C# Tag in Windows Forms

Tag. The Tag property stores an object reference.

Windows Forms programs can use object models of arbitrary complexity. But the Tag property is a simple way to link a certain object to a certain control. It is useful in certain situations.

Example. To start, this program shows the use of the Tag property inside the object model of the Windows Forms program. The Form1 constructor, which instantiates the control, assigns the Tag property to a new ExampleTag—a custom type.

Then: This object reference can be accessed at any time and in any method later in the event loop of the Windows program.

C# program that uses Tag property

using System;
using System.Windows.Forms;

namespace WindowsFormsApplication1
{
    public partial class Form1 : Form
    {
	public Form1()
	{
	    InitializeComponent();
	    // Set tag on constructor.
	    this.Tag = new ExampleTag(1, 2);
	}

	private void Form1_Load(object sender, EventArgs e)
	{
	    // When form loads, render tag to the title bar.
	    this.Text = this.Tag.ToString();
	}
    }

    class ExampleTag
    {
	public int _a;
	public int _b;
	public ExampleTag(int a, int b)
	{
	    // Store the fields.
	    this._a = a;
	    this._b = b;
	}
	public override string ToString()
	{
	    // Write the fields to a string.
	    return string.Format("Tag a = {0}, b = {1}", this._a, this._b);
	}
    }
}

Output

The window is displayed with the title bar reading the tag ToString output.

In the Form1 constructor, the ExampleTag constructor is called. It returns an ExampleTag object reference, which is copied to the Tag reference field. The ExampleTag remains in memory. It is pointed to by the Tag property's backing store.

Next: In the Form1_Load event handler, we see that you can access the Tag property on the enclosing control.

And: This is the same Tag that we previously set. We could also modify the ExampleTag object at this point.

The Form1_Load event accesses the Tag reference as the base type object. However, when you invoke the ToString method upon a base class, the most derived method is actually called, typically through a matrix data structure.

ToString Method

Thus: You can see that the title bar of the program is equal to the result of the ToString method.

When planning a program, the object model is an important part of the program's architecture. If you use the Tag property too much, you will conflate the user interface specific information in the program with the data model.

Caution: This makes portability of your program to new interfaces more difficult.

Summary. We explored an example usage of the Tag property in the Windows Forms widget layout system. The Tag property essentially provides a user-defined field that can store any form of object in a persistent way.

And: It provides a convenient hook to your data. But it conflates user interface concepts and more abstract object models.

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