When should use Readonly and Get only properties

Creating a property with only a getter makes your property read-only for any code that is outside the class.

You can however change the value using methods provided by your class :

public class FuelConsumption {
    private double fuel;
    public double Fuel
    {
        get { return this.fuel; }
    }
    public void FillFuelTank(double amount)
    {
        this.fuel += amount;
    }
}

public static void Main()
{
    FuelConsumption f = new FuelConsumption();

    double a;
    a = f.Fuel; // Will work
    f.Fuel = a; // Does not compile

    f.FillFuelTank(10); // Value is changed from the method's code
}

Setting the private field of your class as readonly allows you to set the field value only in the constructor of the class (using an inline assignment or a defined constructor method).
You will not be able to change it later.

public class ReadOnlyFields {
    private readonly double a = 2.0;
    private readonly double b;

    public ReadOnlyFields()
    {
        this.b = 4.0;
    }
}

readonly class fields are often used for variables that are initialized during class construction, and will never be changed later on.

In short, if you need to ensure your property value will never be changed from the outside, but you need to be able to change it from inside your class code, use a “Get-only” property.

If you need to store a value which will never change once its initial value has been set, use a readonly field.