Code - readonly modifier in c#

If you want to create fields in a class, weather public or private and want them to be un-mutable from anywhere within or outside of the class except from the class constructor or at field level declaration then you can make use of readonly modifier:

using System;

public class SomeClass
{
    public readonly string FieldA = string.Empty;

    public SomeClass(string valueForFieldA)
    {
       this.FieldA = valueForFieldA;
    }
    
    public void SomeFunction(string someValue)
    {
       //this.FieldA = someValue; This will not work
    }
}

Points to remember:

1. Trying to modify the value of FieldA after instantiating SomeClass will not work. 
2. Remember readonly keyword (modifier) is only for fields not for properties!