ReactiveUI binding crash Xamarin.Android

[RLittlesII]

Bug

Despite our best efforts, bugs can slip into releases or corner cases forgotten about. We will try our best to remedy the situation
and/or provide workarounds. Note that certain (odd) behaviors are by design and as such are not considered bugs.

Which library version?

Was reported against 4.1.6, still present in 4.3.2

What are the platform(s), environment(s) and related component version(s)?

Xamarin.Android

What is the use case or problem?

In ReacitveUI we use Reflection and Tuples to provide Two Way Binding to UI elements. It seems that the reflection spits a specific error now when it tries to bind on Xamarin.Android. The issue was reported after we migrated to Value Tuples.

This is related to reactiveui/ReactiveUI#2170

What is the expected outcome?

That the bindings are found and Android handles the Two Way binding.

What is the actual outcome?

The following error is produced. The Android App crashes.

What is the stacktrace of the exception(s) if any?

https://gist.github.com/RLittlesII/13c15d8047ae906aee0ad585c097f0f2

Do you have a code snippet or project that reproduces the problem?

reactiveui.2170.zip

[glennawatson]

This error seems to be related to android 9.0. It doesn't seem to happen with 8.1.

[clintonrocksmith-skedulo]

We also have experienced this error and have had to refactor our code to use non-reactive approaches with Native Android Xamarin/Mono using .NetStandard. It fails at runtime and caused us delays in our release cycles.

[clintonrocksmith-skedulo]

I've also created a Xamarin project solution that demonstrates the bug

https://github.com/clintonrocksmith/RxCrash

[glennawatson]

It also appears the ValueTuple is a red herring and it was the upgrade to Andoid 9 for rxui that caused the binding to break.

[KevinvdBurg]

@glennawatson I have tested it on Android 8, 8.1 and 9 and every version has this issue.

[akarnokd]

I'm not experienced with .NET at this depth. Could you create a test case that fails on a desktop runtime?

[glennawatson]

@akarnokd Only happens on xamarin android runtime. Works fine on other platforms

[danielcweber]

It seems there's a creation of an observable from an event pattern, and the event handler delegate doesn't match the actual method signature of the method to wrap.

If the method to create a delegate from is EditText.Text and ReactiveUI tries to get the right method from the x => x.Text expression, a good guess is that it just got the wrong method. But from what I see there haven't been any changes in signature or any additional signatures in Android 9.

Anyway, if ReactiveUI finds the wrong delegate for a given property expression, it's more probably due to a change in the Xamarin bindings vs. a change in Android. But there doesn't seem to be a change either.

[RLittlesII]

@danielcweber That's as far as I've been able to gather. I will see if I can get closer to which method it is attempting to use.

[gerardvanderkruijs]

Any update on this issue?

[Lapinou42]

I got the same issue here on Xamarin.Android.

• Android target SDK: 10.0
• VS for Mac : up to date
• ReactiveUI: up to date

I tried to debug and I the exception is thrown here:

If I checked this two var:

ParameterInfo[] delargs = invoke.GetParametersInternal ();
ParameterInfo[] args = method.GetParametersInternal ();

delargs has 2 values and args has 1 value.

Then, if we check the condition:

if (target != null)
{
    // delegate closed over target
    if (!method.IsStatic)
        // target is passed as this
        argLengthMatch = (args.Length == delargs.Length);
    else
        // target is passed as the first argument to the static method
        argLengthMatch = (args.Length == delargs.Length + 1);
}
else
{
    if (!method.IsStatic)
    {
        //
        // Net 2.0 feature. The first argument of the delegate is passed
        // as the 'this' argument to the method.
        //
        argLengthMatch = (args.Length + 1 == delargs.Length);

        if (!argLengthMatch)
            // closed over a null reference
            argLengthMatch = (args.Length == delargs.Length);
    }
    else
    {
        argLengthMatch = (args.Length == delargs.Length);

        if (!argLengthMatch)
            // closed over a null reference
            argLengthMatch = args.Length == delargs.Length + 1;
    }
}

=> target is not null
=> the method is not static.
So it goes to the condition:

// target is passed as this
argLengthMatch = (args.Length == delargs.Length);

So argLenghtMatch would be false and exception will be throw.

But I see few lines down this comment:

 //
// Net 2.0 feature. The first argument of the delegate is passed
// as the 'this' argument to the method.
//
argLengthMatch = (args.Length + 1 == delargs.Length);

If this code was executed, argLengthMatch will be true and no exception will be throw.

What about a code that seems like this, is this a possible solution ?

if (target != null)
{
    // delegate closed over target
    if (!method.IsStatic)
        // target is passed as this
        //
        // Net 2.0 feature. The first argument of the delegate is passed
        // as the 'this' argument to the method.
        //
        argLengthMatch = (args.Length + 1 == delargs.Length);

        if (!argLengthMatch)
            // closed over a null reference
            argLengthMatch = (args.Length == delargs.Length);
    else
        // target is passed as the first argument to the static method
        argLengthMatch = (args.Length == delargs.Length + 1);
}
else
{
    if (!method.IsStatic)
    {
        //
        // Net 2.0 feature. The first argument of the delegate is passed
        // as the 'this' argument to the method.
        //
        argLengthMatch = (args.Length + 1 == delargs.Length);

        if (!argLengthMatch)
            // closed over a null reference
            argLengthMatch = (args.Length == delargs.Length);
    }
    else
    {
        argLengthMatch = (args.Length == delargs.Length);

        if (!argLengthMatch)
            // closed over a null reference
            argLengthMatch = args.Length == delargs.Length + 1;
    }
}

Is this an issue of Xamarin teams ? (File: System.Delegate.cs)

[lwschan]

No reply from anyone? :(

[lwschan]

Do we have someone from Rx.NET or ReactiveUI looking into this? :(