InvalidCastException in Xamarin iOS

Question

I'm receiving an InvalidCastException when running my code in Xamarin iOS. This doesn't happen for .NET or Android - only for iOS. Here's the exception details:

at Microsoft.Scripting.Interpreter.EqualInstruction+EqualInt64.DoCalculate (System.Object l, System.Object r) [0x00000] in /Users/builder/data/lanes/3426/6c3fee4d/source/xamarin-macios/_ios-build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/dlr/Runtime/Microsoft.Dynamic/Interpreter/Instructions/EqualInstruction.cs:84 at Microsoft.Scripting.Interpreter.ComparisonInstruction.Calculate (System.Object l, System.Object r) [0x00015] in /Users/builder/data/lanes/3426/6c3fee4d/source/xamarin-macios/_ios-build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/dlr/Runtime/Microsoft.Dynamic/Interpreter/Instructions/ComparisonInstruction.cs:44 at Microsoft.Scripting.Interpreter.ArithmeticInstruction.Run (Microsoft.Scripting.Interpreter.InterpretedFrame frame) [0x00020] in /Users/builder/data/lanes/3426/6c3fee4d/source/xamarin-macios/_ios-build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/dlr/Runtime/Microsoft.Dynamic/Interpreter/Instructions/ArithmeticInstruction.cs:45 at Microsoft.Scripting.Interpreter.Interpreter.Run (Microsoft.Scripting.Interpreter.InterpretedFrame frame) [0x0001b] in /Users/builder/data/lanes/3426/6c3fee4d/source/xamarin-macios/_ios-build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/dlr/Runtime/Microsoft.Dynamic/Interpreter/Interpreter.cs:126

--- End of stack trace from previous location where exception was thrown --- at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw () [0x0000c] in /Users/builder/data/lanes/3426/6c3fee4d/source/xamarin-macios/_ios-build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/referencesource/mscorlib/system/runtime/exceptionservices/exceptionservicescommon.cs:143 at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess (System.Threading.Tasks.Task task) [0x00047] in /Users/builder/data/lanes/3426/6c3fee4d/source/xamarin-macios/_ios-build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:187 at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification (System.Threading.Tasks.Task task) [0x0002e] in /Users/builder/data/lanes/3426/6c3fee4d/source/xamarin-macios/_ios-build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:156 at System.Runtime.CompilerServices.TaskAwaiter.ValidateEnd (System.Threading.Tasks.Task task) [0x0000b] in /Users/builder/data/lanes/3426/6c3fee4d/source/xamarin-macios/_ios-build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:128 at System.Runtime.CompilerServices.TaskAwaiter`1[TResult].GetResult () [0x00000] in /Users/builder/data/lanes/3426/6c3fee4d/source/xamarin-macios/_ios-build/Library/Frameworks/Xamarin.iOS.framework/Versions/git/src/mono/mcs/class/referencesource/mscorlib/system/runtime/compilerservices/TaskAwaiter.cs:357 at XamarinFormsPortable.Models.TweetingViewModel+d__4.MoveNext () [0x00328] in C:\Projects\LinqToTwitter\Samples\Xamarin4\XamarinFormsPortable\XamarinFormsPortable\XamarinFormsPortable\Models\TweetingViewModel.cs:54

The code that causes the problem is a LINQ to Twitter query:

            var ctx = new TwitterContext(auth);
            ulong InitialTweetId = 1;
            int MaximumTweetsToRetrieve = 20;

            var tweetsQuery = from tweet in ctx.Status
                              where tweet.Type == StatusType.User &&
                              tweet.ScreenName == "reinoso_alvaro" &&
                              tweet.Text.Contains("#buenosdias") &&
                              tweet.IncludeRetweets == false &&
                              tweet.TrimUser == true &&
                              tweet.ExcludeReplies == true &&
                              tweet.SinceID == InitialTweetId &&
                              tweet.Count == MaximumTweetsToRetrieve
                              select tweet;
            var tweetList = await tweetsQuery.ToListAsync();

The condition that causes the problem is SinceID, which is ulong. However, you can see from the stack trace that the last method is EqualInt64, which is long. I looked for every place where I worked with SinceID and they are all ulong. The exception occurs on the last line: awaiting tweetsQuery.ToListAsync()

The implementation of ToListAsync is:

    public static async Task<List<T>> ToListAsync<T>(this IQueryable<T> query)
    {
        var provider = query.Provider as TwitterQueryProvider;

        IEnumerable<T> results = (IEnumerable<T>)await provider.ExecuteAsync<IEnumerable<T>>(query.Expression).ConfigureAwait(false);

        return results.ToList();
    }

ExecuteAsync runs and returns results. Then the exception occurs when materializing via results.ToList().

This is only happening on iOS.

@Sven-MichaelStübe Without SinceID, It doesn't throw an exception, but logically it won't work because it's a necessary for paging. — Joe Mayo

Jonathan Derrough Jonathan Derrough · Accepted Answer · 2017-02-08T14:00:31

A possible workaround is to cast the Status.SinceID property in the expression:

[...]
&& (long)tweet.SinceID == sinceStatusId // sinceStatusId is a long
[...]

Interesting point, Mono's Interpreter object comparison instructions for unsigned integers are assigned instances of their signed counterparts (https://github.com/mono/mono/blob/mono-4.8.0-branch/mcs/class/dlr/Runtime/Microsoft.Dynamic/Interpreter/Instructions/EqualInstruction.cs , line 155 to 157):

case TypeCode.UInt16: return _UInt16 ?? (_UInt16 = new EqualInt16());
case TypeCode.UInt32: return _UInt32 ?? (_UInt32 = new EqualInt32());
case TypeCode.UInt64: return _UInt64 ?? (_UInt64 = new EqualInt64());

Not sure if this is intentional or not.

InvalidCastException in Xamarin iOS

1 Answers