The command ICSharpCode.SharpDevelop.Commands.SaveFileAs won't save the Form.Designer.cs as well as the Form.cs, which is done by visual studio by default. It also results that the FormsDesigner report "ICSharpCode.FormsDesigner.FormsDesignerLoadException: Could not find InitializeComponent method in any part of the open class" after I save the "MainForm.cs" as "Form.cs".
This bug exists for quite a few years, through #develop 2.2 to #develop 4.3.