Add Item to Binded Combo Box

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Problem Scenario

The Windows Forms ComboBox control is used to display data in a drop-down combo box. When the data source property of the combo box is binded with data, then you will not be able to add any item through the Items.Add() function.

So, let's go through a demo by the help of which you can understand the problem scenario and its solution. Suppose we have a class named Person and we will create an IList to store this Person list.

Person p = null;
IList list = new ArrayList();

p = new Person();
p.PersonID = 1;
p.PersonName = "Ali";
list.Add(p);

p = new Person();
p.PersonID = 2;
p.PersonName = "Raza";
list.Add(p);

p = new Person();
p.PersonID = 3;
p.PersonName = "Shaikh";
list.Add(p);

We have added three persons information in the list, now we want to bind the combo box with this data.

this.comboBox1.DataSource = list;
this.comboBox1.DisplayMember = "PersonName";
this.comboBox1.ValueMember = "PersonID";

Now, the data is binded with the Combo Box, and we will see the following kind of look on the interface.

Now, what's the problem here? In most of the professional applications, there is a select option at the top of each combo box, so that the user can choose an option if he desires, or else he can leave the combo box without selecting anything. So, as a common practice, one will write this line of code:

this.comboBox1.Items.Add("< Select Person >");
//OR
this.comboBox1.Items.Insert(0, "< Select Person >");

Which ends up giving this error:

Solution

This thing can be achieved by using the following two classes of System.Reflection:

• Activator: Contains methods to create types of objects locally or remotely, or obtain references to existing remote objects
• PropertyInfo: Discovers the attributes of a property and provides access to property metadata.

Now, simply call this function to add the required item on the list.

private void AddItem(IList list, Type type, string valueMember, 
	string displayMember, string displayText)
{
            //Creates an instance of the specified type 
            //using the constructor that best matches the specified parameters.
            Object obj = Activator.CreateInstance(type);

            // Gets the Display Property Information
            PropertyInfo displayProperty = type.GetProperty(displayMember);

            // Sets the required text into the display property
            displayProperty.SetValue(obj, displayText, null);

            // Gets the Value Property Information
            PropertyInfo valueProperty = type.GetProperty(valueMember);

            // Sets the required value into the value property
            valueProperty.SetValue(obj, -1, null);

            // Insert the new object on the list
            list.Insert(0, obj);
}

Then, add the following line of code just before the data binding with the combo box:

AddItem(list, typeof(Person), "PersonID", "PersonName", "< Select Option >");

In the end, the interface looks like this, looking like a professional effort to deal with the data inside the combo box.

History

• 7^th October, 2006: Initial post

This article, along with any associated source code and files, is licensed under The Code Project Open License (CPOL)