What is the difference between const and readonly in C#?

Apart from the apparent difference of

• 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;
  public Const_V_Readonly()
  {
     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

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!

Constants

• Constants are static by default
• They must have a value at compilation-time (you can have e.g. 3.14 * 2, but cannot call methods)
• Could be declared within functions
• Are copied into every assembly that uses them (every assembly gets a local copy of values)
• Can be used in attributes

Readonly instance fields

• Must have set value, by the time constructor exits
• Are evaluated when instance is created

Static readonly fields

• Are evaluated when code execution hits class reference (when new instance is created or a static method is executed)
• Must have an evaluated value by the time the static constructor is done
• It's not recommended to put ThreadStaticAttribute on these (static constructors will be executed in one thread only and will set the value for its thread; all other threads will have this value uninitialized)

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
  }
}

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

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
    }
}

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.

Here's another link demonstrating how const isn't version safe, or relevant for reference types.

Summary:

• The value of your const property is set at compile time and can't change at runtime
• Const can't be marked as static - the keyword denotes they are static, unlike readonly fields which can.
• Const can't be anything except value (primitive) types
• The readonly keyword marks the field as unchangeable. However the property can be changed inside the constructor of the class
• The readonly only keyword can also be combined with static to make it act in the same way as a const (atleast on the surface). There is a marked difference when you look at the IL between the two
• const fields are marked as "literal" in IL while readonly is "initonly"

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...

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; }
  }
}

One of the team members in our office provided the following guidance on when to use const, static, and readonly:

• Use const when you have a variable of a type you can know at runtime (string literal, int, double, enums,...) that you want all instances or consumers of a class to have access to where the value should not change.
• Use static when you have data that you want all instances or consumers of a class to have access to where the value can change.
• Use static readonly when you have a variable of a type that you cannot know at runtime (objects) that you want all instances or consumers of a class to have access to where the value should not change.
• Use readonly when you have an instance level variable you will know at the time of object creation that should not change.

One final note: a const field is static, but the inverse is not true.

CONST

1. const keyword can be applied to fields or local variables
2. We must assign const field at time of declaration
3. No Memory Allocated Because const value is embedded in IL code itself after compilation. It is like find all occurrences of const variable and replace by its value. So the IL code after compilation will have hard-coded values in place of const variables
4. Const in C# are by default static.
5. The value is constant for all objects
6. There is dll versioning issue - This means that whenever we change a public const variable or property , (In fact, it is not supposed to be changed theoretically), any other dll or assembly which uses this variable has to be re-built
7. Only C# built-in types can be declared as constant
8. Const field can not be passed as ref or out parameter

ReadOnly

1. readonly keyword applies only to fields not local variables
2. We can assign readonly field at the time of declaration or in constructor,not in any other methods.
3. dynamic memory allocated for readonly fields and we can get the value at run time.
4. Readonly belongs to the object created so accessed through only instance of class. To make it class member we need to add static keyword before readonly.
5. The value may be different depending upon constructor used (as it belongs to object of the class)
6. If you declare a non-primitive types (reference type) as readonly only reference is immutable not the object it contains.
7. Since the value is obtained at run time, there is no dll versioning problem with readonly fields/ properties.
8. We can pass readonly field as ref or out parameters in the constructor context.

• when to use const or readonly

  • const

    • compile-time constant: absolute constant, value is set during declaration, is in the IL code itself

  • readonly

    • run-time constant: can be set in the constructor/init via config file i.e. App.config, but once it initializes it can't be changed

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.

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).

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).

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.

Const: Absolute constant value during the application life time.

Readonly: It can be changed in running time.

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.