Dynamically define C# value types (structs) at runtime with a dynamic assembly

gistfile1.cs

using System;
using System.Collections.Generic;
using System.Linq;
using System.Reflection;
using System.Reflection.Emit;
using System.Runtime.InteropServices;
using System.Text;

namespace DevConsole
{
    class Program
    {
        static void Main(string[] args)
        {
            // Create assembly and module
            var assemblyName = new AssemblyName("DynamicAssemblyExample");
            var assemblyBuilder = AppDomain.CurrentDomain.
                DefineDynamicAssembly(assemblyName, AssemblyBuilderAccess.RunAndSave);
            var moduleBuilder = assemblyBuilder.
                DefineDynamicModule(assemblyName.Name, assemblyName.Name + ".dll");

            // Open a new Type for definition
            var typeBuilder = moduleBuilder.DefineType("MyDynamicType",
                TypeAttributes.Public |
                TypeAttributes.Sealed |
                TypeAttributes.ExplicitLayout |
                TypeAttributes.Serializable |
                TypeAttributes.AnsiClass,
                typeof(ValueType));

            // Find MarshalAsAttribute's constructor by signature, then invoke
            var ctorParameters = new Type[] { typeof(UnmanagedType) };
            var ctorInfo = typeof(MarshalAsAttribute).GetConstructor(ctorParameters);

            var fields = typeof(MarshalAsAttribute).GetFields(BindingFlags.Public | BindingFlags.Instance);
            var sizeConst = (from f in fields
                            where f.Name == "SizeConst"
                            select f).ToArray();
            var marshalAsAttr = new CustomAttributeBuilder(ctorInfo,
                new object[] { UnmanagedType.ByValTStr }, sizeConst, new object[] { 5 });

            var fbString = typeBuilder.DefineField(
                "m_string",
                typeof(string),
                FieldAttributes.Public);
            fbString.SetOffset(0);
            fbString.SetCustomAttribute(marshalAsAttr);

            var fbNumber = typeBuilder.DefineField(
                "m_number",
                typeof(int),
                FieldAttributes.Public);
            fbNumber.SetOffset(8);

            var type = typeBuilder.CreateType();
            bool isVal = type.IsValueType; // true
        }
    }
}