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POWhy can't I define a default constructor for a struct in .NET?
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  1. POWhy can't I define a default constructor for a struct in .NET?
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    copied!<p>In .NET, a value type (C# <code>struct</code>) can't have a constructor with no parameters. According to <a href="https://stackoverflow.com/questions/203695/structure-vs-class-in-c#204009">this post</a> this is mandated by the CLI specification. What happens is that for every value-type a default constructor is created (by the compiler?) which initialized all members to zero (or <code>null</code>).</p> <p>Why is it disallowed to define such a default constructor?</p> <p>One trivial use is for rational numbers:</p> <pre><code>public struct Rational { private long numerator; private long denominator; public Rational(long num, long denom) { /* Todo: Find GCD etc. */ } public Rational(long num) { numerator = num; denominator = 1; } public Rational() // This is not allowed { numerator = 0; denominator = 1; } } </code></pre> <p>Using current version of C#, a default Rational is <code>0/0</code> which is not so cool.</p> <p><strong>PS</strong>: Will default parameters help solve this for C#&nbsp;4.0 or will the CLR-defined default constructor be called?</p> <hr> <p><a href="https://stackoverflow.com/questions/333829/why-cant-i-define-a-default-constructor-for-a-struct-in-net#333840">Jon Skeet</a> answered:</p> <blockquote> <p>To use your example, what would you want to happen when someone did:</p> <pre><code> Rational[] fractions = new Rational[1000]; </code></pre> <p>Should it run through your constructor 1000 times?</p> </blockquote> <p>Sure it should, that's why I wrote the default constructor in the first place. The CLR should use the <em>default zeroing</em> constructor when no explicit default constructor is defined; that way you only pay for what you use. Then if I want a container of 1000 non-default <code>Rational</code>s (and want to optimize away the 1000 constructions) I will use a <code>List&lt;Rational&gt;</code> rather than an array.</p> <p>This reason, in my mind, is not strong enough to prevent definition of a default constructor.</p>
 

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