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Asked: 2026-05-10T15:16:43+00:00

In a recent question on stubbing, many answers suggested C# interfaces or delegates for

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In a recent question on stubbing, many answers suggested C# interfaces or delegates for implementing stubs, but one answer suggested using conditional compilation, retaining static binding in the production code. This answer was modded -2 at the time of reading, so at least 2 people really thought this was a wrong answer. Perhaps misuse of DEBUG was the reason, or perhaps use of fixed value instead of more extensive validation. But I can’t help wondering:

Is the use of conditional compilation an inappropriate technique for implementing unit test stubs? Sometimes? Always?

Thanks.

Edit-add: I’d like to add an example as a though experiment:

class Foo {     public Foo() { .. }     private DateTime Now {        get { #if UNITTEST_Foo         return Stub_DateTime.Now; #else         return DateTime.Now; #endif       }     }     // .. rest of Foo members }

comparing to

interface IDateTimeStrategy {      DateTime Now { get; } } class ProductionDateTimeStrategy : IDateTimeStrategy {   public DateTime Now { get { return DateTime.Now; } } } class Foo {     public Foo() : Foo(new ProductionDateTimeStrategy()) {}     public Foo(IDateTimeStrategy s) { datetimeStrategy = s; .. }     private IDateTime_Strategy datetimeStrategy;     private DateTime Now { get { return datetimeStrategy.Now; } } }

Which allows the outgoing dependency on ‘DateTime.Now’ to be stubbed through a C# interface. However, we’ve now added a dynamic dispatch call where static would suffice, the object is larger even in the production version, and we’ve added a new failure path for Foo’s constructor (allocation can fail).

Am I worrying about nothing here? Thanks for the feedback so far!

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1 Answer

  1. Try to keep production code separate from test code. Maintain different folder hierarchies.. different solutions/projects.

    Unless.. you’re in the world of legacy C++ Code. Here anything goes.. if conditional blocks help you get some of the code testable and you see a benefit.. By all means do it. But try to not let it get messier than the initial state. Clearly comment and demarcate conditional blocks. Proceed with caution. It is a valid technique for getting legacy code under a test harness.

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