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copied!<p>The member declared as readonly gives the possibility to be changed in the (static) constructor of the class, while the const member cannot be changed at runtime. </p> <p>Declaring a field as const makes it automatically static, quoting from §10.3.7:</p> <blockquote> <p>When a field, method, property, event, operator, or constructor declaration includes a static modifier, it declares a static member. In addition, a constant or type declaration implicitly declares a static member.</p> </blockquote> <p>The third is just a read-only property which happens to always return 5. </p> <p>You should never use such a property and prefer const members where possible in order to allow the compiler and/or the jitter to perform their optimizations and to help other people reading your code (that property is kind of a WTF to me). The static readonly member has to be used if it is required a constant value initialized during the program start-up (like, for example, the number of cores of a machine). </p> <p>This is a great example from the C# specs (§10.5.2.1):</p> <blockquote> <p>A static readonly field is useful when a symbolic name for a constant value is desired, but when the type of the value is not permitted in a const declaration, or when the value cannot be computed at compile-time. In the example</p> </blockquote> <pre><code>public class Color { public static readonly Color Black = new Color(0, 0, 0); public static readonly Color White = new Color(255, 255, 255); public static readonly Color Red = new Color(255, 0, 0); public static readonly Color Green = new Color(0, 255, 0); public static readonly Color Blue = new Color(0, 0, 255); private byte red, green, blue; public Color(byte r, byte g, byte b) { red = r; green = g; blue = b; } } </code></pre> <blockquote> <p>the Black, White, Red, Green, and Blue members cannot be declared as const members because their values cannot be computed at compile-time. However, declaring them static readonly instead has much the same effect. </p> </blockquote> <p>And yet another difference (§10.5.2.2):</p> <blockquote> <p>Constants and readonly fields have different binary versioning semantics. When an expression references a constant, the value of the constant is obtained at compile-time, but when an expression references a readonly field, the value of the field is not obtained until run-time.</p> </blockquote> <p>So, summing it up, they are very different even if at a first glance they might look similar and you should use the one which best suits your intent.</p>

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