Why overriden ToString() do not return what I want when item added to ComboBox?

Question

public partial class TestConrol : UserControl
{
    public TestConrol()
    {
        InitializeComponent();
    }

    public override string ToString()
    {
        return "asd";
    }
}

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

    private void Form1_Load(object sender, EventArgs e)
    {
        TestConrol tc1 = new TestConrol();
        comboBox1.Items.Add(tc1);

        TestConrol tc2 = new TestConrol();
        comboBox1.Items.Add(tc2);
    }
}

When form loaded, I see combobox has two items with empty names, instead of "asd" :/
But this work if I override ToString() in common class, not derived from anything:

public class TestClass
{
    public override string ToString()
    {
        return "bla bla bla";
    }
}

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

    private void Form1_Load(object sender, EventArgs e)
    {
        TestClass tcl = new TestClass();
        comboBox1.Items.Add(tcl);
    }
}

After that I see in combobox "bla bla bla"

Answer 1

I tried to understand the source code(!). This is not a simple call to ToString().

There's an internal class System.Windows.Forms.Formatter doing some stuff. It eventually creates a converter. This is roughly equivalent to saying:

var conv = System.ComponentModel.TypeDescriptor.GetConverter(tc1.GetType());

where tc1 is the TestContol from your question. Now, had we used the TestClass tcl which doesn't implement any interfaces, this would have given us a converter which would eventually call ToString().

But in this example we use tc1, and it is a System.ComponentModel.IComponent. Our conv therefore becomes a System.ComponentModel.ComponentConverter. It uses the Site of the IComponent. When we say:

string result = conv.ConvertTo(tc1, typeof(string));

and the Site is null, we get the empty string "" you saw in your combo box. Had there been a Site it would have used its Name instead.

To demonstrate that, put the following into your TestControl instance constructor:

public TestConrol()
{
    InitializeComponent();
    Site = new DummySite(); // note: Site is public, so you can also
                            // write to it from outside the class.
                            // It is also virtual, so you can override
                            // its getter and setter.
}

where DummySite is something like:

class DummySite : ISite
{
    public IComponent Component
    {
        get { throw new NotImplementedException(); }
    }

    public IContainer Container
    {
        get { throw new NotImplementedException(); }
    }

    public bool DesignMode
    {
        get { throw new NotImplementedException(); }
    }

    public string Name
    {
        get
        {
            return "asd";  // HERE'S YOUR TEXT
        }
        set
        {
            throw new NotImplementedException();
        }
    }

    public object GetService(Type serviceType)
    {
        return null;
    }
}