public CriteriaOperator GetFilterCriteria()

@using DevExpress.Data.Filtering
<DxToolbar ItemClick="@OnItemClick">
    <Items>
        <DxToolbarItem Name="_setFilterCriteria" Text="Set Filter Criteria" Tooltip="Set Filter Criteria" />
        <DxToolbarItem Name="_clearFilterCriteria" Text="Clear Filter Criteria" Tooltip="Clear Filter Criteria" />
    </Items>
</DxToolbar>


<DxListBox @ref="@_listBox" TData=Person TValue=Person Data="Staff.DataSource"
            ShowCheckboxes="true"
            SelectionMode="@ListBoxSelectionMode.Multiple">
    <Columns>
        <DxListEditorColumn FieldName="FirstName"></DxListEditorColumn>
        <DxListEditorColumn FieldName="LastName"></DxListEditorColumn>
        <DxListEditorColumn FieldName="Department"></DxListEditorColumn>
        <DxListEditorColumn FieldName="Salary"></DxListEditorColumn>
    </Columns>
</DxListBox>
<p class="cw-480 mt-3">
    Filter Criteria: <b>@_listBox?.GetFilterCriteria().ToString()</b>
</p>

@code {
    IListBox<Person, Person> _listBox;
    void OnItemClick(ToolbarItemClickEventArgs e) {
        switch(e.ItemName) {
            case "_setFilterCriteria":
                _listBox.SetFilterCriteria(new BinaryOperator(nameof(Person.Salary), 2000, BinaryOperatorType.Greater));
                break;
            case "_clearFilterCriteria":
                _listBox.ClearFilter();
                break;
        }
    }
}

namespace StaffData {
    public static class Staff {
        private static readonly Lazy<List<Person>> dataSource = new Lazy<List<Person>>(() =>
        {
            Random rnd = new Random();
            var dataSource = new List<Person>() {
                new Person() { Id = 0 , FirstName = "John John", LastName = "Heart", Department = Department.Electronics, Salary = GetRandomSalary(rnd) },
                new Person() { Id = 1 , FirstName = "Samantha", LastName = "Bright", Department = Department.Motors, Salary = GetRandomSalary(rnd) },
                new Person() { Id = 2 , FirstName = "Arthur", LastName = "Miller", Department = Department.Software, Salary = GetRandomSalary(rnd) },
                new Person() { Id = 3 , FirstName = "Robert", LastName = "Reagan", Department = Department.Electronics, Salary = GetRandomSalary(rnd) },
                new Person() { Id = 4 , FirstName = "Greta", LastName = "Sims", Department = Department.Motors, Salary = GetRandomSalary(rnd) },
                new Person() { Id = 5 , FirstName = "Brett", LastName = "Wade", Department = Department.Software, Salary = GetRandomSalary(rnd) },
                new Person() { Id = 6 , FirstName = "Sandra", LastName = "Johnson", Department = Department.Electronics, Salary = GetRandomSalary(rnd) },
                new Person() { Id = 7 , FirstName = "Edward", LastName = "+Holmes", Department = Department.Motors, Salary = GetRandomSalary(rnd) },
                new Person() { Id = 8 , FirstName = "Barbara", LastName = "Banks", Department = Department.Software, Salary = GetRandomSalary(rnd) },
                new Person() { Id = 9 , FirstName = "Kevin", LastName = "Carter", Department = Department.Electronics, Salary = GetRandomSalary(rnd) },
                new Person() { Id = 10, FirstName = "Cynthia", LastName = "Stanwick", Department = Department.Motors, Salary = GetRandomSalary(rnd) },
                new Person() { Id = 11, FirstName = "Sam", LastName = "Hill", Department = Department.Electronics, Salary = GetRandomSalary(rnd) },
            };
            return dataSource;
        });
        public static List<Person> DataSource { get { return dataSource.Value; } }
    }

    public class Person {
        public int Id { get; set; }
        public string FirstName { get; set; }
        public string LastName { get; set; }
        public Department Department { get; set; }
        public string Text => $"{FirstName} {LastName} ({Department} Dept.)";
        public decimal Salary { get; set; }
        public DateTime BirthDate { get; set; }
        public override bool Equals(object obj) {
            if (obj is Person typedObj) {
                return (this.Id == typedObj.Id) && (this.FirstName == typedObj.FirstName) && (this.LastName == typedObj.LastName)
                && (this.Department == typedObj.Department);
            }
            return base.Equals(obj);
        }
    }

    static decimal GetRandomSalary(Random rnd) {
        return rnd.Next(1000, 3000);
    }

    public enum Department { Motors, Electronics, Software }
}