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copied!<p>The reason you are not allowed to define a variable with the same name in both the for-loop as well as outside the for-loop is because variables in the outer-scope are valid in the inner-scope. Meaning that there would be two 'i' variables within the for-loop if this was allowed.</p> <p>See: <a href="http://msdn.microsoft.com/en-us/library/aa691132%28v=vs.71%29.aspx" rel="nofollow">MSDN Scopes</a></p> <p>Specifically:</p> <blockquote> <p>The scope of a local variable declared in a local-variable-declaration (Section 8.5.1) is the block in which the declaration occurs.</p> </blockquote> <p>and</p> <blockquote> <p>The scope of a local variable declared in a for-initializer of a for statement (Section 8.8.3) is the for-initializer, the for-condition, the for-iterator, and the contained statement of the for statement.</p> </blockquote> <p>And also: <a href="http://msdn.microsoft.com/en-us/library/aa664742%28v=vs.71%29.aspx" rel="nofollow">Local variable declarations</a> (Section 8.5.1 of the C# specification)</p> <p>Specifically:</p> <blockquote> <p><strong>The scope of a local variable declared in a local-variable-declaration is the block in which the declaration occurs.</strong> It is an error to refer to a local variable in a textual position that precedes the local-variable-declarator of the local variable. <strong>Within the scope of a local variable, it is a compile-time error to declare another local variable or constant with the same name.</strong></p> </blockquote> <p>(Emphasis mine.)</p> <p>Which means that the scope of the <code>i</code> inside your for-loop, is the for-loop. Whereas the scope of the <code>i</code> outside of your for-loop is the entire main method <strong>plus</strong> the for-loop. Meaning you'd have two occurrences of <code>i</code> inside the loop which is invalid according to the above.</p> <p>The reason why you're not allowed to do <code>int A = i;</code> is because <code>int i</code> is only scoped for use within the <code>for</code> loop. Thus it is no longer accessible outside of the <code>for</code> loop. </p> <p>As you can see both of these issues are a result of scoping; the first issue (<code>int i = 4;</code>) would result in two <code>i</code> variables within the <code>for</code> loop scope. Whereas <code>int A = i;</code> would result in access to a variable that is out of scope.</p> <p>What you could do instead is declare <code>i</code> to be scoped to the entire method, and then use it in both the method as well as the for-loop scope. This will avoid breaking either rule.</p> <pre><code>public static void Main() { int i; for (i = 0; i < 5; i++) { } // 'i' is only declared in the method scope now, // no longer in the child scope -> valid. i = 4; // 'i' is declared in the method's scope -> valid. int A = i; } </code></pre> <p><strong>EDIT</strong>:</p> <p>The C# compiler could of course be changed to allow this code to compile quite validly. After all this is valid: </p> <pre><code>for (int i = 0; i < 5; i++) { Console.WriteLine(i); } for (int i = 5; i > 0; i--) { Console.WriteLine(i); } </code></pre> <p>But would it really be beneficial to your code readability and maintainability to be able to write code such as:</p> <pre><code>public static void Main() { int i = 4; for (int i = 0; i < 5; i++) { Console.WriteLine(i); } for (int i = 5; i > 0; i--) { Console.WriteLine(i); } Console.WriteLine(i); } </code></pre> <p>Think about the potential for mistakes here, does the last <code>i</code> print out 0 or 4? Now this is a very small example, one which is quite easy to follow and track but it is definitely a lot less maintainable and readable than having declared the outer <code>i</code> by a different name. </p> <p><strong>N.B:</strong></p> <p>Please note, C#'s scoping rules differ from <a href="http://msdn.microsoft.com/en-us/library/b7kfh662%28v=vs.80%29.aspx" rel="nofollow">C++'s scoping rules</a>. In C++ variables are only in scope from where they are declared until the end of the block. Which would make your code a valid construct in C++. </p>

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