c# - How to get WinForms custom control's default value to be respected when first dropped on a form

Question

I have a class library with a custom control in it:

using System.ComponentModel;
using System.Windows.Forms;

namespace ClassLibrary1
{
    public sealed class CustomLabel : Label
    {
        [DefaultValue(false), Browsable(false), EditorBrowsable(EditorBrowsableState.Never), DesignerSerializationVisibility(DesignerSerializationVisibility.Hidden)]
        public override bool AutoSize
        {
            get => base.AutoSize;
            set => base.AutoSize = value;
        }

        public CustomLabel()
        {
            AutoSize = false;
        }
    }
}

Notice that AutoSize is set to false in both the constructor and the designer attribute on the overridden method.

I have a winforms project where I want to use the control. I drag/drop it from the toolbox, but it doesn't have AutoSize set to false:

If I save and close the form and then re-open it, now it's set correctly:

How can I make it respect the property value when first dropped on the form?

Answer 1

The default values which you assign in constructor are respected in general. But for some cases the default values will be changed using designer, for example by the CreateComponentsCore method of ToolboxItem of the control.

The default value for AutoSize property for Label is false and you even don't need to override it or set it in constructor. But an AutoSizeToolboxItem has been assigned to Label which sets AutoSize to true when you drop an instance of Label on designer. To remove this behavior, it's enough to assign a new ToolboxItemto your control:

using System.ComponentModel;
using System.Drawing.Design;
using System.Windows.Forms;

namespace ClassLibrary1
{
    [ToolboxItem(typeof(ToolboxItem))]
    public sealed class CustomLabel : Label
    {
    }
}

Note 1: Just for your information, the ToolboxItem has a CreateComponentsCore method which you can use it to to some initialization tasks when dropping control on design surface.

Note 2 I should also add, the CreateComponentCore method will just run when you drop the component from toolbox to design surface. It describes why after dropping it on form, it's auto-size, because it's set by CreateComponentCore after your constructor. But after you open the form again, this time, just your constructor will run and set the property to false.