using System;
using System.Reflection;

public class Test
{
	public static void Main()
	{
		var yoba = new Yoba();
		Console.WriteLine(yoba.Size);
		
		// error CS0200: Property or indexer `Yoba.Size' cannot be assigned to (it is read-only)
		// yoba.Size = 5;
		
		var field = typeof (Yoba).GetField("<Size>k__BackingField", 
			BindingFlags.Instance | BindingFlags.NonPublic);
			
		field.SetValue(yoba, 5);
		Console.WriteLine(yoba.Size);
	}
}

public class Yoba
{
	public int Size { get; } = 2;
}

public class OldYoba
{
	private readonly int _size = 2;
	
	public int Size
	{
		get { return _size; }
	}
}