Does adding optional parameters change method signatures and would it trigger method missing exception?

Question

We have several projects that are referencing library assembly lets call "myUtil", now one of the method which is referenced in several projects is,

GetData(int p1, string p2, object p3, bool p4 = false);

Now if I change above method to this,

GetData(int p1, string p2, object p3, bool p4 = false, bool p5 = false);

Do I must need to update "myUtil.dll" assembly to projects referencing it ?

WIthout referencing it I get this error which does makes sense and goes away when I update reference, but I am NOT ABLE TO VALID FIND reference where it says you must need to update references

System.MissingMethodException: Method not found: 'Void GetData(....

Please be a bit more clear with what you are asking. It sounds like you have a C# project that has a reference to MyUtil.dll. In side of a completely separate project, you add a new method to MyUtil.dll and recompile it. You then expect MyUtil.dll to magically expose the new method to your first project that references it. Any time you add a method to the .dll project, you must recompile and re-add it to any other projects referencing it. — Johnathon Sullinger, May 19 '15 at 05:47
If the projects are sharing the same solution, then this would be a different problem. Your question wasn't clear on that. — Johnathon Sullinger, May 19 '15 at 05:47
Once you rebuild your `myUtil.dll` and your referencing projects point to your new assembly you're good to go. — Dunken, May 19 '15 at 05:47
@Dunken what am really looking for is a valid reference for it, does adding optional parameter makes it a different method then without one ? — Mathematics, May 19 '15 at 05:49
Yes, adding a new parameter, optional or not, changes the method signature. This requires you to redeploy the reference to the projects referencing it. — Johnathon Sullinger, May 19 '15 at 05:50

score 15 · Accepted Answer · answered May 19 '15 at 05:48

15

All references must be updated.

Optional parameters are just syntactic sugar.

When you have this:

GetData(int p1, string p2, object p3, bool p4 = false);

And call it like this:

GetData(1, "p2", obj);

The compiler does this:

GetData(1, "p2", obj, false);

You'll need to recompile and redeploy all assemblies/projects that reference it.

answered May 19 '15 at 05:48

Brendan Green

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Thanks Brendan for confirmation, Would you like to add a valid reference please as I couldn't find one myself – Mathematics May 19 '15 at 05:49

Jon Skeet · Answer 2 · 2015-05-19T05:55:32.920

You don't need to remove and re-add the reference, but you do need to rebuild all of the projects that compile against the DLL. Adding an optional parameter is a source-compatible but not a binary-compatible change, because the compiler emits different IL at the call site, to include the default value as normal - the call itself looks the same in the IL whether the optional parameter was omitted or not.

For example, consider the following code:

class Test
{
    static void Main()
    {
        Foo(3);
        Foo();
    }

    static void Foo(int x = 5)
    {
    }
}

The IL for Main looks like this:

  .method private hidebysig static void  Main() cil managed
  {
    .entrypoint
    // Code size       16 (0x10)
    .maxstack  8
    IL_0000:  nop
    IL_0001:  ldc.i4.3
    IL_0002:  call       void Test::Foo(int32)
    IL_0007:  nop
    IL_0008:  ldc.i4.5
    IL_0009:  call       void Test::Foo(int32)
    IL_000e:  nop
    IL_000f:  ret
  } // end of method Test::Main

As you can see, the constant 5 is loaded in the Main method and passed as an argument, just as 3 is explicitly. The same thing happens across assembly boundaries.

The same rules also applies to changing the values of constants, and changing the default value of an optional parameter - all of these require client rebuilds.

Thanks Skeet, I am struggling to find any valid read or reference for this subject - Does adding optional parameters change method signatures and would it trigger method missing exception ? maybe i am just after a valid reference to keep other developers happy — Mathematics, May 19 '15 at 05:52
@THE: I can't see anything *very* clear in the C# language specification, but yes, the optional parameter is part of the signature. I've updated my answer to give an example of how a call to a method with an optional parameter is compiled, so you can see that the default value is baked into the call site. — Jon Skeet, May 19 '15 at 05:56

score 4 · Answer 3 · answered May 19 '15 at 08:29

You can maintain compatibility if you stop using the optional parameters and use method overloading instead:

public void GetData(int p1, string p2, object p3, bool p4 = false, bool p5 = false)
{
    //Do something
}

Becomes

public void GetData(int p1, string p2, object p3)
{
    GetData(p1, p2, p3, false);
}

public void GetData(int p1, string p2, object p3, bool p4)
{
    GetData(p1, p2, p3, p4, false);
}

public void GetData(int p1, string p2, object p3, bool p4, bool p5)
{
    //Do something
}

This however won't reflect a change in the default value unless you recompile all the projects. For example any old GetData(1, "2", null) will always call the new GetData(1, "2", null, false) even if you change

public void GetData(int p1, string p2, object p3)
{
    GetData(p1, p2, p3, false);
}

to

public void GetData(int p1, string p2, object p3)
{
    GetData(p1, p2, p3, true);
}

+1 I like this coding pattern as it doesnt require a client recompile. (just for the new param add; I realize changing default values does require client recompile) — Sat Thiru, Jun 10 '16 at 17:49

Does adding optional parameters change method signatures and would it trigger method missing exception?

3 Answers3

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