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    <p>Apart from the apparent difference of </p> <ul> <li>having to declare the value at the time of a definition for a <code>const</code> VS <code>readonly</code> values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen.</li> <li>'const's are implicitly <code>static</code>. You use a <code>ClassName.ConstantName</code> notation to access them.</li> </ul> <p>There is a subtle difference. Consider a class defined in <code>AssemblyA</code>.</p> <pre><code>public class Const_V_Readonly { public const int I_CONST_VALUE = 2; public readonly int I_RO_VALUE; public Const_V_Readonly() { I_RO_VALUE = 3; } } </code></pre> <p><code>AssemblyB</code> references <code>AssemblyA</code> and uses these values in code. When this is compiled, </p> <ul> <li>in the case of the <code>const</code> value, it is like a find-replace, the value 2 is 'baked into' the <code>AssemblyB</code>'s IL. This means that if tomorrow I'll update <code>I_CONST_VALUE</code> to 20 in the future. <em><code>AssemblyB</code> would still have 2 till I recompile it</em>.</li> <li>in the case of the <code>readonly</code> value, it is like a <code>ref</code> to a memory location. The value is not baked into <code>AssemblyB</code>'s IL. This means that if the memory location is updated, <code>AssemblyB</code> gets the new value without recompilation. So if <code>I_RO_VALUE</code> is updated to 30, you only need to build <code>AssemblyA</code>. All clients do not need to be recompiled.</li> </ul> <p>So if you are confident that the value of the constant won't change use a <code>const</code>. </p> <pre><code>public const int CM_IN_A_METER = 100; </code></pre> <p>But if you have a constant that may change (e.g. w.r.t. precision).. or when in doubt, use a <code>readonly</code>. </p> <pre><code>public readonly float PI = 3.14; </code></pre> <p><em>Update: Aku needs to get a mention coz he pointed this out first. Also I need to plug where I learned this.. <a href="http://rads.stackoverflow.com/amzn/click/0321245660" rel="noreferrer">Effective C# - Bill Wagner</a></em></p>
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    1. POWhat is the difference between const and readonly?
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    1. COThe `static` point seems to be the most important and useful point - `consts are implicitly static`
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    2. COThe part about reference values is the most important one. Const values can be optimized away.
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    3. CO`readonly` variables can be changed outside the constructor (reflection). It's only the compiler that tries to hinder you from modifying the var outside the constructor.
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