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POPattern for Creating a Simple and Efficient Value type
 

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  1. POPattern for Creating a Simple and Efficient Value type
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    <p><strong>Motivation:</strong></p> <p>In reading Mark Seemann’s blog on <a href="http://blog.ploeh.dk/2011/05/26/CodeSmellAutomaticProperty.aspx" rel="noreferrer">Code Smell: Automatic Property</a> he says near the end:</p> <blockquote> <p>The bottom line is that automatic properties are rarely appropriate. In fact, they are only appropriate when the type of the property is a value type and all conceivable values are allowed.</p> </blockquote> <p>He gives <code>int Temperature</code> as an example of a bad smell and suggests the best fix is unit specific value type like Celsius. So I decided to try writing a custom Celsius value type that encapsulates all the bounds checking and type conversion logic as an exercise in being more <a href="http://en.wikipedia.org/wiki/Solid_%28object-oriented_design%29" rel="noreferrer">SOLID</a>.</p> <p><strong>Basic requirements:</strong></p> <ol> <li>Impossible to have an invalid value</li> <li>Encapsulates conversion operations</li> <li>Effient coping (equivalent to the int its replacing) </li> <li>As intuitive to use as possible (trying for the semantics of an int)</li> </ol> <p><strong>Implementation:</strong></p> <pre><code>[System.Diagnostics.DebuggerDisplay("{m_value}")] public struct Celsius // : IComparable, IFormattable, etc... { private int m_value; public static readonly Celsius MinValue = new Celsius() { m_value = -273 }; // absolute zero public static readonly Celsius MaxValue = new Celsius() { m_value = int.MaxValue }; private Celsius(int temp) { if (temp &lt; Celsius.MinValue) throw new ArgumentOutOfRangeException("temp", "Value cannot be less then Celsius.MinValue (absolute zero)"); if (temp &gt; Celsius.MaxValue) throw new ArgumentOutOfRangeException("temp", "Value cannot be more then Celsius.MaxValue"); m_value = temp; } public static implicit operator Celsius(int temp) { return new Celsius(temp); } public static implicit operator int(Celsius c) { return c.m_value; } // operators for other numeric types... public override string ToString() { return m_value.ToString(); } // override Equals, HashCode, etc... } </code></pre> <p><strong>Tests:</strong></p> <pre><code>[TestClass] public class TestCelsius { [TestMethod] public void QuickTest() { Celsius c = 41; Celsius c2 = c; int temp = c2; Assert.AreEqual(41, temp); Assert.AreEqual("41", c.ToString()); } [TestMethod] public void OutOfRangeTest() { try { Celsius c = -300; Assert.Fail("Should not be able to assign -300"); } catch (ArgumentOutOfRangeException) { // pass } catch (Exception) { Assert.Fail("Threw wrong exception"); } } } </code></pre> <p><strong>Questions:</strong></p> <ul> <li><strong>Is there a way to make MinValue/MaxValue const instead of readonly?</strong> Looking at the BCL I like how the meta data definition of <a href="http://msdn.microsoft.com/en-us/library/system.int32.aspx" rel="noreferrer">int</a> clearly states MaxValue and MinValue as compile time constants. How can I mimic that? I don’t see a way to create a Celsius object without either calling the constructor or exposing the implementation detail that Celsius stores an int.</li> <li><strong>Am I missing any usability features?</strong></li> <li><strong>Is there a better pattern for creating a custom single field value type?</strong></li> </ul>
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    1. COCheck out this question (somewnat answering you "missing usability features" part) - http://stackoverflow.com/questions/441309/why-are-mutable-structs-evil and links out of it. Usefull for all value types.
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    2. CO+1 for the question on becoming more SOLID.
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    3. CO@Alexei – I’ve read all the “mutable structs are evil” posts before. I agree. The problem is that if I make the private field readonly then Celcius.MaxValue calls the constructor which requires Celsius.MaxValue to already be defined. This is circular and results in a runtime exception. That’s why I’m using a default constructor in the MaxValue definition. Do you know a way around this? A special purpose “don’t check bounds” private constructor feels wrong.
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