Bug 16900 - Nullable cast does strict cast first causing NullReferenceException
Summary: Nullable cast does strict cast first causing NullReferenceException
Alias: None
Product: Compilers
Classification: Mono
Component: C# ()
Version: 3.2.x
Hardware: PC Windows
: --- normal
Target Milestone: ---
Assignee: Marek Safar
Depends on:
Reported: 2013-12-19 15:46 UTC by Jason Morse
Modified: 2014-01-02 12:53 UTC (History)
1 user (show)

Is this bug a regression?: ---
Last known good build:

Sample program illustration issue (140.34 KB, application/x-zip-compressed)
2013-12-30 15:33 UTC, Jason Morse

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Description Jason Morse 2013-12-19 15:46:12 UTC
Upcasting from interface to nullable struct implementation causes Mono compiler to perform strict cast *before* nullable cast. When executing against standard Microsoft .NET 2.0 CLR this causes NullReferenceException (unsure about behavior when used against Mono runtime).

Sample project attached. Solution configurations are available for Microsoft (Debug)and Mono (Debug-Mono).

            IMyStruct myStruct = null;
            MyStruct? structValue;

            structValue = (Nullable<MyStruct>)myStruct;  // <-- NullReferenceException when compiled with Mono

            // Equivalent generated by csc.exe (v4.0)
            structValue = (MyStruct?)myStruct;

            // Equivalent generated by msc.exe (v3.2.3)
            structValue = new MyStruct?((MyStruct)myStruct);

    public struct MyStruct : IMyStruct { }

    public interface IMyStruct { }
Comment 1 Jason Morse 2013-12-19 15:59:40 UTC
Verified mono runtime exhibits same behavior.
Comment 2 Jason Morse 2013-12-30 15:33:08 UTC
Created attachment 5740 [details]
Sample program illustration issue

Visual Studio 2010 project/solution.
Comment 3 Marek Safar 2014-01-02 12:53:22 UTC
Fixed in master