Simple and Complex Data Binding
What is DataBinding?
DataBinding is a powerful feature provided by the .NET framework that enables visual elements in a client to connect to a datasource such as DataSets, DataViews, Arrays etc. Some of the visual elements in the client can be TextBox, Datagrid etc. A two-way connection is established such that any changes made to the datasource are reflected immediately in the visual element and vice versa.
Below is a graphical description of the concept of databinding:
DataBinding Before .NET
In the earlier databinding models, the datasource that could be used was usually limited to a database. All DBMS systems provided their own API's to help in building GUI applications and quickly bind them to the data. Programmer did not have the flexibility to control the databinding process with the result that most developers avoided the use of databinding.
DataBinding with .NET
The .NET framework provides a very flexible and powerful approach to databinding and allows the programmer to have a fine control over the steps involved in the whole process. One of the biggest improvements with .Net has been the introduction of databinding to web pages through the use of .Net server-side web controls. Hence, building data driven web applications has been greatly simplified. Please note that this article only deals with data binding in .NET windows forms.
Advantages of DataBinding
1. Databinding in .NET can be used to write data driven applications quickly. .NET data binding allows you to write less code with fast execution but still get the work done in the best way.
2. .NET automatically writes a lot of databinding code for you in the background (you can see it in "Windows Generated Code" section), so the developer does not have to spend time writing code for basic databinding, but still has the flexibility of modifying any code that he would like to. We get the benefits of bound as well as unbound approach.
3. Control over the Databinding process by using events. This is discussed in more detail later in the article.
Disadvantages of DataBinding
1. More optimized code can be written by using the unbound or traditional methods.
2. Complete flexibility can only be achieved by using the unbound approach.
For databinding to take place data provider and a data consumer should exist so that a synchronized link is established between the two. Data providers contain the data and the data consumers use the data exposed by the data providers and display them.
.NET has expanded the scope of possible data providers. In .NET any class or component that implements the IList interface is a valid DataSource. If a component implements the IList interface then it is transformed into an index based collection.
Some of the classes that support the IList interface in the NET framework are given below. Please note that any class that implements the IList interface is a valid data provider.
Please note that IList interface only allows you to bind at run time. If you want to support DataBinding at design time you will have to implement the IComponent interface as well. Also note that you cannot bind to DataReaders in windows forms (you can in web forms).
The .NET framework supports simple and complex DataBinding. Simple databinding is supported by controls like TextBoxes. In Simple databinding, only one data value can be displayed by the control at a time. In complex databinding, which is supported by controls like the DataGrid, more than one data value from the DataSource can be displayed.
Dataflow during DataBinding
A good understanding of the dataflow from the control to the datasource is very important. The diagram below gives an overview of the dataflow and the objects involved.
In .NET, controls can have many properties that can be bound to a DataSource. Each databound property has an associated Binding object. Since a control can have many Binding objects, the control has a collection (instance of ControlBindingsCollection class) of all the Binding objects. Also remember that different properties of the same control can be bound to different datasource's.
Each Binding object talks to a CurrencyManager or a PropertyManager. CurrencyManager and PropertyManager classes merit a little explanation, as they are important. CurrencyManager and PropertyManager are derived from the base class BindingManagerBase. The purpose of BindingManagerBase class is to maintain the concurrency between the datasource and the control. Of the two classes, the CurrencyManager is used when the datasource implements the IList Interface. Examples of such datasources are DataView, DataSet, ArrayList etc. The CurrencyManager can be used for simple as well as complex databinding. However, the PropertyManager is used when the datasource is an instance of a user-defined class. The Control's property is bound to the property exposed by this object. PropertyManager can only be used for simple databinding.
As a rule of thumb if you want your class to be a datasource, you should use CurrencyManager when your class is a data container. However, if you are interested in binding a control to properties exposed by your own class, then using a PropertyManager is easier, since you do not have to implement the IList Interface.
Since a form can contain many controls each binding to a different datasource, a class is needed to manage the CurrencyManager and PropertyManager objects. Therefore, each windows form in .NET has a default BindingContext object associated with it. But, you can always create more BindingContext objects on the form. The BindingContext object is a collection of CurrencyManager and PropertyManager objects.