Overriding a read/write property to make it readonly

Question

I'm writing a custom control in C# based on TextBox. I want the Text property of my control to be read-only (since only certain values are allowed in the textbox and I don't want the calling program to be able to put invalid values in there.)

I don't see how to do this. If I use public override Text {} the compiler inserts default get and set accessors. If I comment out the set accessor, the code compiles and runs, which I assume means the base accessor is being used. If I put readonly in the definition of my property, the compiler throws an error.

Any help would be greatly appreciated.

Answer 1

The issue here is that by trying to make a read/write property read-only, you're violating the contract of your base class. Your base class explicitly states that this property can be both retrieved and modified. Derived classes cannot break the contracts of their base classes, otherwise polymorphism would fail. Remember that if B derives from A, anywhere an A object is accepted, B must perform.

If that should not be the case for the derived class, then I would first question the design -- should this truly be derived from the base class? Is this truly an "is-a" relationship?

Assuming that we passed that test and truly should be derived, one approach -- which I don't personally like, but will share anyway -- may be to simply have the setter throw an exception. I personally don't like this, it still violates the contract, and feels like an excessive amount of cognitive friction in using your derived class, but if you truly have that solid a reason to do so ... well ... OK, I guess. But be sure you know WHY you're hacking around this.

But go back to the first point: Is this truly a scenario where a derived class is the right answer, from a design standpoint? Without seeing your code, my gut reaction is a no.

EDIT

Somehow in my initial read of this, I missed the fact that we're talking about a UI control here. I stand by what I wrote above this edit, in the general property overriding case. However, in the case of a UI control, where the polymorphism opportunities (I hope) will be somewhat limited, and where the need may well justify this design, then my recommendation would be to have the Text property setter throw an exception.

A better design, in my opinion, would be composition. Make a user control, add the TextBox to the user control design surface with Dock=Fill, and then expose a new Text property which is readonly. You can also now expose only those properties you really want to.