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POuse runtime type (in c#) for logic flow
 

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  1. POuse runtime type (in c#) for logic flow
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    <p>This question is similar to (a subset of) <a href="https://stackoverflow.com/questions/447947/is-type-checking-ever-ok">this question</a></p> <p>In this case it is using runtime type to distinguish between a success and a failure returned results.</p> <p>Quite often I see following pattern:</p> <pre><code>public struct Result { public boolean IsSuccess { get;set;} public string ErrorMessage {get;set;} public int Value {get;set;} } ... Result result = someObject.SomeMethod(); if (result.IsSuccess) DoSomething(result.Value); else handleError(result.ErrorMessage); </code></pre> <p>I think that following is more natural, and expresses the intention more clear (in my opinion):</p> <pre><code>public abstract class Result { } public sealed class Failure : Result { public string ErrorMessage { get; set; } } public sealed class Success : Result { public int Value { get; set; } } ... Result result = someObject.SomeMethod(); if (result is Success) DoSomething((result as Success).Value); else if (result is Failure) handleError((result as Failure).ErrorMessage); </code></pre> <p>Also note that .Net (and many other languages) use this pattern in try-catch block with multiple catch clauses (where exception type selects a catch block).</p> <p><strong>EDIT:</strong> this pattern (i.e. relying on runtime type) is identical to discriminated unions of F#, the difference is that in F# it is native, and in C# it is emulated using constructs meant for different purposes.</p> <p><strong>EDIT:</strong> I think my main issue with first code is a code smell "partially initialized object". In 100% of cases only half of the object will be initialized. It also <em>almost</em> violates ISP, "almost" because once .IsSuccess is evaluated only part of the object will be used henceforth (if it is success only .Result is used, if it is an error - only error properties are used). Run-type checking solution does not have those issues.</p> <p><strong>So the question is:</strong> what are the issues with using this pattern? I am especially interested in issues from following perspectives: maintainability, readability, testability, conceptual purity, and OOP/OOD.</p>
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    1. COIf you care about intent then never use an "as" cast for something you're certain of. If result is success then ((Success)result).Value
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