gistfile1.txt

using System.Linq;
using Mono.Cecil;
using Mono.Cecil.Cil;

namespace FixActionBar
{
    internal class Program
    {
        private static void Main(string[] args)
        {
            var fileName = @"_SOURCE_FOLDER_\lib\MonoAndroid10\Xamarin.Forms.Platform.Android.dll";
            var typeName = "Xamarin.Forms.Platform.Android.Platform";


            var assembly = AssemblyDefinition.ReadAssembly(fileName);
            var types = assembly.Modules.SelectMany(_ => _.Types).ToDictionary(_ => _.Namespace + "." + _.Name);

            var platformType = types[typeName];

            var updateActionBarHomeAsUp = platformType.Methods.First(_ => _.Name == "UpdateActionBar");
            var actionBarGetter = platformType.Properties.First(_ => _.Name == "ActionBar").GetMethod;

            var ilProcessor = updateActionBarHomeAsUp.Body.GetILProcessor();

            var firstInstruction = ilProcessor.Body.Instructions[0];
            ilProcessor.InsertBefore(firstInstruction, Instruction.Create(OpCodes.Ldarg_0));
            ilProcessor.InsertBefore(firstInstruction, Instruction.Create(OpCodes.Call, actionBarGetter));
            ilProcessor.InsertBefore(firstInstruction, Instruction.Create(OpCodes.Brtrue_S, firstInstruction));
            ilProcessor.InsertBefore(firstInstruction, Instruction.Create(OpCodes.Ret));

            assembly.Write(@"_DEST_FOLDER_\Xamarin.Forms.Platform.Android.dll");
        }
    }
}