The Readonly property in C#

The Readonly property in C# Explained

In C#, readonly is a keyword which is useful to define read-only fields in our applications.  The read-only field values need to be initialized either at the declaration or in a constructor of the same class unlike constant keyword. If we use readonly keyword with fields, then those field values will be evaluated at the runtime.

                

To define read-only fields in C#, we need to use readonly keyword during the declaration of fields in our application and we can use readonly modifier with the numbers, boolean values, strings or with null references.

 

If we use readonly keyword to define the read-only field, then that field value cannot be changed once the constructor execution has finished so we should not use readonly keyword with the fields whose value will be changed at any time.

 

It’s mandatory to initialize read-only field values either at the declaration or in a constructor otherwise we will get compile-time errors.

 

Readonly Keyword Syntax in C#

The following is the syntax of defining read-only fields using readonly keyword in C#.

                 

readonly datatype fieldname=”value”;

The following are the different ways of declaring and initializing read-only fields in C# programming language.

    class User

    {

        // Initialize Read Only Fields

        public readonly string Name = "Musa Sule";

        public readonly string Location;

        public readonly int Age;

        public User()

        {

            Location = "Abuja";

            Age = 42;

        }

        public void SetUserDetails()

        {

            // Compile error if uncommented

            //location = "Guntur";

            //age = 30;

        }

    }  

In case if we uncomment the commented code in SetUserDetails() method, then we will get compile errors because, in C#, the read-only field values can be initialized either during declaration or in a constructor.

 

Readonly Property Example

The following code is an example of defining and using read-only fields in C# with readonly keyword.

 

using System;

namespace CsharpnaijaTutorial

{

    class Person

    {

        // Initialize Read Only Fields

        public readonly string Name = "Musa Sule";

        public readonly string Location;

        public readonly int Age;

        public Person()

        {

            Location = "Abuja";

            Age = 32;

        }

    }

    class Program

    {

        static void Main(string[] args)

        {

            Person person = new Person();

            // This will throw compile time error

            //person.Name = "Sakinat Musa";

            Console.WriteLine($"Name: {person.Name}");

            Console.WriteLine($"Location: {person.Location}");

            Console.WriteLine($"Age: {person.Age}");

            Console.WriteLine("\nPress Enter Key to Exit..");

            Console.ReadLine();

        }

    }

}

 

As we earlier discussed, once the read-only field is declared and initialized, then that field value must be same throughout the application.

ReadOnly Property Features

The following are the important features of the read-only variables in C#.

 

• Read-only fields in C# can be created by using readonly keyword.
• The readonly fields can be initialized either at the declaration or in a constructor.
• The readonly field values will be evaluated during the run time in C#.
• Once values assigned to read-only fields, then those values must be the same throughout the application.

Difference between Constant and ReadOnly

The following are the difference between constant and readonly properties in C#.

·        The constant fields can be created by using const keyword and read-only fields can be created by using readonly keyword.

• The constant fields can only be initialized during the time of declaration but read-only fields can be initialized either at the declaration or in a constructor.
• Constant field values will be evaluated during the compile time but read-only field values will be evaluated at run time in c#.

 

Thank you

References

1.     Tutlane

2.     Geeks For Geeks