Fix/issue 441

[gyurebalint]

Fixes #441

Problem

When a configuration method has a params array parameter (e.g. params string[] ids) and the Args section is omitted entirely from config, the library fails to invoke the method at startup with:
System.ArgumentException: Object of type 'System.DBNull' cannot be converted to type 'System.String[]'.

This happens because:

1. HasImplicitValueWhenNotSpecified does not recognise params parameters as optional — so SelectConfigurationMethod rejects the method entirely when no args are supplied
2. Even if the method were selected, GetImplicitValueForNotSpecifiedKey falls through to return parameter.DefaultValue which is DBNull for params parameters on .NET 10 — causing a crash at invocation

Changes

ConfigurationReader.cs

• Added ParamArrayAttribute check to HasImplicitValueWhenNotSpecified so params parameters are treated as optional during method selection
• Added ParamArrayAttribute check to GetImplicitValueForNotSpecifiedKey to return an empty array of the correct element type instead of DBNull
• Made GetImplicitValueForNotSpecifiedKey
  internal to allow direct unit testing

ConfigurationReaderTests.cs

• Added test asserting that a method with a params string[] parameter is selected when no args are supplied
• Added test asserting that the implicit value for a params string[] parameter is an empty array, not DBNull

Root cause

In .NET 10, params parameters report HasDefaultValue = true but DefaultValue is DBNull — not a valid value for an array type. The fix intercepts params parameters before falling through to return parameter.DefaultValue.

[gyurebalint]

Hi everyone!
Note: I changed GetImplicitValueForNotSpecifiedKey to internal to allow for isolated unit testing of the params logic.
If you'd prefer to keep it private and have this tested strictly through the public API, let me know and I'll adjust the tests.
Any feedback is appreciated.

[nblumhardt]

Thanks for sending this!

params is now supported on spans, enumerables, and other collections:

https://devblogs.microsoft.com/dotnet/csharp13-calling-methods-is-easier-and-faster/#params-collections

It would be great to extend test coverage to make sure these cases work, or at least degrade gracefully.

[nblumhardt]

If this is modified to:

if (parameter.IsDefined(typeof(ParamArrayAttribute), false) &&
    parameter.ParameterType.GetElementType() is {} elementType)

then elementType can be used in the CreateInstance call without !, and we'll also not fail in (as yet unknown) cases where ParamArrayAttribute is present but the parameter type isn't an array.

[gyurebalint]

I’ve updated the PR!

This new params support begs the question, should this library handle these cases, and if so how?

Ideas:

• IEnumerable could just return an array, closest type to it, and further handling can happen on the client side
• List and other generics I think can be handled generically, by returning an instance of the collection with T type
• ReadOnlySpan is a whole different beast.. and could be a big change since now we are returning object? in GetImplicitValueForNotSpecifiedKey so this method won't be able to handle it.

[nblumhardt]

LGTM, thanks. It would be nice to extend support to different params types - though things might get tricky in the Span case. Extending to the generic enumerable/list types seems like a win, though.