TryCast should support nullable value types as its target type

[AnthonyDGreen]

[AnthonyDGreen]

We rejected this in the LDM on 8/9/2017. It turns out that while the IL generated for TryCast(obj, Integer?) is shorter than If TypeOf obj Is Integer Then CInt(obj, Integer) the JIT recognizes the later and generates more efficient native code. When we used the former pattern equivalent in C# pattern matching we actually had to revert the optimization after measuring it found worse performance. As the purpose of TryCast is to make code more performant we'd rather not relax a restriction that would lead people to write less efficient code.

[reduckted]

Very interesting. The closest we can get to TryCast(obj, Integer?) right now is this:

Return If(TypeOf obj Is Integer, CInt(obj), New Integer?)

That generates different IL to the more verbose form that you mentioned:

If TypeOf obj Is Integer Then
    Return CInt(obj)
Else
    Return New Integer?
End If

@AnthonyDGreen, do you happen to know how the JIT handles the first technique?

For the record, here's the code I'm testing and the IL I'm seeing:

Public Class Test

    Public Function Inline(obj As Object) As Integer?
        Return If(TypeOf obj Is Integer, CInt(obj), New Integer?)
    End Function

    Public Function Normal(obj As Object) As Integer?
        If TypeOf obj Is Integer Then
            Return CInt(obj)
        Else
            Return New Integer?
        End If
    End Function

End Class

.method public 
	instance valuetype [mscorlib]System.Nullable`1<int32> Inline (
		object obj
	) cil managed 
{
	// Method begins at RVA 0x2130
	// Code size 30 (0x1e)
	.maxstack 1
	.locals init (
		[0] valuetype [mscorlib]System.Nullable`1<int32>
	)

	IL_0000: ldarg.1
	IL_0001: isinst [mscorlib]System.Int32
	IL_0006: brtrue.s IL_0012

	IL_0008: ldloca.s 0
	IL_000a: initobj valuetype [mscorlib]System.Nullable`1<int32>
	IL_0010: ldloc.0
	IL_0011: ret

	IL_0012: ldarg.1
	IL_0013: call int32 [Microsoft.VisualBasic]Microsoft.VisualBasic.CompilerServices.Conversions::ToInteger(object)
	IL_0018: newobj instance void valuetype [mscorlib]System.Nullable`1<int32>::.ctor(!0)
	IL_001d: ret
} // end of method Test::Inline

.method public 
	instance valuetype [mscorlib]System.Nullable`1<int32> Normal (
		object obj
	) cil managed 
{
	// Method begins at RVA 0x215c
	// Code size 33 (0x21)
	.maxstack 2
	.locals init (
		[0] valuetype [mscorlib]System.Nullable`1<int32> Normal
	)

	IL_0000: ldarg.1
	IL_0001: isinst [mscorlib]System.Int32
	IL_0006: brfalse.s IL_0017

	IL_0008: ldloca.s Normal
	IL_000a: ldarg.1
	IL_000b: call int32 [Microsoft.VisualBasic]Microsoft.VisualBasic.CompilerServices.Conversions::ToInteger(object)
	IL_0010: call instance void valuetype [mscorlib]System.Nullable`1<int32>::.ctor(!0)
	IL_0015: br.s IL_001f

	IL_0017: ldloca.s Normal
	IL_0019: initobj valuetype [mscorlib]System.Nullable`1<int32>

	IL_001f: ldloc.0
	IL_0020: ret
} // end of method Test::Normal