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    <p>The most time you inherit of <code>System.Array</code> is using a single dimension array. like:</p> <pre><code>int[] a1 = new int[2]; </code></pre> <p>When you define <code>int[]</code> this implicitly derived from <code>System.Array</code> type like @Sergey Rybalkin says. In this case the method <code>IndexOf</code> would surely be best implemented as a instance method and not as static method. </p> <p>But there is another types that inherit from System.Array like mult dimension arrays. In this case (mult dimension) the method <code>IndexOf</code> does not make sense.</p> <p>Test this:</p> <pre><code>int[,] arr = new int[2, 2]; arr[0, 0] = 3; arr[1, 0] = 4; arr[0, 1] = 5; arr[1, 1] = 6; Array.IndexOf(arr, 4); </code></pre> <p>The last like throws a RankException with the message <code>"Only single dimension arrays are supported here."</code></p> <p>Perhaps, and most probably, because of that this method is implemented as static.</p> <p>...</p> <p>About the comment <code>Is there a main principle behind the decision, whether to make a method static or not?</code></p> <p>There is, and the principle is quite simple. The instance method represents a action or behavior of a object. The static method is a function of the system that is logic related with the class, or in some cases a method you want to call without creating an instance of the class.</p> <p>Think in <code>System.Math</code> class how mess will be if you need instance math every time you want call a method like <code>Sqrt</code>or <code>Pow</code>?</p> <p>The final example I will give to you is the <code>System.Text.RegularExpressions.Regex</code> class. This class have a <code>Match</code> method implemented as instance and an overload implemented as static.</p> <p>Each one is used in a diferent context. The instance is used when you use the same pattern multiple times. The static when you use the pattern a unique time in your code.</p>
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    1. POWhy are most methods of System.Array static?
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    1. USJonny Piazzi
    1. USJonny Piazzi
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    1. CO"If the answer was enlightening please mark..."? :-P No no.. Mark as correct answer *if this is the correct answer*.
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      1. USPatrick
    2. COI don't think your additional examples are quite fitting. There would be no point in making functions such as `Sqrt` or `Pow` instance methods of `System.Math` because those functions *do not require a `System.Math` instance*. On the other hand, making them instance methods of the various number types such as `double` etc. would be conceivable. As for the regular expressions, the static method does not *perform better* for unique invocations IMO; it just helps shorten the code.
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      1. USO. R. Mapper
    3. CO@O. R. Mapper you right instance method perform better. I corrected the text.
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      1. USJonny Piazzi
 

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