Using readonly property as target of ByRef

[jrmoreno1]

Microsoft Visual Studio Enterprise 2015
Version 14.0.25123.00 Update 2
Microsoft .NET Framework
Version 4.6.01590

Visual Basic 2015 00322-90150-04967-AA372
Microsoft Visual Basic 2015

Steps to Reproduce:
Compile

Dim nlDate As Date? = #1/01/2015#
If (Date.TryParse("07/04/2016", nlDate.Value)) Then
Console.WriteLine(nlDate)
End If

Expected Behavior:
Compiler error ('ReadOnly' Property cannot be the target of an assignment)
Actual Behavior:
Compiles, and when run the parse succeeds, but the nullable date is unchanged. Checked with VS 2013, and same behavior, so it's not a change due to Roslyn, but I do think it's an error.

[gafter]

I believe the VB language spec specifically permits this, instead sending a ref to the temporary value that it has read the value into. @AnthonyDGreen Can you please confirm?

[markhurd]

Because VB.NET doesn't acknowledge Out parameters as more than ByRef, this makes sense: as far as VB is concerned you might just be expecting to pass in the property as an argument.

[paul1956]

Please someone fix VB so it acknowledge "Out" parameters even if it is only for new code. Currently the "Out" attribute is completely ignored. Lack of this makes code less clear and for readonly variables being passed to Out parameters subtle bugs.

[jrmoreno1]

@markhurd : ref or out, it doesn't make a difference. Either way you are potentially making an assigment to a readonly value. If you only need the value, then the argument should be ByVal.

[jrmoreno1]

@gafter: VB.NET does allow using properties for ByRef arguments, but it shouldn't allow doing so for ReadOnly properties. Your comment prompted me to check readonly fields, and it has the same behavior. I also looked at the IL and there is no attempt to assign to the Property (or the readonly field).

[jrmoreno1]

I don't know if VB is scheduled to get readonly locals, but if so it's going to get even more confusing if readonly local variables can't be passed to as a ref argument but readonly field/properties can. And if readonly locals are allowed, that's going to cause a lot of confusion between vb and C#.

[gafter]

Issue moved to dotnet/vblang #169 via ZenHub