[Proposal]: breaking point for C# Primary Constructor

[ndepend]

I was investigating C# 12 class and struct primary constructors and figured out that no breakpoint can be set, unless a parameter is used within a property initialization. It can certainly be useful to break upon a primary constructor, the same way it is useful to be able to break upon a property get; or set;:

[tmat]

There are 3 issues here:

1. The compiler is not generating sequence points correctly for primary constructors of non-records. It works well for records.

Repro:

var s = new S(1, 2);
var p = new P(1, 2);
Console.WriteLine();

record struct S(int A, int B);
struct P(int A, int B) { }

Run and keep pressing F11 to step into all debuggable code (with "Just My Code" enabled):

1

2

3

4

Compiler issue - missing sequence points (initializer of P is skipped):

5

6

2. The IDE is not allowing to place breakpoints for primary constructors (both records and non-records).

Repro: place cursor on int A in S or P and try to place a breakpoint (F9).

3. Not sure why, but in step 2 and 3 the grey highlight should be on var s = new S(1, 2);, not on var p = new P(1, 2);. It could be a debugger issue.

The sequence points of <Main>$ look correct:

Method 'Program.<Main>$' (0x06000003)
  Locals: S, P (#22)
{
  // Code size       24 (0x18)
  .maxstack  3
 -IL_0000:  ldloca.s   V_0
  IL_0002:  ldc.i4.1
  IL_0003:  ldc.i4.2
  IL_0004:  call       'S..ctor' (0x06000005)

 -IL_0009:  ldloca.s   V_1
  IL_000b:  ldc.i4.1
  IL_000c:  ldc.i4.2
  IL_000d:  call       'P..ctor' (0x06000012)

 -IL_0012:  call       'System.Console.WriteLine' (0x0a00000f)
  IL_0017:  ret
}

Sequence points of S..ctor are:

Method 'S..ctor' (0x06000005)
{
  // Code size       15 (0xf)
  .maxstack  8
 -IL_0000:  ldarg.0
  IL_0001:  ldarg.1
  IL_0002:  stfld      '<A>k__BackingField' (0x04000002)

 -IL_0007:  ldarg.0
  IL_0008:  ldarg.2
  IL_0009:  stfld      '<B>k__BackingField' (0x04000003)
  IL_000e:  ret
}

I think we could add hidden sequence point to ret here, but not sure if necessary.

[tmat]

@AlekseyTs

[AlekseyTs]

It works well for records.

I wouldn't say that. It looks to me that the issue is not specific to a particular flavor of a primary constructor, record or not. If a primary constructor doesn't have any user code, no sequence points generated for its body. For example, debugger doesn't stop for this record.

        record struct S(int A, int B)
        {
            public int A;
            public int B;
        }

Bottom line, this doesn't look like a new issue to me.

[tmat]

Ok, it may work accidentally for some records and not for others, so might not be new. That doesn't mean it shouldn't be fixed (possibly by removing the existing sequence points generated for records, if we don't want to step into the ctor).

[AlekseyTs]

That doesn't mean it shouldn't be fixed

No argument here.

[ndepend]

@tmat indeed I didn't notice one can step into record primary constructor parameter declaration.

Issue 1) No breaking point can be set with F9 in this context:

Issue 2) I cannot step-into with F11 in this context:

Issue 3) No breaking point can be set with F9 in this context:

[egvijayanand]

In my case, the object instantiation would be from the DI container, so it is necessary to place a breakpoint at the type definition level to inspect it during the debugging process.

Otherwise, I need to traverse through a hierarchy of instances to start from the invocation point (Application -> View -> ViewModel). This is just for the sample created. A real-world example would be much more complex. As we assess this feature to be useful esp for Services where the constructor parameter just gets assigned to the read-only field for its lifetime.

And most of the time, the parameter would be used in another command, which is independent of instantiation, unlike the property where the value would be set instantly.

So need an option that addresses this issue before this feature hits GA.

[tmat]

Breakpoints in primary constructors are implemented in VS 17.8 Preview 1: #68765

[egvijayanand]

Breakpoints in primary constructors are implemented in VS 17.8 Preview 1

Glad that it's getting addressed. Clarify whether v17.8 is planned to release before/alongside .NET 8 GA.