How does C# choose with ambiguity and params

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It’s easy to test – the second method gets called.

As to why – the C# language specification has some pretty detailed rules about how ambiguous function declarations get resolved. There are lots of questions on SO surrounding interfaces, inheritance and overloads with some specific examples of why different overloads get called, but to answer this specific instance:

C# Specification – Overload Resolution

7.5.3.2 Better function member

For the purposes of determining the
better function member, a
stripped-down argument list A is
constructed containing just the
argument expressions themselves in the
order they appear in the original
argument list.

Parameter lists for each of the
candidate function members are
constructed in the following way:

  • The expanded form is used if
    the function member was applicable
    only in the expanded form.

  • Optional parameters with no
    corresponding arguments are removed
    from the parameter list

  • The parameters are reordered
    so that they occur at the same
    position as the corresponding argument
    in the argument list.

And further on…

In case the parameter type sequences {P1, P2, …, PN} and {Q1, Q2, …, QN} are equivalent > (i.e. each Pi has an identity conversion to the corresponding Qi), the following
tie-breaking rules are applied, in order, to determine the better function member.

  • If MP is a non-generic method and MQ is a generic method, then MP is better than MQ.

  • Otherwise, if MP is applicable in its normal form and MQ has a params array and is
    applicable only in its expanded form, then MP is better than MQ.

  • Otherwise, if MP has more declared parameters than MQ, then MP is better than MQ.
    This can occur if both methods have params arrays and are applicable only in their
    expanded forms.

The bolded tie-breaking rule seems to be what is applying in this case. The specification goes into detail about how the params arrays are treated in normal and expanded forms, but ultimately the rule of thumb is that the most specific overload will be called in terms of number and type of parameters.

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