I have a condition
if (driver?.VistrackId == 555)
it equivalent (in IL code, not C# code):
if (driver != null && driver.VistrackId == 555)
right?
3 Answers
3 Comments
RMH
I think the thread-safety in the article is directed towards handling delegates, not necessarily towards handling variables. It depends on the situation if it is actually thread-safe. See this answer for a discussion on the thread-safety of the operator.
Oleksii Klipilin
@RMH but as I understand this operation is kind of atomic, and you can't set
null your variable during null propagation is processed, even you set object to null you still be referencing initial objectRMH
my apologies, I misinterpreted your meaning (and the doc's) of 'thread safe' here. I agree that the variable cannot become null during its null-check exactly as you stated. In that sense it is a 'thread-safe null-check'. However, this does not hold for the called property / method (
VistrackId in the post's case).That's correct. The "?" checks for null, and if it's not null, the "." continues the interrogation of the object
Comments
This code generates the following IL
var driver = new {VistrackId = 1};
if (driver?.VistrackId == 555)
{
Console.WriteLine("?. operator");
}
IL:
IL_0000: nop
IL_0001: ldc.i4.1
IL_0002: newobj <>f__AnonymousType0<System.Int32>..ctor
IL_0007: stloc.0 // driver
IL_0008: ldloc.0 // driver
IL_0009: brtrue.s IL_000E
IL_000B: ldc.i4.0
IL_000C: br.s IL_001B
IL_000E: ldloc.0 // driver
IL_000F: call <>f__AnonymousType0<System.Int32>.get_VistrackId
IL_0014: ldc.i4 2B 02 00 00
IL_0019: ceq
IL_001B: stloc.1
IL_001C: ldloc.1
IL_001D: brfalse.s IL_002C
IL_001F: nop
IL_0020: ldstr "?. operator"
IL_0025: call System.Console.WriteLine
IL_002A: nop
IL_002B: nop
IL_002C: ret
Code with == null
var driver = new {VistrackId = 1};
if (driver != null && driver.VistrackId == 555)
{
Console.WriteLine("== null");
}
IL
IL_0000: nop
IL_0001: ldc.i4.1
IL_0002: newobj <>f__AnonymousType0<System.Int32>..ctor
IL_0007: stloc.0 // driver
IL_0008: ldloc.0 // driver
IL_0009: brfalse.s IL_001A
IL_000B: ldloc.0 // driver
IL_000C: callvirt <>f__AnonymousType0<System.Int32>.get_VistrackId
IL_0011: ldc.i4 2B 02 00 00
IL_0016: ceq
IL_0018: br.s IL_001B
IL_001A: ldc.i4.0
IL_001B: stloc.1
IL_001C: ldloc.1
IL_001D: brfalse.s IL_002C
IL_001F: nop
IL_0020: ldstr "== null"
IL_0025: call System.Console.WriteLine
IL_002A: nop
IL_002B: nop
IL_002C: ret
They are slightly different
if (driver?.VistrackId == 555)is this compiling ?if((driver == null ? null : driver.VistrackId) == 555)IL, this is stillC#, but without syntactic sugar.