Methods to use constructor injection in user controls?

Question

Are there any ways to enforce or allow constructor required dependencies for a user control without breaking the designer? What work arounds exist?

a default constructor with EditorBrowsable.Never or property injection, but I prefer constructor injection.

other work-arounds or solutions?

Answer 1

You can overload constructors. The designer is going to require the default constructor at design time so be sure to supply that one as well.

    public UserControl1() {
        InitializeComponent();
    }
    public UserControl1(Foo arg) : this() {
        // Do something with arg
        //...
    }

Of course, the client code has to create that user control itself. Favor properties to keep the user control useful in the designer. Throw an exception in OnLoad() if DesignMode is false and you're not happy about the way the client code used your control.

Answer 2

Design user controls so that they don't use dependencies.

Look at the controls (ASP.NET, Windows Forms, WPF, etc.) supplied by Microsoft. None of them use dependencies. Instead, you assign data to them - often by way of writable properties.

This is a more SOLID design because a control's single responsibility should be to render data. Thus, if you also give it the responsibility of retrieving or formatting data, you are breaking the Single Responsibility Principle.

When you design controls like that, a default constructor becomes natural.