InvalidCastException in Xamarin iOS - xamarin

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.

Answer 1

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.