DxComboBox<TData, TValue> Class

Namespace: DevExpress.Blazor

Assembly: DevExpress.Blazor.v21.1.dll

Declaration

public class DxComboBox<TData, TValue> :
    DxDropDownBase<TData, ComboBoxJSInteropProxy>,
    IDataSourceSettings<TData>,
    IServiceProviderAccessor,
    IRequireSelfCascading,
    IDropDownWidthModeAccessor,
    ITextInputEditor<TData>,
    IInputBase,
    IComboBoxJSInteropProxyServer,
    IJSCallback

Type Parameters

Remarks

The DevExpress ComboBox for Blazor (<DxComboBox>) component displays a drop-down window with a list of items. Users can click an item in the drop-down list or use the Up arrow, Down arrow, and Enter keys to navigate between items and select them. When a user presses and holds an arrow key, the editor’s window continuously navigates between items.

Run Demo Watch Video

Add a ComboBox to a Project

Follow the steps below to add the ComboBox component to an application:

1. Use a DevExpress Project Template to create a new Blazor Server or Blazor WebAssembly application. If you use a Microsoft project template or already have a Blazor project, configure your project to incorporate DevExpress Blazor components.
2. Add the <DxComboBox>…</DxComboBox> markup to a Razor page.
3. Bind the component to data.
4. Configure the component (see the sections below).

Bind to Data

Use the Data property to bind the ComboBox to a strongly typed collection or enumeration. Initialize this object in the OnInitialized lifecycle method or before this method is invoked. Use the DataAsync property instead of the Data property if a strongly typed collection is loaded asynchronously (for instance, from an HTTP request).

Use the Value property to specify the component’s selected value or item. You can use the @bind attribute to bind the Value property to a data field. For more information, refer to the following help topic: Two-Way Data Binding.

<DxComboBox Data="@Cities" @bind-Value="@Value" ></DxComboBox>

@code {
  IEnumerable<string> Cities = new List<string>() {
    "London",
    "Berlin",
    "Paris",
  };

  string Value { get; set; }
}

The ComboBox can detect IQueryable<T> collections and use benefits of the corresponding LINQ query providers (such as Entity Framework).

If you bind the ComboBox to a data collection that stores custom objects (IEnumerable<CustomType>), override the object’s Equals method and specify the TextFieldName property. This property specifies the custom object’s field name that returns string values to be shown within the ComboBox’s drop-down window.

@using BlazorApp.Data

<DxComboBox Data="@Staff.DataSource"
            TextFieldName="@nameof(Person.Text)"
            @bind-Value="@Value" >
</DxComboBox>

@code {
  Person Value { get; set; } = Staff.DataSource[0];
}

namespace BlazorApp.Data {
    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 enum Department { Motors, Electronics, Software }

    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);
    }
}

using System;
using System.Collections.Generic;

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

Otherwise, the ComboBox items are populated with CustomType.ToString() values.

Run Demo: ComboBox - Overview

If your data is stored on a remote service and is loaded through a Web API, assign the data type to the component’s T parameter and use the CustomData property to implement data load logic.

Items

The <DxComboBox> drop-down window displays a list of predefined items. Set the AllowUserInput property to true to allow users to type values into the ComboBox’s edit box.

<DxComboBox Data="@Cities"
            AllowUserInput="true"
            NullText="Select City ..."
            @bind-Value="@Value"
            @bind-Text="@Text">
</DxComboBox>

@code {
  IEnumerable<string> Cities = new List<string>() {
    "London",
    "Berlin",
    "Paris",
  };

  string Text { get; set; } = "New York";
  string Value { get; set; }
}

Use the Text property to specify an editor value in code. To respond to an editor’s text change, handle the TextChanged event.

The table below lists API members related to the ComboBox item collection:

Clear Button and Placeholder

Set the ClearButtonDisplayMode property to Auto to show the Clear button when the editor has a non-null value. Users can click this button to clear the editor’s value (set it to null).

Use the NullText property to display the prompt text (placeholder) in the editor when its value is null.

<DxComboBox NullText="Select a country..."
            Data="@CountryData.Countries"
            @bind-Value="@CurrentCountry"
            TextFieldName="@nameof(Country.CountryName)"
            ClearButtonDisplayMode="DataEditorClearButtonDisplayMode.Auto">
</DxComboBox>

@code {
    Country CurrentCountry { get; set; } = CountryData.Countries[1];
}

public class Country {
    public int Id { get; set; }
    public string CountryName { get; set; }
}

public static class CountryData {
    private static readonly Lazy<List<Country>> countries = new Lazy<List<Country>>(() => {
        return new List<Country>()
        {
            new Country() { Id = 0, CountryName = "USA" },
            new Country() { Id = 1, CountryName = "Germany" },
            new Country() { Id = 2, CountryName = "Japan" }
        };
    });
    public static List<Country> Countries { get { return countries.Value; } }
}

Run Demo: ComboBox - Clear Button and Placeholder

Cascading Comboboxes

The <DxComboBox> allows you to create cascade lists - populate one ComboBox with items based on the user selection from another ComboBox. Use the Value property to access the selected item. To respond to the selected item change, handle the ValueChanged event.

<DxComboBox Data="@Countries"
            TextFieldName="@nameof(Country.CountryName)"
            Value="@CurrentCountry"
            ValueChanged="@((Country country) => SelectedCountryChanged(country))">
</DxComboBox>

<DxComboBox Data="@CurrentCountryCities"
            TextFieldName="@nameof(City.CityName)"
            @bind-Value="@CurrentCity">
</DxComboBox>

@code {
    List<Country> Countries { get; set; } = CountryCity.Countries;
    List<City> CurrentCountryCities { get; set; } = new List<City>();
    Country CurrentCountry { get; set; } = CountryCity.Countries[1];
    City CurrentCity { get; set; } = CountryCity.Cities[4];

    void SelectedCountryChanged(Country country) {
        CurrentCountry = country;
        CurrentCountryCities = CountryCity.Cities.FindAll(city => city.CountryId == CurrentCountry.Id);
        CurrentCity = CurrentCountryCities[0];
    }

    protected override void OnInitialized() {
        base.OnInitialized();
        SelectedCountryChanged(CurrentCountry);
    }
}

Run Demo: ComboBox - Cascading Lists

Watch Video: ComboBox - Multiple Columns and Cascade Lists

Filter Data

<DxComboBox> allows you to dynamically filter list items based on the text typed into the editor. Use the FilteringMode property to enable filtering and specify the filter mode (Contains or StartsWith).

<DxComboBox FilteringMode="DataGridFilteringMode.Contains" Data="@Data" @bind-Value="@Value"></DxComboBox>

@code {
    IEnumerable<Country> Data;
    Country Value { get; set; }
}

Run Demo: ComboBox - Incremental Filtering

Multiple Columns

The ComboBox can display data across multiple columns. To create columns, use DxListEditorColumn objects that include the following options for column customization:

• FieldName - Specifies the data source field that populates column items.
• Caption - Specifies the column header.
• Visible - Specifies the column visibility.
• VisibleIndex - Specifies the column display order.
• Width - Specifies the column width.

To format an edit box’s value, use the EditFormat property.

<DxComboBox Data="@Staff.DataSource"
            @bind-Value="@SelectedPerson"
            EditFormat ="{1} {2}">
    <DxListEditorColumn FieldName="Id" Width="50px" />
    <DxListEditorColumn FieldName="FirstName" Caption="Name"/>
    <DxListEditorColumn FieldName="LastName" Caption="Surname"/>
</DxComboBox>

@code {
    Person SelectedPerson { get; set; } = Staff.DataSource[0];
}

Run Demo: ComboBox – Multiple Columns

Watch Video: ComboBox - Multiple Columns and Cascade Lists

Appearance Customization

Use the SizeMode property to specify the size of a ComboBox. The code below applies different sizes to ComboBox components.

<DxComboBox Data="@Cities"
            NullText="Select City ..."
            @bind-Value="@Value"
            SizeMode="SizeMode.Small">
</DxComboBox>

<DxComboBox Data="@Cities"
            NullText="Select City ..."
            @bind-Value="@Value"
            SizeMode="SizeMode.Medium">
</DxComboBox>

<DxComboBox Data="@Cities"
            NullText="Select City ..."
            @bind-Value="@Value"
            SizeMode="SizeMode.Large">
</DxComboBox>

@code {
    IEnumerable<string> Cities = new List<string>() {
        "London",
        "Berlin",
        "Paris",
    };

    string Value { get; set; }
}

To customize ComboBox input, use the InputCssClass property. The following code snippet applies a custom style to input borders:

<style>
    .my-style {
        border: 2px dotted orange;
    }
</style>

<DxComboBox Data="@Cities"
            NullText="Select City ..."
            @bind-Value="@Value"
            InputCssClass="my-style">
</DxComboBox>

@code {
    IEnumerable<string> Cities = new List<string> () {
        "London",
        "Berlin",
        "Paris",
    };

    string Value { get; set; }
}

For more information, refer to the following help topics:

• Size Modes
• CSS Classes

Virtual Scrolling

When virtual scrolling is activated (ListRenderMode is set to Virtual), the ComboBox loads data on demand when a user scrolls its items.

<DxComboBox ListRenderMode="ListRenderMode.Virtual"
            Data="@Strings"
            @bind-Value="@Value" >
</DxComboBox>

Run Demo: ComboBox - Virtual Scrolling

Input Validation

You can add a standalone ComboBox or Form Layout component to Blazor’s standard EditForm. This form validates user input based on data annotation attributes defined in a model and indicates errors. To specify which ComboBox property takes part in input validation (Text or Value), use the ValidateBy property. If you handle the ValueChanged event and cannot use two-way binding, specify the ValueExpression property to identify the value passed to the event handler.

<EditForm Model="@starship" Context="EditFormContext">
    <DataAnnotationsValidator />
    <DxFormLayout >
        <DxFormLayoutItem Caption="Primary Classification:" ColSpanMd="6" >
            <Template >
                <DxComboBox
                            NullText="Select classification ..."
                            ClearButtonDisplayMode="DataEditorClearButtonDisplayMode.Auto"
                            @bind-Value="@starship.Classification"
                            Data="@(new List<string>() { "Defense", "Exploration", "Diplomacy" })"
                            ValidateBy="ComboBoxValidateBy.Value"
                />
            </Template >
        </DxFormLayoutItem >
        @*...*@
    </DxFormLayout>
</EditForm>

@code {
    private Starship starship=new Starship();
}

using System.ComponentModel.DataAnnotations;

public class Starship
{
    [Required]
    public string Classification { get; set; }
    // ...
}

For more information, refer to the following help topic: Validate Input.

Run Demo: Form Validation

Read-Only State

<DxComboBox> supports a read-only state. Set the ReadOnly property to true to activate this option.

<DxComboBox ReadOnly="true" 
            Data="@Strings" 
            @bind-Value="@Value" > 
</DxComboBox>

Run Demo: ComboBox - Read-Only and Disabled Modes

Drop-Down Window Width

Use the DropDownWidthMode property to specify the width of the drop-down list. The following values are available:

• ContentOrEditorWidth (Default) - The list displays item text completely. The minimum list width matches the editor.

  

• ContentWidth - The list width is equal to the width of the longest list item.

  

• EditorWidth - The list width matches the editor. List items are cut if they do not fit.

  

The following code sets the width of the drop-down list to the editor width.

@using BlazorApp.Data

<DxComboBox Data="@Staff.DataSource"
            @bind-Value="@Value"
            TextFieldName="@nameof(Person.Text)"
            DropDownWidthMode="DropDownWidthMode.EditorWidth">
</DxComboBox>

@code {
  Person Value { get; set; } = Staff.DataSource[0];
}

namespace BlazorApp.Data {
    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 enum Department { Motors, Electronics, Software }

    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);
    }
}

using System;
using System.Collections.Generic;

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

Run Demo: ComboBox – Drop-Down List Width

Drop-Down Window Direction

Use the DropDownDirection property to specify the direction in which the ComboBox’s drop-down window is displayed relative to the input element. The default value is Down. The following code changes the direction to Up:

<DxComboBox Data="@Cities"
            @bind-Value="@Value"
            NullText="Select City ..."
            DropDownDirection="DropDownDirection.Up">
</DxComboBox>

@code {
    IEnumerable<string> Cities = new List<string>() {
        "London",
        "Berlin",
        "Paris",
    };

    string Value { get; set; }
}

HTML Attributes and Events

You can use HTML attributes and events to configure the ComboBox.

<DxComboBox Data="@Strings" 
            @bind-Value="@Value" 
            maxlength="10" 
            @onclick="MyFunction">
</DxComboBox>

@code {
    void MyFunction(){
        //...
    }
}