PR Summary

Fix .NET method invocation with an argument that has an [Optional] attribute but no default value.

Fixes: #21372

PR Context

While not common .NET has the ability to set an optional argument but no explicit value https://learn.microsoft.com/en-us/dotnet/csharp/programming-guide/classes-and-structs/named-and-optional-arguments#optional-arguments

You can also declare optional parameters by using the .NET OptionalAttribute class. OptionalAttribute parameters do not require a default value. However, if a default value is desired, take a look at DefaultParameterValueAttribute class.

The docs for F# even recommend this attribute when it comes to F# assemblies that could be used in C# and other .NET languages https://learn.microsoft.com/en-us/dotnet/fsharp/language-reference/parameters-and-arguments#optional-parameters

For the purposes of C# and Visual Basic interop you can use the attributes [<Optional; DefaultParameterValue<(...)>] in F#, so that callers will see an argument as optional. This is equivalent to defining the argument as optional in C# as in MyMethod(int i = 3).

When it comes to the compiled assembly the ParameterInfo.DefaultValue for an argument with only [Optional] is System.Reflection.Missing but this value cannot be passed as an argument unless the type is object because it cannot be casted to the argument type. Instead using null or the default value for that type should be used.

This is a good candidate to backport.

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