Remove button border on tab c# winforms

Question

I have a button on my form that has flat style applied and uses a background image, I have removed all borders from the button, but when I tab onto the button from another control a black border appears around the button.

This can be seen in the image below. On the left is the button with black border on the right is a different button but shows how the cancel button should look.

Answer 1

You have to make a new button class using IButtonControl and change NotifyDefault to false:

base.NotifyDefault(false);

Answer 2

I do not get this border, if I set the BoderSize to 0 in the FlatAppearance section.

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Further investigation shows that this border appears only when the button is the default button. You can create your own button, which does never show this border like this

public class NoNotifyButton: System.Windows.Forms.Button
{
    public override void NotifyDefault(bool value)
    {
    }
}

Note: NotifyDefault remains intentionally empty.