Calling a C# interface method crashes Godot immediately

[johnfn]

Godot version

4.0 beta10

System information

MacOS Catalina

Issue description

Calling an interface method crashes Godot immediately. The game closes with no stack trace.

Steps to reproduce

Put this code on any node:

using Godot;
using System;

interface IFoo {
  public void TestMethod() {
    GD.Print("Hello");
  }
}

public partial class Test : Node2D, IFoo {
  public override void _Ready() {
    TestMethod();
  }
  
  public void TestMethod() {
    (this as IFoo).TestMethod();
  }
}

Minimal reproduction project

https://github.com/johnfn/CsharpGodotCrashRepro

[raulsntos]

The code in the MRP results in a stackoverflow because TestMethod calls itself. It seems your intention was for the Test.TestMethod implementation to call the default interface implementation but by implementing the method you are essentially overriding the interface method.

This is the output I get in the terminal when launching the Godot project in Linux:

Stack overflow.
Repeat 261612 times:
--------------------------------
   at Test.TestMethod()
--------------------------------
   at Test._Ready()
   at Godot.Node.InvokeGodotClassMethod(Godot.NativeInterop.godot_string_name ByRef, Godot.NativeInterop.NativeVariantPtrArgs, Godot.NativeInterop.godot_variant ByRef)
   at Godot.CanvasItem.InvokeGodotClassMethod(Godot.NativeInterop.godot_string_name ByRef, Godot.NativeInterop.NativeVariantPtrArgs, Godot.NativeInterop.godot_variant ByRef)
   at Godot.Node2D.InvokeGodotClassMethod(Godot.NativeInterop.godot_string_name ByRef, Godot.NativeInterop.NativeVariantPtrArgs, Godot.NativeInterop.godot_variant ByRef)
   at Test.InvokeGodotClassMethod(Godot.NativeInterop.godot_string_name ByRef, Godot.NativeInterop.NativeVariantPtrArgs, Godot.NativeInterop.godot_variant ByRef)
   at Godot.Bridge.CSharpInstanceBridge.Call(IntPtr, Godot.NativeInterop.godot_string_name*, Godot.NativeInterop.godot_variant**, Int32, Godot.NativeInterop.godot_variant_call_error*, Godot.NativeInterop.godot_variant*)
zsh: IOT instruction (core dumped)  godot4

I think what you are looking for is this:

public partial class Test : IFoo
{
	public void TestMethod()
	{
		base(IFoo).TestMethod();
	}
}

This feature was cut from C# 8.0 but the intention is to bring it back in a future version of the language, see https://github.com/dotnet/csharplang/blob/main/meetings/2019/LDM-2019-04-29.md#default-interface-implementations-and-base-calls.

You can follow the base(T) feature in the champion issue: dotnet/csharplang#2337.

Since this is not a Godot issue but a limitation of the C# language I'll close this issue.

[johnfn]

Hmm, even if I got the code wrong, I disagree that this is not an issue at all: the game should certainly not crash without so much as an error message.

[raulsntos]

Unfortunately we don't have any control over stackoverflows, it's a special type of exception that can't really be catched.