MudPaper Height="auto" Width="auto">
                                                    <MudCard @ondblclick="@OnChapterCardDoubleClick">
                                                        <MudCardHeader>
                                                            <CardHeaderContent>
                                                                <MudText Class="d-flex" Typo="Typo.h6">@chapters.Name</MudText>
                                                            </CardHeaderContent>
                                                            <CardHeaderActions>                                                                
                                                                <MudIconButton OnClick="@(async () => await GetComputedTemplateChapter())" Icon="@Icons.Material.Outlined.Functions" />                                                                
                                                                <MudIconButton OnClick="@OnSaveChapterClick" Icon="@Icons.Material.Filled.Save" Color="Color.Success" />
                                                            </CardHeaderActions>
                                                        </MudCardHeader>
                                                        <MudCardContent>
                                                            @if (rteVisible)
                                                            {
                                                                <TelerikEditor @ref="chapterEditor" @bind-Value="@chapters.HtmlString"
                                                                               Tools="@Tools"
                                                                               Height="300px">
                                                                </TelerikEditor>
                                                            }
                                                            else
                                                            {
                                                                @((MarkupString)chapters.HtmlString)
                                                            }
                                                        </MudCardContent>
                                                    </MudCard>
                                                </MudPaper>                     
                                            </MudContainer>
                                        }