Windows forms combobox значение по умолчанию

Suppose you bound your combobox to a List<Person>

List<Person> pp = new List<Person>();
pp.Add(new Person() {id = 1, name="Steve"});
pp.Add(new Person() {id = 2, name="Mark"});
pp.Add(new Person() {id = 3, name="Charles"});

cbo1.DisplayMember = "name";
cbo1.ValueMember = "id";
cbo1.DataSource = pp;

At this point you cannot set the Text property as you like, but instead you need to add an item to your list before setting the datasource

pp.Insert(0, new Person() {id=-1, name="--SELECT--"});
cbo1.DisplayMember = "name";
cbo1.ValueMember = "id";
cbo1.DataSource = pp;
cbo1.SelectedIndex = 0;

Of course this means that you need to add a checking code when you try to use the info from the combobox

if(cbo1.SelectedValue != null && Convert.ToInt32(cbo1.SelectedValue) == -1)
    MessageBox.Show("Please select a person name");
else
    ...... 

The code is the same if you use a DataTable instead of a list. You need to add a fake row at the first position of the Rows collection of the datatable and set the initial index of the combobox to make things clear. The only thing you need to look at are the name of the datatable columns and which columns should contain a non null value before adding the row to the collection

In a table with three columns like ID, FirstName, LastName with ID,FirstName and LastName required you need to

DataRow row = datatable.NewRow();
row["ID"] = -1;
row["FirstName"] = "--Select--";    
row["LastName"] = "FakeAddress";
dataTable.Rows.InsertAt(row, 0);

Последнее обновление: 31.10.2015

Элемент ComboBox образует выпадающий список и совмещает функциональность компонентов ListBox и TextBox. Для хранения элементов списка в ComboBox также предназначено свойство
Items.

Подобным образом, как и с ListBox, мы можем в окне свойств на свойство Items и нам отобразится окно для добавления элементов ComboBox:

И как и с компонентом ListBox, здесь мы также можем программно управлять элементами.

Добавление элементов:

// добавляем один элемент
comboBox1.Items.Add("Парагвай");
// добавляем набор элементов
comboBox1.Items.AddRange(new string[] { "Уругвай", "Эквадор" });
// добавляем один элемент на определенную позицию
comboBox1.Items.Insert(1, "Боливия");

При добавлении с помощью методов Add / AddRange все новые элементы помещаются в конец списка. Однако если мы зададим у ComboBox свойство
Sorted равным true, тогда при добавлении будет автоматически производиться сортировка.

Удаление элементов:

// удаляем один элемент
comboBox1.Items.Remove("Аргентина");
// удаляем элемент по индексу
comboBox1.Items.RemoveAt(1);
// удаляем все элементы
comboBox1.Items.Clear();

Мы можем получить элемент по индексу и производить с ним разные действия. Например, изменить его:

comboBox1.Items[0] = "Парагвай";

Настройка оформления ComboBox

С помощью ряда свойств можно настроить стиль оформления компонента. Так, свойство DropDownWidth задает ширину выпадающего списка.
С помощью свойства DropDownHeight можно установить высоту выпадающего списка.

Еще одно свойство MaxDropDownItems позволяет задать число видимых элементов списка — от 1 до 100. По умолчанию это число равно 8.

Другое свойство DropDownStyle задает стиль ComboBox. Оно может принимать три возможных значения:

• Dropdown: используется по умолчанию. Мы можем открыть выпадающий список вариантов при вводе значения в текстовое поле или нажав на кнопку со стрелкой
  в правой части элемента, и нам отобразится собственно выпадающий список, в котором можно выбрать возможный вариант

• DropdownList: чтобы открыть выпадающий список, надо нажать на кнопку со стрелкой в правой стороне элемента

• Simple: ComboBox представляет простое текстовое поле, в котором для перехода между элементами мы можем использовать клавиши
  клавиатуры вверх/вниз

Событие SelectedIndexChanged

Наиболее важным событием для ComboBox также является событие SelectedIndexChanged, позволяющее отследить выбор элемента в списке:

public partial class Form1 : Form
{
    public Form1()
    {
        InitializeComponent();

        comboBox1.SelectedIndexChanged += comboBox1_SelectedIndexChanged;    
    }

    void comboBox1_SelectedIndexChanged(object sender, EventArgs e)
    {
        string selectedState = comboBox1.SelectedItem.ToString();
        MessageBox.Show(selectedState);
    }
}

Здесь также свойство SelectedItem будет ссылаться на выбранный элемент.

C# ComboBox Control

The ComboBox control provides combined functionality of a text box and a listbox in a single control. Only one list item is displayed at one time in a ComboBox and rest of the available items are loaded in a drop down list.

The following code snippet creates a ComboBox control object.

In the next step, you set properties of a ComboBox control.

The following code snippet sets location, width, height, background color, foreground color, Text, Name, and Font properties of a ComboBox.

Once the ComboBox control is ready with its properties, the next step is to add the ComboBox to a Form.

To do so, we use Form.Controls.Add method.

The following code snippet adds a ComboBox control to the current Form.

Name

Name property represents a unique name of a ComboBox control. It is used to access control in the code. The following code snippet sets and gets the name and text of a ComboBox control.

Location, Height, Width and Size

The Location property is a type of Point that specifies the starting position of the ComboBox on a Form. You may also use Left and Top properties to specify the location of a control from the left top corner of the Form. The Size property specifies the size of the control. We can also use Width and Height property instead of Size property. The following code snippet sets Location, Width, and Height properties of a ComboBox control.

DropDownHeight and DropDownWidthYou can control the size of the dropdown area of a ComboBox. The DropDownHeight and DropDownWidth properties represent the height and width of the dropdown area in pixel respectively. If the DropDownWidth and DropDownHeight properties are less than the Width and Height values, they will not be applicable. If all the items do not fit in the size of the dropdown area, the scrollbars will appear as you can see from Figure 3.

The following code snippet sets the height and width of the dropdown area of a ComboBox.

Font

Font property represents font of text of a ComboBox control. If you click on the Font property in Properties window, you will see Font name, size and other font options.

Background and Foreground

BackColor and ForeColor properties are used to set background color and foreground color of a ComboBox respectively. If you click on these properties in Properties window, the Color Dialog pops up.

Alternatively, you can set background color and foreground color at run-time. The following code snippet sets BackColor and ForeColor properties.

The new ComboBox with background and foreground looks like Figure 4.

When you click on the Collections, the String Collection Editor window will pop up where you can type strings. Each line added to this collection will become a ComboBox item. I add four items as you can see from Figure 6.

The ComboBox looks like Figure 7.

Alternatively, you can add same items at run-time by using the following code snippet.

Getting All Items

To get all items of a control, we use Items property that is a collection of items.

The following code snippet loops through all items and adds item contents to a StringBuilder and displays in a MessageBox.

Selected Text and Item

Text property is used to set and get text of a ComboBox. The following code snippet sets and gets current text of a ComboBox.

We can also get text associated with currently selected item by using Items property.

Why the value of ComboBox.SelectedText is Empty?

SelectedText property gets and sets the selected text in a ComboBox only when a ComboBox has focus on it. If the focus moves away from a ComboBox, the value of SelectedText will be an empty string. To get current text in a ComboBox when it does not have focus, use Text property.

DataSource

A ComboBox can be used to bind to a collection of items. DataSource property is used to get and set a data source to a ComboBox. The data source can be a collection or object that implements IList interface such as an array, a collection, or a DataSet.

The following code snippet binds an enumeration converted to an array to a ComboBox.

DropDownStyle

DropDownStyle property is used to gets and sets the style of a ComboBox. It is a type of ComboBoxStyle enumeration.

The ComboBoxStyle enumeration has following three values.

The following code snippet sets the DropDownStyle property of a ComboBox to DropDownList. 

DroppedDown

If set true, the dropped down portion of the ComboBox is displayed. By default, this value is false.

Sorting Items

The Sorted property set to true, the ComboBox items are sorted. The following code snippet sorts the ComboBox items.

Find Items

FindString method is used to find a string or substring in a ComboBox. The following code snippet finds a string in a ComboBox and selects it if found.

The following code snippet defines and implements these events and their respective event handlers.

Summary

In this article, we discussed discuss how to create a ComboBox control in Windows Forms at design-time as well as run-time. After that, we discussed how to use its various properties and methods to build real world applications.