Passing null for array of blittable types results in NullReferenceException

[elinor-fung]

Generated code doesn't handle null when marshalling an array of blittable types.

[GeneratedDllImport(NativeExportsNE_Binary, EntryPoint = "sum_int_array")]
public static partial int Sum(int[] values, int numValues);
...
NativeExportsNE.Sum(null, 0);

In the generated code, we pass null to MemoryMarshal.GetArrayDataReference throws NullReferenceException:

fixed (int* __values_gen_native = &System.Runtime.InteropServices.MemoryMarshal.GetArrayDataReference(values))
    __retVal = Sum__PInvoke__(__values_gen_native, numValues);

cc @AaronRobinsonMSFT @jkoritzinsky

[jkotas]

Also, this should generate just fixed (int* __values_gen_native = values) for concrete arrays.

[jkoritzinsky]

We can't just do fixed (int* __values_gen_native = values) because that results in a null pointer for a zero-length array, which breaks P/Invokes to libraries like GDI+.

[AaronRobinsonMSFT]

@jkoritzinsky I recall this being an issue we wanted to fix but given the compat burden we decided not to do that. We should have a conversation around the GDI+ API surface and see if it is something we want to support. I'm not convinced a single API surface should force this decision.

[jkotas]

These bogus null pointer checks are not unusual in the Windows APIs. For example here: https://github.com/dotnet/runtime/blob/01b7e73cd378145264a7cb7a09365b41ed42b240/src/libraries/System.Private.CoreLib/src/System/Globalization/CompareInfo.Icu.cs#L733 or here https://github.com/dotnet/runtime/pull/52030/files#diff-4b9bcd238c75b08d184c630e8e90372464f848bbb2ed0466c0c3bafe74ee27b7R23

The eventual marshaling of Spans will hit similar problem.

[jkoritzinsky]

Fixed by #1063