readonly fields in C# class

C# language has a keyword to mention fields in a class can be readonly. In this tutorial let's discuss readonly keyword and how it can be used to convey class author intention that a field value shouldn't be modified once the class instance is created.

C# sample demonstrating readonly keyword

public class Employee
    {
        private readonly string _id;
        public string _deptId;

        public Employee(string id)
        {
            _id = id;
            DeptId = deptId;
        }

        public string ID
        {
            get
            {
                return _id;
            }
        }
        
        public string DeptID
        {
           get
             {
               return _deptID;
             }
           set
             {
              _deptID =value;
             }
         }
    }

Notes on above C# program

1. "_id" has been declared as readonly.
2. "_id" member has been assigned a value inside the Employee class constructor.

readonly class members and value assignment

1. With readonly members properties, you can assign value only during initialization and inside the class constructors.
2. If you try to assign value to members anywhere else, you will get the below error:

   A readonly field cannot be assigned to (except in a constructor or a variable initializer)

When to define readonly fields in a class

If a member of a class whose value you(class author) don't want anyone to change post object construction, then consider marking that member as readonly.

By declaring a member as readonly you are indicating to other fellow developers that such member value cannot be changed once the object is created (post constructor call).