Anatomy-of-a-Breaking-Change

Client.cs

using System;
using BreakingChangeAnatomy.Library;
using BreakingChangeAnatomy.Library.ChildNs;

namespace BreakingChangeAnatomy.Client
{
	class Program
	{
		static void Main(string[] args)
		{
			var o = new TheType { TheProperty = 42m };
			var result = o.TheMethod(new TheArgument { Postfix = "!" });
			Console.WriteLine(result.Something);
			Console.ReadLine();
		}
	}
}

Library.v1.cs

using BreakingChangeAnatomy.Library.ChildNs;

namespace BreakingChangeAnatomy.Library
{
	public class TheType
	{
		public decimal TheProperty { get; set; }
	
		public TheReturn TheMethod(TheArgument argument)
		{
			// pretend doing something
			var result = new TheReturn
			{
				Something = $"{TheProperty} {argument.Postfix}"
			};
			return result;
		}
	}
}

Library.v1.layout

├───Library
    │   TheType.cs
    │   
    └───ChildNs
            TheArgument.cs
            TheReturn.cs

Library.v1_1.cs

using BreakingChangeAnatomy.Library.ChildNs;

namespace BreakingChangeAnatomy.Library
{
	public class TheType
	{
		public decimal TheProperty { get; set; }
	
		public TheReturn TheMethod(TheArgument argument)
		{
			// pretend doing something
			var result = new TheReturn
			{
        //  compatible change
				Something = $"{TheProperty}{argument.Postfix}"
			};
			return result;
		}
	}
}

Library.v2.cs

namespace BreakingChangeAnatomy.Library
{
	public class TheType
	{
		public decimal TheProperty { get; set; }
	}
}

namespace BreakingChangeAnatomy.Library.ChildNs
{
	public static class Operation
	{
		public static TheReturn TheMethod(this TheType o, TheArgument argument)
		{
			// fake doing something
			var result = new TheReturn
			{
				Something = $"{o.TheProperty}{argument.Postfix}"
			};
			return result;
		}
	}
}

Library.v2.layout

├───Library
    │   TheType.cs
    │   
    └───ChildNs
            Operation.cs
            TheArgument.cs
            TheReturn.cs