inject byte[] using dnlib

using System;
using System.Collections.Generic;
using System.IO;
using dnlib.DotNet;
using dnlib.DotNet.Emit;

class Program
{
    // returns FieldDef of injected byte[]
    static FieldDef InjectArray(ModuleDefMD mod, byte[] injectedData, string injectedName)
    {
        // we'll have to import lots of new stuff into our module
        Importer importer = new Importer(mod);

        // add class with layout
        ITypeDefOrRef valueTypeRef = importer.Import(typeof(System.ValueType));
        TypeDef classWithLayout = new TypeDefUser("dummyClass", valueTypeRef);
        classWithLayout.Attributes |= TypeAttributes.Sealed | TypeAttributes.ExplicitLayout;
        classWithLayout.ClassLayout = new ClassLayoutUser(1, (uint)injectedData.Length);
        mod.Types.Add(classWithLayout);

        // add field with proper InitialValue
        FieldDef fieldWithRVA = new FieldDefUser("dummyField", new FieldSig(classWithLayout.ToTypeSig()), FieldAttributes.Static | FieldAttributes.Assembly | FieldAttributes.HasFieldRVA);
        fieldWithRVA.InitialValue = injectedData;
        mod.GlobalType.Fields.Add(fieldWithRVA);

        // add byte[] field
        ITypeDefOrRef byteArrayRef = importer.Import(typeof(System.Byte[]));
        FieldDef fieldInjectedArray = new FieldDefUser(injectedName, new FieldSig(byteArrayRef.ToTypeSig()), FieldAttributes.Static | FieldAttributes.Public);
        mod.GlobalType.Fields.Add(fieldInjectedArray);

        // and finally add code to global .cctor to initialize array.
        /*
          ldc.i4     XXXsizeofarrayXXX
          newarr     [mscorlib]System.Byte
          dup
          ldtoken    field valuetype className fieldName
          call       void [mscorlib]System.Runtime.CompilerServices.RuntimeHelpers::InitializeArray(class [mscorlib]System.Array, valuetype [mscorlib]System.RuntimeFieldHandle)
          stsfld     uint8[] bla
         */
        ITypeDefOrRef systemByte = importer.Import(typeof(System.Byte));
        ITypeDefOrRef runtimeHelpers = importer.Import(typeof(System.Runtime.CompilerServices.RuntimeHelpers));
        IMethod initArray = importer.Import(typeof(System.Runtime.CompilerServices.RuntimeHelpers).GetMethod("InitializeArray", new Type[] { typeof(System.Array), typeof(System.RuntimeFieldHandle) }));

        MethodDef cctor = mod.GlobalType.FindOrCreateStaticConstructor();
        IList<Instruction> instrs = cctor.Body.Instructions;
        instrs.Insert(0, new Instruction(OpCodes.Ldc_I4, injectedData.Length));
        instrs.Insert(1, new Instruction(OpCodes.Newarr, systemByte));
        instrs.Insert(2, new Instruction(OpCodes.Dup));
        instrs.Insert(3, new Instruction(OpCodes.Ldtoken, fieldWithRVA));
        instrs.Insert(4, new Instruction(OpCodes.Call, initArray));
        instrs.Insert(5, new Instruction(OpCodes.Stsfld, fieldInjectedArray));

        return fieldInjectedArray;
    }

    static void Main(string[] args)
    {
        ModuleDefMD mod = ModuleDefMD.Load(args[0]);
        FieldDef x = InjectArray(mod, new byte[] { 1, 2, 3, 4, 5, 6 }, "hello world!");
        mod.Write(Path.ChangeExtension(args[0], "patched" + Path.GetExtension(args[0])));
    }
}