using System.Reflection;
using System.Reflection.Emit;
using System.Runtime.Loader;

PersistedAssemblyBuilder ab = new PersistedAssemblyBuilder(new AssemblyName("MyAssembly"), typeof(object).Assembly);
ModuleBuilder mob = ab.DefineDynamicModule("MyModule");

TypeBuilder exp = mob.DefineType("ExplicitLayoutType", TypeAttributes.Public | TypeAttributes.Class | TypeAttributes.ExplicitLayout, typeof(ValueType), 8);
var f1 = exp.DefineField("first", typeof(int), FieldAttributes.Public);
f1.SetOffset(0);
var f2 = exp.DefineField("second", typeof(int), FieldAttributes.Public);
f2.SetOffset(4);
exp.CreateType();

TypeBuilder tb = mob.DefineType("MyType", TypeAttributes.Public | TypeAttributes.Class);
tb.DefineField("structfield", exp, FieldAttributes.Public | FieldAttributes.Static);

MethodBuilder meb = tb.DefineMethod("SumMethod", MethodAttributes.Public | MethodAttributes.Static,
                                                        typeof(int), new Type[] { typeof(int), typeof(int) });
ILGenerator il = meb.GetILGenerator();
il.Emit(OpCodes.Ldarg_0);
il.Emit(OpCodes.Ldarg_1);
il.Emit(OpCodes.Add);
il.Emit(OpCodes.Ret);

tb.CreateType();

using var stream = new MemoryStream();
ab.Save(stream); 
stream.Seek(0, SeekOrigin.Begin);
Assembly assembly = AssemblyLoadContext.Default.LoadFromStream(stream);
MethodInfo method = assembly.GetType("MyType").GetMethod("SumMethod");
Console.WriteLine(method.Invoke(null, new object[] { 5, 10 }));