The following table lists the difference between Static , Readonly, and constant in C. C ASP. Skill Tests ASP. How to combine two arrays without duplicate values in C? Difference between String and string in C. How to get a comma separated string from an array in C? How to remove duplicate values from an array in C? How to sort an array in C?
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How to write file using StreamWriter in C? Declared using the readonly keyword. Declred using the const keyword. By default a const is static that cannot be changed. 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. 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. Can I change a private readonly inherited field in C using reflection? 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. One of the team members in our office provided the following guidance on when to use const, static, and readonly:.
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. 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. 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 -. Here, instance objOne will have value of readonly field as 5 and objTwo has Which is not possible using const. 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. A const has to be hard-coded , where as readonly can be set in the constructor of the class.
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.
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. However, consider the case where you define a type and want to provide some pre-fab instances of it.
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:. 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. 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:. Now, can the code that is generated rely on the fact that A. C is a compile-time constant? 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. ReadOnly :The value will be initialized only once from the constructor of the class. 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.
Constant variables are declared and initialized at compile time. Read-only variables will be initialized only from the Static constructor of the class. Read only is used only when we want to assign the value at run time.
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.
One thing to add to what people have said above. If you have an assembly containing a readonly value e. 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. The value of the constant field is the same throughout the program or in other words, once the constant field is assigned the value of this field is not be changed.
In C , constant fields and locals are not variables, a constant is a number, string, null reference, boolean values. This readonly keyword shows that you can assign the variable only when you declare a variable or in a constructor of the same class in which it is declared.
ReadOnly is a runtime constant. Const is a compile time constant. The value of readonly field can be changed. The value of the const field can not be changed. It cannot be declared inside the method. It can be declared inside the method. In readonly fields, we can assign values in declaration and in the constructor part.
In const fields, we can only assign values in declaration part. It can be used with static modifiers.
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