Building the Right Environment to Support AI, Machine Learning and Deep Learning
Recently, I ran across an issue where I needed to control the background color of the selected items in a Windows Forms ListBox control. Imagine my surprise when I discovered that not only was such a capability not present in the properties, but that the authors of the control seemed to go out of their way to make it difficult to do so. It may be that they had valid reasons for doing it this way, but I fail to see them. That being said, this is the solution I found. It is simple and straightforward, but took a little time to track it down.
I realized that I had to get involved in the item drawing process, so the first thing was to set the control to Owner drawn and write the event handler for the DrawItem event. I won't go into the details of writing a DrawItem event handler because that is topic is amply covered by others, so I'll simply show the lines that need to be placed at the start of the DrawItem method. My first thought was that I could simply test to see whetherf it was a selected item and instruct the DrawBackground method to use my BackColor if true.
Well, problem number one! The DrawBackground() method of the DrawItemEventArgs takes no parameters and there are no overrides. Hmmm, this means that it is using the BackColor property of the DrawItemEventArgs object.
Okay, let me change that... Whoops, no dice. They made it readonly for no apparent reason. I'll step around that issue by creating a new DrawItemEventArgs using the original's properties and changing the BackColor in the constructor. So far, so good...
if ((e.State & DrawItemState.Selected) == DrawItemState.Selected) e = new DrawItemEventArgs(e.Graphics, e.Font, e.Bounds, e.Index, e.State, e.ForeColor, Color.Red);
Nope, it's still drawing using the 'Highlight' named color. Apparently, the DrawBackground method is ignoring the BackColor property when it's a selected item, even though the control does set the BackColor to 'HighLight' for selected item(s)...
Okay, I verified that if it's not a selected item, the above code changes the BackColor as hoped, so now I need to fool the DrawBackground() method so it doesn't see an item as selected.
if ((e.State & DrawItemState.Selected) == DrawItemState.Selected) e = new DrawItemEventArgs(e.Graphics, e.Font, e.Bounds, e.Index, e.State ^ DrawItemState.Selected, e.ForeColor, Color.Red);
By XORing(^) the current State bitmask with the DrawItemState.Selected flag, I have fooled the DrawBackground() method into thinking that it is a normal item and it uses the Backcolor I specified.
I hope this helps.