I am not here mentioning any new thing , just orgainising this; is one of the most frequently asked questions for entry level .net programmers/developers
Const :
1 A Const variable value is initialized at Compile time and Cant be changed afterwards once value is assigned.
2 A Const variable is initiailized at declaration only cant be changed once initialised.
3 It is to be said that Const variable/member is considered as Static by compiler at time of compilation. but Const cant be declared as static.
4 Constants must be of an integral type (sbyte, byte, short, ushort, int, uint, long, ulong, char, float, double, decimal, bool, or string), an enumeration, or a reference to null.
5 Const cant be of type class / structure bcoz both are initialized at runtime ( with new keyword ) & const expected to assign value at compile time.
e.g.
public class MyClass
{
public const double e = 2.71828; // Need to assign value at the time of declaration.
}
To use a constant outside of the class that it is declared in, you must fully qualify it using the class name.
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Readonly :
1 A Readonly variable can be either instance-level or static.
2 A Readonly variable value is initialized at run time and Cant be changed afterwards once value is assigned.
3 A Readonly variable can be initialized at declaration or by code in the constructor, Can have different values depending upon which constructor is getting called.(Also variable can be modified/initialized at Static Instructor if Variable is static readonly.)
4 A Readonly members cannot be of type enumerations.
e.g.
public class MyClass
{
public readonly double e = 2.71828;
}
OR
public class MyClass
{
public readonly double e; // Can use Default value ,initialization at the runtime.
public MyClass()
{
e = 2.71828;
}
}
The readonly field can be used for runtime constants as in the following example:
public static readonly uint l1 = (uint)DateTime.Now.Ticks;
REF Details : Readonly & Static
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Static Readonly:
1 We can create ‘static readonly’ member when we want member to be accessed without need to create instance of the class.
2 static readonly is generally used if the type of the field is not allowed in a const declaration, or when the value is not known at compile t ime.
3 For static readonly member we can modify/initialize at Static Instructor.
Note :
1. I would like to borrow an examples from MSDN Blogs –
Remember that for reference types, in both cases (static and instance) the readonly modifier only prevents you from assigning a new reference to the field. It specifically does not make immutable the object pointed to by the reference.
class Program
{
public static readonly Test test = new Test();
static void Main(string[] args)
{
test.Name = “Program”;
test = new Test(); // Error: A static readonly field cannot be assigned to (except in a static constructor or a variable initializer)
}
}
class Test
{
public string Name;
}
On the other hand, if Test were a value type, then assignment to test.Name would be an error.
2. If the readonly member is of type int then.
class Test
{
readonly int testValue;
readonly int testnew = 12;
Test(int TestValue)
{
testValue = TestValue;
}
void ChangeYear()
{
testValue = 10; // Will get compile time error.
testnew = 23; // Will get compile time error.
}
}
we will get the compiler error message: “The left-hand side of an assignment must be an l-value”
We will get the same error when we try to assign a value to a constant.
CollectedDotNet 