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    <p>This is frequently confusing, and I think the MSDN documentation actually is a bit "clear only if already known". That is, it is correct, but it really only makes sense if you already understand the concept.</p> <p>Here's how I think of it.</p> <p>A regular parameter makes a <em>copy</em> of the <em>value</em> of the <em>argument</em>. When you say:</p> <pre><code>static int M(int z) { z = z + 1; return z; } ... int x = 123; int y = M(x); </code></pre> <p>That is just like you said:</p> <pre><code>int x = 123; int z = x; // make a copy of x z = z + 1; int y = z; </code></pre> <p>A ref or out parameter <em>make an alias for an existing variable</em>. When you say</p> <pre><code>static void N(ref int q) { q = q + 1; } ... int x = 123; N(x); </code></pre> <p>That is the same as saying:</p> <pre><code>int x = 123; // MAGIC: q is now an another name for variable x q = q + 1; </code></pre> <p><code>q</code> and <code>x</code> are <em>two different names</em> that refer to <em>the same variable</em>. Incrementing <code>q</code> also increments <code>x</code> because they are the same. <code>z</code> and <code>x</code> in the previous example are <em>two different names</em> that refer to <em>two different variables</em>. Incrementing <code>z</code> does not change <code>x</code>.</p> <p>Summing up: "out" and "ref" just mean "do not make a new variable; rather, temporarily make a second name for an existing variable". </p> <p>Is that now clear?</p> <p>UPDATE: I did not say what the difference between "out" and "ref" is. The difference is simple. On the "caller" side, a "ref" must be a definitely assigned variable <em>before</em> the method is called. An "out" need not be. On the "callee" side, a "ref" may be read before it is written to, but an "out" must be written to before it is read. Also, an "out" must be written to before control leaves the method normally.</p>
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    1. CONot bit, i created a sample example project, where i created below method static void N(out int q) { q = q + 1; } now it throws the error saying use of unassigned local variable q, now how to assign, i have already assinged value to q.
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    2. COAn out parameter need not be definitely assigned in the caller before the call site. Because of this, it must be assigned before first use in the callee (the declaring method). You can correct the method in your comment by changing `out` to `ref`.
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    3. CO@Abbas: You are right; I was not clear because I did not say what the difference is between "out" and "ref". Is the update now clear?
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