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    <p>Lets start out with a silly but illustrative example:</p> <pre><code>Object o = 15; o = "apples"; </code></pre> <p>At no point do we get the impression that we just turned the number 15 into a string of apples. We know that <code>o</code> is simply a pointer. Now lets do this in iterator form.</p> <pre><code>int[] nums = { 15, 16, 17 }; foreach (Object o in nums) { o = "apples"; } </code></pre> <p>Again, this really accomplishes nothing. Or at least it <em>would</em> accomplish nothing were it to compile. It certainly wouldn't insert our string into the int array -- that's not allowed, and we know that <code>o</code> is just a pointer anyway.</p> <p>Let's take your example:</p> <pre><code>foreach (Position Location in Map) { //We want to fudge the position to hide the exact coordinates Location = Location + Random(); //Compiler Error Plot(Location); } </code></pre> <p>Were this to compile, the <code>Location</code> in your example stars out referring to a value in <code>Map</code>, but then you change it to refer to a new <code>Position</code> (implicitly created by the addition operator). Functionally it's equivalent to this (which DOES compile):</p> <pre><code>foreach (Position Location in Map) { //We want to fudge the position to hide the exact coordinates Position Location2 = Location + Random(); //No more Error Plot(Location2); } </code></pre> <p>So, why does Microsoft prohibit you from re-assigning the pointer used for iteration? Clarity for one thing -- you don't want people assigning to it thinking they've changed your position within the loop. Ease of implementation for another: The variable might hide some internal logic indicating the state of the loop in progress. </p> <p>But more importantly, there is no reason for you to <em>want</em> to assign to it. It represents the current element of the looping sequence. Assigning a value to it breaks the "Single Responsibility Principle" or <a href="http://www.codinghorror.com/blog/archives/000805.html" rel="nofollow noreferrer">Curly's Law</a> if you follow Coding Horror. A variable should mean one thing only.</p>
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    1. POWhy is The Iteration Variable in a C# foreach statement read-only?
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    1. POWhy is The Iteration Variable in a C# foreach statement read-only?
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    1. COTyler, I may be wrong but I don't agree with your assertion that "Postion Location" or "Object O" are iterators. Instead, they are iteration variables which have iterators that operate on them. If that is indeed the case, then I think your reasoning may be a little skewed. I do agree that changing the value of the iteration variable can cause clarity issues but I do wish Microsoft had left that for the developer to decide. I have found a number of instances where it would have been useful to change the value of a foreach iteration variable.
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      1. USAnthony Gatlin
    2. CO@AnthonyGatlin I don't agree that they should let the developer decide in this case. There's a very real cost to confusing code, and Microsoft has enough people coding in C# that they have a very real incentive to create a language that, as much as possible, prevents the developer from creating bugs.
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    3. CO"*A variable should mean one thing only*". Good concept. In my head, though, it'd be better if it simply meant "*current element*" rather than a "*reference*" or a "*pointer*". Or, in the op's instance `Map[i]`, which is exactly what we intuitively expect.
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