Guide to the Language

CHAPTER 29
Constants
A variable in C# can be made into a compile-time constant by adding the 
const keyword before the data type. This modifier means that the variable 
cannot be changed and it must therefore be assigned a value at the same 
time as it is declared. Any attempts to assign a new value to the constant 
will result in a compile-time error.
 Local Constants
A local constant must always be initialized at the same time as it is 
declared.
static void Main()
{
const int a = 10; // compile-time constant
}
The const modifier creates a compile-time constant, and so the 
compiler will replace all usage of the constant with its value. The assigned 
value must therefore be known at compile time. As a result of this, the 
const modifier may only be used together with the simple types, as well as 
with enum and string types.

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 Constant Fields
The const modifier can be applied to a field to make the field 
unchangeable.
class MyClass
{
const int b = 5; // compile-time constant field
}
Constant fields cannot have the static modifier. They are implicitly 
static and are accessed in the same way as static fields.
int a = MyClass.b;
 Readonly
Another variable modifier similar to const is readonly, which creates a 
runtime constant. This modifier can be applied to fields, and like const, it 
makes the field unchangeable.
class MyClass
{
readonly int c = 3; // run-time constant field
}
Since a readonly field is assigned at runtime, it can be assigned a 
dynamic value that is not known until runtime.
readonly int d = System.DateTime.Now.Hour;
Unlike const, readonly can be applied to any data type.
readonly int[] e = { 1, 2, 3 }; // readonly array
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Additionally, a readonly field cannot only be initialized when it is 
declared. It can also be assigned a value in the constructor.
class MyClass
{
readonly string s;
public MyClass() { s = "Hello World"; }
}
As of C# 7.2, the readonly modifier can be applied to not just fields but 
also to structs. Declaring a struct as readonly will enforce immutability 
on the members of the struct, requiring all fields and properties to be 
made readonly.
readonly struct MyStruct
{
public readonly int myVar;
public int myProperty { get; }
public MyStruct(int var, int prop)
{
myVar = var;
myProperty = prop;
}
}
Another addition in C# 7.2 is the ability to mark a method’s return 
value as readonly when returning a value type by reference with the ref 
modifier. This will disallow the caller from modifying the returned value, 
provided that the returned value is also assigned as a readonly reference 
and not just a copy.
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class MyClass
{
readonly static int i;
static ref readonly int GetValue() { return ref i; }
static void Main()
{
ref readonly int a = ref GetValue();
a = 5; // error: readonly variable
}
}