System.Reflection.Emit namespace

using System;
using System.Reflection;
using System.Reflection.Emit;

class DemoAssemblyBuilder
{
    public static void Main()
    {
        // An assembly consists of one or more modules, each of which
        // contains zero or more types. This code creates a single-module
        // assembly, the most common case. The module contains one type,
        // named "MyDynamicType", that has a private field, a property 
        // that gets and sets the private field, constructors that 
        // initialize the private field, and a method that multiplies 
        // a user-supplied number by the private field value and returns
        // the result. In C# the type might look like this:
        /*
        public class MyDynamicType
        {
            private int m_number;

            public MyDynamicType() : this(42) {}
            public MyDynamicType(int initNumber)
            {
                m_number = initNumber;
            }

            public int Number
            {
                get { return m_number; }
                set { m_number = value; }
            }

            public int MyMethod(int multiplier)
            {
                return m_number * multiplier;
            }
        }
        */

        AssemblyName aName = new AssemblyName("DynamicAssemblyExample");
        AssemblyBuilder ab =
            AppDomain.CurrentDomain.DefineDynamicAssembly(
                aName,
                AssemblyBuilderAccess.RunAndSave);

        // For a single-module assembly, the module name is usually
        // the assembly name plus an extension.
        ModuleBuilder mb =
            ab.DefineDynamicModule(aName.Name, aName.Name + ".dll");

        TypeBuilder tb = mb.DefineType(
            "MyDynamicType",
             TypeAttributes.Public);

        // Add a private field of type int (Int32).
        FieldBuilder fbNumber = tb.DefineField(
            "m_number",
            typeof(int),
            FieldAttributes.Private);

        // Next, we make a simple sealed method.
        MethodBuilder mbMyMethod = tb.DefineMethod(
            "MyMethod",
            MethodAttributes.Public,
            typeof(int),
            new[] { typeof(int) });

        ILGenerator il = mbMyMethod.GetILGenerator();
        il.Emit(OpCodes.Ldarg_0); // Load this - always the first argument of any instance method
        il.Emit(OpCodes.Ldfld, fbNumber);
        il.Emit(OpCodes.Ldarg_1); // Load the integer argument
        il.Emit(OpCodes.Mul); // Multiply the two numbers with no overflow checking
        il.Emit(OpCodes.Ret); // Return

        // Next, we build the property. This involves building the property itself, as well as the
        // getter and setter methods.
        PropertyBuilder pbNumber = tb.DefineProperty(
            "Number", // Name
            PropertyAttributes.None, 
            typeof(int), // Type of the property
            new Type[0]); // Types of indices, if any

        MethodBuilder mbSetNumber = tb.DefineMethod(
            "set_Number", // Name - setters are set_Property by convention
            // Setter is a special method and we don't want it to appear to callers from C#
            MethodAttributes.PrivateScope | MethodAttributes.HideBySig | MethodAttributes.Public | MethodAttributes.SpecialName,
            typeof(void), // Setters don't return a value
            new[] { typeof(int) }); // We have a single argument of type System.Int32

        // To generate the body of the method, we'll need an IL generator
        il = mbSetNumber.GetILGenerator();
        il.Emit(OpCodes.Ldarg_0); // Load this
        il.Emit(OpCodes.Ldarg_1); // Load the new value
        il.Emit(OpCodes.Stfld, fbNumber); // Save the new value to this.m_number
        il.Emit(OpCodes.Ret); // Return

        // Finally, link the method to the setter of our property
        pbNumber.SetSetMethod(mbSetNumber);

        MethodBuilder mbGetNumber = tb.DefineMethod(
            "get_Number",
            MethodAttributes.PrivateScope | MethodAttributes.HideBySig | MethodAttributes.Public | MethodAttributes.SpecialName,
            typeof(int),
            new Type[0]);

        il = mbGetNumber.GetILGenerator();
        il.Emit(OpCodes.Ldarg_0); // Load this
        il.Emit(OpCodes.Ldfld, fbNumber); // Load the value of this.m_number
        il.Emit(OpCodes.Ret); // Return the value

        pbNumber.SetGetMethod(mbGetNumber);
        
        // Finally, we add the two constructors.
        // Constructor needs to call the constructor of the parent class, or another constructor in the same class
        ConstructorBuilder intConstructor = tb.DefineConstructor(
            MethodAttributes.Public, CallingConventions.Standard | CallingConventions.HasThis, new[] { typeof(int) });
        il = intConstructor.GetILGenerator();
        il.Emit(OpCodes.Ldarg_0); // this
        il.Emit(OpCodes.Call, typeof(object).GetConstructor(new Type[0])); // call parent's constructor
        il.Emit(OpCodes.Ldarg_0); // this
        il.Emit(OpCodes.Ldarg_1); // our int argument
        il.Emit(OpCodes.Stfld, fbNumber); // store argument in this.m_number
        il.Emit(OpCodes.Ret);

        var parameterlessConstructor = tb.DefineConstructor(
            MethodAttributes.Public, CallingConventions.Standard | CallingConventions.HasThis, new Type[0]);
        il = parameterlessConstructor.GetILGenerator();
        il.Emit(OpCodes.Ldarg_0); // this
        il.Emit(OpCodes.Ldc_I4_S, (byte)42); // load 42 as an integer constant
        il.Emit(OpCodes.Call, intConstructor); // call this(42)
        il.Emit(OpCodes.Ret);

        // And make sure the type is created
        Type ourType = tb.CreateType();

        // The types from the assembly can be used directly using reflection, or we can save the assembly to use as a reference
        object ourInstance = Activator.CreateInstance(ourType);
        Console.WriteLine(ourType.GetProperty("Number").GetValue(ourInstance)); // 42
        
        // Save the assembly for use elsewhere. This is very useful for debugging - you can use e.g. ILSpy to look at the equivalent IL/C# code.
        ab.Save(@"DynamicAssemblyExample.dll");
        // Using newly created type
        var myDynamicType = tb.CreateType();
        var myDynamicTypeInstance = Activator.CreateInstance(myDynamicType);

        Console.WriteLine(myDynamicTypeInstance.GetType()); // MyDynamicType

        var numberField = myDynamicType.GetField("m_number", BindingFlags.NonPublic | BindingFlags.Instance);
        numberField.SetValue (myDynamicTypeInstance, 10);

        Console.WriteLine(numberField.GetValue(myDynamicTypeInstance)); // 10
    }
}