How to extend class with an extra property

Without having access to the class definition, the best you could do is create a class which is derived from the target class. Unless the original is Sealed.

as my comments were getting very verbose, I thought I'd add a new answer. this answer is completely Mario's work and thinking, only has my minor addition to exemplify what I'm trying to put across.

There are a few minor changes to mario's example that would make this work very well, namely, just changing the fact that the existing properties are added as the class object, rather than duplicating the entire class. Anyway, here's how this looks (only amended sections added, all else remains as per mario's answer):

public class Person
{
    public int Age { get; set; }
    public string FaveQuotation { get; set; }
}

for the IPerson interface, we add the actual Person class, rather than copying the properties:

public interface IPerson
{
    // extended property(s)
    string Name { get; set; }
    // base class to extend - tho we should try to overcome using this
    Person Person { get; set; }
}

This translates to an updated usage of:

static void Main(string[] args)
{
    var extended = new DynamicExtension<Person>().ExtendWith<IPerson>();

    var pocoPerson = new Person
    {
        Age = 25,
        FaveQuotation = "2B or not 2B, that is the pencil"
    };

    // the end game would be to be able to say: 
    // extended.Age = 25; extended.FaveQuotation = "etc";
    // rather than using the Person object along the lines below
    extended.Person = pocoPerson;
    extended.Name = "Billy";

    Console.WriteLine(extended.Name + " is " + extended.Person.Age 
        + " loves to say: '" + extended.Person.FaveQuotation + "'");

    Console.ReadKey();
}

Hope this helps the original OP, I know it made me think, tho the jury is still out as to whether the Person class should be flattened to the same level in the method as the new properties!! So in effect, using the line new DynamicExtension<Person>().ExtendWith<IPerson>(); SHOULD return a fully extended new object -intellisence included. Tough call - hmmm...

Your problem can relatively easily be solved by using Reflection.Emit and run-time code generation.

Suppose now you have the following class that you would like to extend.

public class Person
{
    public int Age { get; set; }
}

This class represents a person, and contains a property named Age to represent the person's age.

In your case, you would also like to add a Name property of type string to represent the person's name.

The simplest and most streamlined solution would then be to define the following interface.

public interface IPerson
{   
    string Name { get; set; }
    int Age { get; set; }
}

This interface, which will be used to extend your class, should contain all the old properties your current class contains, and the new ones you would like to add. The reason for this will become clear in a moment.

You can now use the following class definition to actually extend your class by creating a new type at runtime which will also make it derive from the above mentioned interface.

class DynamicExtension<T>
{
    public K ExtendWith<K>()
    { 
        var assembly = AppDomain.CurrentDomain.DefineDynamicAssembly(new AssemblyName("Assembly"), AssemblyBuilderAccess.Run);
        var module = assembly.DefineDynamicModule("Module");
        var type = module.DefineType("Class", TypeAttributes.Public, typeof(T));

        type.AddInterfaceImplementation(typeof(K));

        foreach (var v in typeof(K).GetProperties())
        {
            var field = type.DefineField("_" + v.Name.ToLower(), v.PropertyType, FieldAttributes.Private);
            var property = type.DefineProperty(v.Name, PropertyAttributes.None, v.PropertyType, new Type[0]);
            var getter = type.DefineMethod("get_" + v.Name, MethodAttributes.Public | MethodAttributes.SpecialName | MethodAttributes.Virtual, v.PropertyType, new Type[0]);
            var setter = type.DefineMethod("set_" + v.Name, MethodAttributes.Public | MethodAttributes.SpecialName | MethodAttributes.Virtual, null, new Type[] { v.PropertyType });

            var getGenerator = getter.GetILGenerator();
            var setGenerator = setter.GetILGenerator();

            getGenerator.Emit(OpCodes.Ldarg_0);
            getGenerator.Emit(OpCodes.Ldfld, field);
            getGenerator.Emit(OpCodes.Ret);

            setGenerator.Emit(OpCodes.Ldarg_0);
            setGenerator.Emit(OpCodes.Ldarg_1);
            setGenerator.Emit(OpCodes.Stfld, field);
            setGenerator.Emit(OpCodes.Ret);

            property.SetGetMethod(getter);
            property.SetSetMethod(setter);

            type.DefineMethodOverride(getter, v.GetGetMethod());
            type.DefineMethodOverride(setter, v.GetSetMethod());
        }

        return (K)Activator.CreateInstance(type.CreateType());
    }
}

To actually use this class, simply execute the following lines of code.

class Program
{
    static void Main(string[] args)
    {
        var extended = new DynamicExtension<Person>().ExtendWith<IPerson>();

        extended.Age = 25;
        extended.Name = "Billy";

        Console.WriteLine(extended.Name + " is " + extended.Age);

        Console.Read();
    }
}

You can now see that the reason we used an interface to extend our newly created class is so that we can have a type-safe way of accessing its properties. If we simply returned an object type, we would be forced to access its properties by Reflection.

EDIT

The following modified version is now able to instantiate complex types located inside the interface, and implement the other simple ones.

The definition of the Person class stays the same, while the IPerson interface now becomes the following.

public interface IPerson
{
    string Name { get; set; }

    Person Person { get; set; }
}

The DynamicExtension class definition now changes to the following.

class DynamicExtension<T>
{
    public T Extend()
    {
        var assembly = AppDomain.CurrentDomain.DefineDynamicAssembly(new AssemblyName("Assembly"), AssemblyBuilderAccess.Run);
        var module = assembly.DefineDynamicModule("Module");
        var type = module.DefineType("Class", TypeAttributes.Public);

        type.AddInterfaceImplementation(typeof(T));

        foreach (var v in typeof(T).GetProperties())
        {
            var field = type.DefineField("_" + v.Name.ToLower(), v.PropertyType, FieldAttributes.Private);
            var property = type.DefineProperty(v.Name, PropertyAttributes.None, v.PropertyType, new Type[0]);
            var getter = type.DefineMethod("get_" + v.Name, MethodAttributes.Public | MethodAttributes.SpecialName | MethodAttributes.Virtual, v.PropertyType, new Type[0]);
            var setter = type.DefineMethod("set_" + v.Name, MethodAttributes.Public | MethodAttributes.SpecialName | MethodAttributes.Virtual, null, new Type[] { v.PropertyType });

            var getGenerator = getter.GetILGenerator();
            var setGenerator = setter.GetILGenerator();

            getGenerator.Emit(OpCodes.Ldarg_0);
            getGenerator.Emit(OpCodes.Ldfld, field);
            getGenerator.Emit(OpCodes.Ret);

            setGenerator.Emit(OpCodes.Ldarg_0);
            setGenerator.Emit(OpCodes.Ldarg_1);
            setGenerator.Emit(OpCodes.Stfld, field);
            setGenerator.Emit(OpCodes.Ret);

            property.SetGetMethod(getter);
            property.SetSetMethod(setter);

            type.DefineMethodOverride(getter, v.GetGetMethod());
            type.DefineMethodOverride(setter, v.GetSetMethod());
        }

        var instance = (T)Activator.CreateInstance(type.CreateType());

        foreach (var v in typeof(T).GetProperties().Where(x => x.PropertyType.GetConstructor(new Type[0]) != null))
        {
            instance.GetType()
                    .GetProperty(v.Name)
                    .SetValue(instance, Activator.CreateInstance(v.PropertyType), null);
        }

        return instance;
    }
}

We can now simply execute the following lines of code to get all the appropriate values.

class Program
{
    static void Main(string[] args)
    {
        var extended = new DynamicExtension<IPerson>().Extend();

        extended.Person.Age = 25;
        extended.Name = "Billy";

        Console.WriteLine(extended.Name + " is " + extended.Person.Age);

        Console.Read();
    }
}