In practice, the difference is in the location where the preprocessor searches for the included file.
For #include <filename> the preprocessor searches in an implementation dependent manner, normally in search directories pre-designated by the compiler/IDE. This method is normally used to include standard library header files.
For #include "filename" the preprocessor searches first in the same directory as the file containing the directive, and then follows the search path used for the #include <filename> form. This method is normally used to include programmer-defined header files.
having to declare the value at the time of a definition for a const VS readonly values can be computed dynamically but need to be assigned before the constructor exits.. after that it is frozen.
const's are implicitly static. You use a ClassName.ConstantName notation to access them.
There is a subtle difference. Consider a class defined in AssemblyA.
public class Const_V_Readonly
public const int I_CONST_VALUE = 2;
public readonly int I_RO_VALUE;
I_RO_VALUE = 3;
AssemblyB references AssemblyA and uses these values in code. When this is compiled:
in the case of the const value, it is like a find-replace. The value 2 is 'baked into' the AssemblyB's IL. This means that if tomorrow I update I_CONST_VALUE to 20, AssemblyB would still have 2 till I recompile it.
in the case of the readonly value, it is like a ref to a memory location. The value is not baked into AssemblyB's IL. This means that if the memory location is updated, AssemblyB gets the new value without recompilation. So if I_RO_VALUE is updated to 30, you only need to build AssemblyA and all clients do not need to be recompiled.
So if you are confident that the value of the constant won't change, use a const.
public const int CM_IN_A_METER = 100;
But if you have a constant that may change (e.g. w.r.t. precision).. or when in doubt, use a readonly.
public readonly float PI = 3.14;
Update: Aku needs to get a mention because he pointed this out first. Also I need to plug where I learned this: Effective C# - Bill Wagner