Code Generation for Record.CompareTo(object) lacks type-safety checks

[EBrown8534]

When generating the CompareTo(object) for a record type, there is no type-safety for casting. Instead, the CompareTo(object) simply does a direct-cast to the record type. This means callers from C#/VB.NET to CompareTo(object) will get an InvalidCastException if they send an object that is not the correct type.

Repro steps

F# Code:

type TestType =
    { Name : string }

C# Code:

var testType = new FsLibrary.TestType("Test");
Console.WriteLine(testType.CompareTo("Test 2")); // Throws `System.InvalidCastException`

The generated ILASM for the TestType::CompareTo:

.method public hidebysig virtual final instance int32
        CompareTo(object obj) cil managed
{
  .custom instance void [mscorlib]
    System.Runtime.CompilerServices.CompilerGeneratedAttribute::.ctor() = ( 01 00 00 00 ) 
  // Code size       13 (0xd)
  .maxstack  8
  IL_0000:  ldarg.0
  IL_0001:  ldarg.1
  IL_0002:  unbox.any FsLibrary.TestType
  IL_0007:  callvirt instance int32 FsLibrary.TestType::CompareTo(class FsLibrary.TestType)
  IL_000c:  ret
} // end of method TestType::CompareTo

The obvious problem here is the unconditional unboxing of the argument to the FsLibrary.TestType. There is no attempt made to test the type for compatibility.

This can be more easily seen on sharplab.io:

https://sharplab.io/#v2:DYLgZgzgPgLgngBwKYAIAqSIzY1BeAWACgVSUBvFAOQEMBbVEFLAJwEsA7AcxQF9igA=

The generated C# for CompareTo(object) is:

[CompilerGenerated]
public sealed override int CompareTo(object obj)
{
    return CompareTo((TestType)obj);
}

The culprit here can be easily identified as (TestType)obj.

Expected behavior

The CompareTo(object) should be generated with a type-check, then if the type is compatible it should be casted.

Actual behavior

The generated CompareTo(object) is not properly compatible with other .NET languages.

Known workarounds

None. The C# version must implement it's own comparison.

Related information

• Windows 10 Pro for Workstations (10.0.16299.492)
• Public Visual Studio 2017 Community
• .NET 4.7.2
• Visual Studio 2017 Community 15.7.2
• Low/Medium, for someone like me who does a lot of C# <-> F# Interop it's far more severe