Question
What is the difference between const and readonly in C#?
What is the difference between const
and readonly
in C#?
When would you use one over the other?
Question
What is the difference between const
and readonly
in C#?
When would you use one over the other?
Solution
Apart from the apparent difference of
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;
public Const_V_Readonly()
{
I_RO_VALUE = 3;
}
}
AssemblyB
references AssemblyA
and uses these values in code. When this is compiled:
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.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
Solution
There is a gotcha with consts! If you reference a constant from another assembly, its value will be compiled right into the calling assembly. That way when you update the constant in the referenced assembly it won't change in the calling assembly!
Solution
Solution
Just to add, readonly
for reference types only makes the reference read only not the values. For example:
public class Const_V_Readonly
{
public const int I_CONST_VALUE = 2;
public readonly char[] I_RO_VALUE = new Char[]{'a', 'b', 'c'};
public UpdateReadonly()
{
I_RO_VALUE[0] = 'V'; //perfectly legal and will update the value
I_RO_VALUE = new char[]{'V'}; //will cause compiler error
}
}
Solution
const
: Can't be changed anywhere.
readonly
: This value can only be changed in the constructor. Can't be changed in normal functions.
Solution
This explains it. Summary: const must be initialized at declaration time, readonly can be initialized on the constructor (and thus have a different value depending on the constructor used).
EDIT: See Gishu's gotcha above for the subtle difference
Solution
A constant member is defined at compile time and cannot be changed at runtime. Constants are declared as a field, using the const
keyword and must be initialized as they are declared.
public class MyClass
{
public const double PI1 = 3.14159;
}
A readonly
member is like a constant in that it represents an unchanging value. The difference is that a readonly
member can be initialized at runtime, in a constructor, as well being able to be initialized as they are declared.
public class MyClass1
{
public readonly double PI2 = 3.14159;
//or
public readonly double PI3;
public MyClass2()
{
PI3 = 3.14159;
}
}
const
static
(they are implicitly static)readonly
Solution
There is a small gotcha with readonly. A readonly field can be set multiple times within the constructor(s). Even if the value is set in two different chained constructors it is still allowed.
public class Sample {
private readonly string ro;
public Sample() {
ro = "set";
}
public Sample(string value) : this() {
ro = value; // this works even though it was set in the no-arg ctor
}
}
Solution
A const is a compile-time constant whereas readonly allows a value to be calculated at run-time and set in the constructor or field initializer. So, a 'const' is always constant but 'readonly' is read-only once it is assigned.
Eric Lippert of the C# team has more information on different types of immutability.
Solution
Here's another link demonstrating how const isn't version safe, or relevant for reference types.
Summary:
Solution
Read Only : Value can be changed through Ctor at runtime. But not through member Function
Constant : By default static. Value cannot be changed from anywhere ( Ctor, Function, runtime etc no-where)
Solution
Yet another gotcha: readonly values can be changed by "devious" code via reflection.
var fi = this.GetType()
.BaseType
.GetField("_someField",
BindingFlags.Instance | BindingFlags.NonPublic);
fi.SetValue(this, 1);
Can I change a private readonly inherited field in C# using reflection?
Solution
I believe a const
value is the same for all objects (and must be initialized with a literal expression), whereas readonly
can be different for each instantiation...
Solution
They are both constant, but a const is available also at compile time. This means that one aspect of the difference is that you can use const variables as input to attribute constructors, but not readonly variables.
Example:
public static class Text {
public const string ConstDescription = "This can be used.";
public readonly static string ReadonlyDescription = "Cannot be used.";
}
public class Foo
{
[Description(Text.ConstDescription)]
public int BarThatBuilds {
{ get; set; }
}
[Description(Text.ReadOnlyDescription)]
public int BarThatDoesNotBuild {
{ get; set; }
}
}
Solution
One of the team members in our office provided the following guidance on when to use const, static, and readonly:
One final note: a const field is static, but the inverse is not true.
Solution
when to use const
or readonly
const
readonly
App.config
, but once it initializes it can't be changedSolution
Solution
Variables marked const are little more than strongly typed #define macros, at compile time const variable references are replaced with inline literal values. As a consequence only certain built-in primitive value types can be used in this way. Variables marked readonly can be set, in a constructor, at run-time and their read-only-ness is enforced during run-time as well. There is some minor performance cost associated with this but it means you can use readonly with any type (even reference types).
Also, const variables are inherently static, whereas readonly variables can be instance specific if desired.
Solution
There is notable difference between const and readonly fields in C#.Net
const is by default static and needs to be initialized with constant value, which can not be modified later on. Change of value is not allowed in constructors, too. It can not be used with all datatypes. For ex- DateTime. It can not be used with DateTime datatype.
public const DateTime dt = DateTime.Today; //throws compilation error
public const string Name = string.Empty; //throws compilation error
public readonly string Name = string.Empty; //No error, legal
readonly can be declared as static, but not necessary. No need to initialize at the time of declaration. Its value can be assigned or changed using constructor. So, it gives advantage when used as instance class member. Two different instantiation may have different value of readonly field. For ex -
class A
{
public readonly int Id;
public A(int i)
{
Id = i;
}
}
Then readonly field can be initialised with instant specific values, as follows:
A objOne = new A(5);
A objTwo = new A(10);
Here, instance objOne will have value of readonly field as 5 and objTwo has 10. Which is not possible using const.
Solution
Another gotcha.
Since const really only works with basic data types, if you want to work with a class, you may feel "forced" to use ReadOnly. However, beware of the trap! ReadOnly means that you can not replace the object with another object (you can't make it refer to another object). But any process that has a reference to the object is free to modify the values inside the object!
So don't be confused into thinking that ReadOnly implies a user can't change things. There is no simple syntax in C# to prevent an instantiation of a class from having its internal values changed (as far as I know).
Solution
A const
has to be hard-coded, where as readonly
can be set in the constructor of the class.
Solution
A constant will be compiled into the consumer as a literal value while the static string will serve as a reference to the value defined.
As an exercise, try creating an external library and consume it in a console application, then alter the values in the library and recompile it (without recompiling the consumer program), drop the DLL into the directory and run the EXE manually, you should find that the constant string does not change.
Solution
Const and readonly are similar, but they are not exactly the same. A const field is a compile-time constant, meaning that that value can be computed at compile-time. A readonly field enables additional scenarios in which some code must be run during construction of the type. After construction, a readonly field cannot be changed.
For instance, const members can be used to define members like:
struct Test
{
public const double Pi = 3.14;
public const int Zero = 0;
}
since values like 3.14 and 0 are compile-time constants. However, consider the case where you define a type and want to provide some pre-fab instances of it. E.g., you might want to define a Color class and provide "constants" for common colors like Black, White, etc. It isn't possible to do this with const members, as the right hand sides are not compile-time constants. One could do this with regular static members:
public class Color
{
public static Color Black = new Color(0, 0, 0);
public static Color White = new Color(255, 255, 255);
public static Color Red = new Color(255, 0, 0);
public static Color Green = new Color(0, 255, 0);
public static 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;
}
}
but then there is nothing to keep a client of Color from mucking with it, perhaps by swapping the Black and White values. Needless to say, this would cause consternation for other clients of the Color class. The "readonly" feature addresses this scenario. By simply introducing the readonly keyword in the declarations, we preserve the flexible initialization while preventing client code from mucking around.
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;
}
}
It is interesting to note that const members are always static, whereas a readonly member can be either static or not, just like a regular field.
It is possible to use a single keyword for these two purposes, but this leads to either versioning problems or performance problems. Assume for a moment that we used a single keyword for this (const) and a developer wrote:
public class A
{
public static const C = 0;
}
and a different developer wrote code that relied on A:
public class B
{
static void Main() {
Console.WriteLine(A.C);
}
}
Now, can the code that is generated rely on the fact that A.C is a compile-time constant? I.e., can the use of A.C simply be replaced by the value 0? If you say "yes" to this, then that means that the developer of A cannot change the way that A.C is initialized -- this ties the hands of the developer of A without permission. If you say "no" to this question then an important optimization is missed. Perhaps the author of A is positive that A.C will always be zero. The use of both const and readonly allows the developer of A to specify the intent. This makes for better versioning behavior and also better performance.
Solution
ReadOnly :The value will be initialized only once from the constructor of the class.
const: can be initialized in any function but only once
Solution
The difference is that the value of a static readonly field is set at run time, so it can have a different value for different executions of the program. However, the value of a const field is set to a compile time constant.
Remember: 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.
For details, please refer to C# Frequently Asked Questions on this topic: http://blogs.msdn.com/csharpfaq/archive/2004/12/03/274791.aspx
Solution
Principally; you can assign a value to a static readonly field to a non-constant value at runtime, whereas a const has to be assigned a constant value.
Solution
One thing to add to what people have said above. If you have an assembly containing a readonly value (e.g. readonly MaxFooCount = 4; ), you can change the value that calling assemblies see by shipping a new version of that assembly with a different value (e.g. readonly MaxFooCount = 5;)
But with a const, it would be folded into the caller's code when the caller was compiled.
If you've reached this level of C# proficiency, you are ready for Bill Wagner's book, Effective C#: 50 Specific Ways to Improve Your C# Which answers this question in detail, (and 49 other things).
Solution
The key difference is that Const is the C equivalent of #DEFINE. The number literally gets substituted a-la precompiler. Readonly is actually treated as a variable.
This distinction is especially relevant when you have Project A depending on a Public constant from Project B. Suppose the public constant changes. Now your choice of const/readonly will impact the behavior on project A:
Const: project A does not catch the new value (unless it is recompiled with the new const, of course) because it was compiled with the constants subtituted in.
ReadOnly: Project A will always ask project B for it's variable value, so it will pick up the new value of the public constant in B.
Honestly, I would recommend you use readonly for nearly everything except truly universal constants ( e.g. Pi, Inches_To_Centimeters). For anything that could possibly change, I say use readonly.
Hope this helps, Alan.
Solution
Const: Absolute constant value during the application life time.
Readonly: It can be changed in running time.
Solution
The value of readonly field can be changed. However, The value of the const field can not be changed.
In readonly fields, we can assign values at the time of declaration or in the contructor of that class.Incase of constant we can only assign values at the time of declaration.
Readonly can be used with Static modifiers but constant cannot be used with static.