C# 8 Readonly Members

In C# 8.0, a new feature is introduced which allows you to declare members of a struct is read-only.

• If you do not want to declare the whole structure type as read-only, use the readonly modifier to mark the instance members that don't modify the state of the struct.
• It indicates that the member doesn't modify the state.
• It is more granular than applying the readonly modifier to a struct declaration.

Let's consider the following mutable structure that has two properties Height and Width. It calculates the area in another field Area. The ToString() method which returns a string that contains information about the rectangle object.

public struct Rectangle
{
    public double Height { get; set; }
    public double Width { get; set; }
    public double Area => (Height * Width);
    public Rectangle(double height, double width)
    {
        Height = height;
        Width = width;
    }
    public override string ToString()
    {
        return $"(Total area for height: {Height}, width: {Width}) is {Area}";
    }                
}

As you can see the ToString() method doesn't modify the state, so we can add the readonly modifier to the declaration of ToString().

public struct Rectangle
{
    public double Height { get; set; }
    public double Width { get; set; }
    public double Area => (Height * Width);
    public Rectangle(double height, double width)
    {
        Height = height;
        Width = width;
    }
    public readonly override string ToString()
    {
        return $"(Total area for height: {Height}, width: {Width}) is {Area}";
    }                
}

After adding the readonly modifier to the ToString() declaration, you will see the following warning.

Warning CS8656: Call to non-readonly member 'Rectangle.Area.get' from a 'readonly' member results in an implicit copy of 'this'.

The compiler warns you when it needs to create a defensive copy. The Area property doesn't change state, so you can fix this warning by adding the readonly modifier to the declaration.

public struct Rectangle
{
    public double Height { get; set; }
    public double Width { get; set; }
    public readonly double Area => (Height * Width);
    public Rectangle(double height, double width)
    {
        Height = height;
        Width = width;
    }
    public readonly override string ToString()
    {
        return $"(Total area for height: {Height}, width: {Width}) is {Area}";
    }                
}

• The readonly modifier is necessary on a read-only property.
• The compiler doesn't assume get accessors don't modify state, you must declare readonly explicitly.
• Auto-implemented properties are an exception, the compiler will treat all auto-implemented getters as readonly, so in the above example, there is no need to add the readonly modifier to the Height and Width properties.