Getting the “value” property of a control

Question

I have a method that has a Control parameter. I want to get the value of the control. So if it is a TextBox get the value of the Text property; if it is a NumericUpDown get the value of the Value property and so on.

The problem is that I cannot write something like this:

Method(Control control)
{
    control.Text;
}

or

Method(Control control)
{
    control.Value;
}

Because there is no guarantee that the control has one of these properties, and what is its name if it does have it. Is there a way to do something like that?

Answer 1

There isn't such common Value property in Control class.

You should use some if/else or switch/case or a dictionary approach to get the value from the control. Because you know what property you need. The control just provides properties.

For example for a ComboBox , what is the value? Is it SelectedItem , SelectedIndex , SelectedValue , Text ? It's usage/opinion based.

The nearest thing to what you are looking for, is relying on DefaultProperty attribute of controls to get the value from that property using relfection. For example, having this method:

public object GetDefaultPropertyValue(Control c)
{
    var defaultPropertyAttribute = c.GetType().GetCustomAttributes(true)
        .OfType<DefaultPropertyAttribute>().FirstOrDefault();
    var defaultProperty = defaultPropertyAttribute.Name;
    return c.GetType().GetProperty(defaultProperty).GetValue(c);
}

You can get values this way:

var controls = new List<Control> {
    new Button() { Text = "button1" },
    new NumericUpDown() { Value = 5 },
    new TextBox() { Text = "some text" },
    new CheckBox() { Checked = true }
};

var values = controls.Select(x => GetDefaultPropertyValue(x)).ToList();