Simple WIP Mixin Framework for C# using Mono.Cecil
When working with third-party assemblies, access levels aren't always how you'd want them to be. Sometimes it's necessary to make certain things public or non-final (readonly / sealed).
The PreSharpin class can be used to apply access transformers to a target assembly, which you can then add as a Reference to your Mixin project instead of the original assembly.
PreSharpin.ApplyAccessTransformer("CleanTest.exe", "CleanTest-presharp.exe", @"
# this is a comment
public-f CleanTest.PrivateTest
public System.Boolean CleanTest.PrivateTest::IsThisGud()
public System.Boolean CleanTest.PrivateTest::IsThisGud(System.String)
public System.String CleanTest.PrivateTest::noseepls # I can see!
public-f System.Int32 CleanTest.PrivateTest::notouchpls
");Modifiers can be applied to classes, methods and fields.
publicwill simply make the target public-fwill strip readonly for fields and sealed for classes-nswill add a [NonSerialized] attribute to the field (this is necessary when making fields public that are serialized, like in Unity's MonoBehaviour; otherwise Unity will not be able to load scenes with this behaviour)
[Mixin(typeof(RudeBear))]
public abstract class MixinRudeBear : RudeBear {
[Inject(method = "System.Void RudeBear::CommonDeath(System.Int32)", at = "HEAD", cancellable = true)]
public void CommonDeath(int specialText, CallbackInfo info) {
if(RudeAPIHooks.OnDeath(this)) {
Debug.Log("Not today!");
info.Cancel();
}
}
}will (simplified) result in
public class RudeBear {
// [...]
public void CommonDeath(int specialText) {
if(RudeAPIHooks.OnDeath(this)) {
Debug.Log("Not today!");
return;
}
// [... the rest of CommonDeath ...]
}
// [...]
}[Mixin(typeof(SpearKill))]
public abstract class MixinSpearKill : SpearKill {
[Overwrite]
public void OnTriggerEnter2D(UnityEngine.Collider2D col) {
UnityEngine.Debug.Log("Spears were never meant to be used for killing.");
}
}public interface IMonster {
public int Health { get; set; }
}
[Mixin(typeof(StardewValley.Monsters.Monster))]
[Implements(typeof(IMonster))]
public class MixinMonster : StardewValley.Monsters.Monster, IMonster {
public int Health {
get { return this.health; }
set { this.health = value; }
}
}Pretend we have SomeCharacter with an OnDamage function that takes a base value, and inside it does calculations to get a modified value based on the character's armor stats or something.
These are the locals (with name, pretend it's a Debug build):
.locals init (
[0] int32 armorStat,
[1] int32 modifiedDamage
)
Sadly locals lose their names on Release builds, so we have to access them by index.
The following will grab the modifiedDamage right before it's applied via SomeCharacter::ReduceHealth
[Inject(method = "System.Void SomeCharacter::OnDamage(System.Int32)", at = "IL_0017: call System.Void SomeCharacter::ReduceHealth(System.Int32)")]
public void OnDamage(int baseDamage, [CaptureLocal(1, typeof(int))] int modifiedDamage, [StoreLocal(1, typeof(int))] out int outModifiedDamage) {
if(modifiedDamage == 1336) { // Ugh, so close. Let's just sneakily add one to make it happen.
outModifiedDamage = modifiedDamage + 1;
}
}There are two inbuilt injection points for the at parameter: HEAD and RETURN.
HEADwill always be right before the first statement of the target methodRETURNwill be before any return statement in the target method - this can be multiple, so set the expectedInjections attribute accordingly
Apart from these, specific instructions can be named in a similar manner that you would see them in ILSpy, for example:
IL_0017: call System.Void SomeCharacter::ReduceHealth(System.Int32)
Note that injection points will always be shifted backwards to a state of empty stack. This means that if we have bytecode that looks like
IL_0009: ldloc.0 IL_000a: retand we specify
RETURNorIL_000a: retas injection point, the actual injection point will end up right beforeIL_0009: ldloc.0.
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