Show checkbox for Context MenuStrip or Context Menu of a button

Don't see any of your code so I don't know how you create this menu. But in the most general terms, here is how you access the Checked property.

((ToolStripMenuItem)contextMenuStrip.Items[0]).Checked = true; //false;
((ToolStripMenuItem)contextMenuStrip.Items[1]).Checked = true; //false;
((ToolStripMenuItem)contextMenuStrip.Items[2]).Checked = true; //false;

You can assign them as either true or false. If you have named your ToolStripItems, then you can access them directly rather than going to the Items array.

contextMenuStrip.event1.Checked = true; //false;

As you can see, I am using a ContextMenuStrip.

Change the property CheckOnClick to True

In order for this to work visually, you need to ensure the "ShowCheckMargin" property is ON. Otherwise the ".Checked" property will silently do nothing.