Confused as to why this C# code compiles, while similar code does not

Question

Let's take the following extension method:

static class Extensions {    public static bool In<T>(this T t, params T[] values)    {       return false;    } } 

I'm curious as to why this code compiles and runs:

var x = new Object(); IEnumerable<int> p = new List<int> { 1, 2, 3 }; var t2 = x.In(p); 

Within In, values is an Object[], as if the List<int> gets converted on the fly to an array. To me, it seems that params T[] does not match IEnumerable<int>, which is why I'm surprised this even runs.

Now this code:

var x = 5; IEnumerable<int> p = new List<int> { 1, 2, 3 }; var t2 = x.In(p); 

Does not run and generates the compiler error:

Error 2 Argument 2: cannot convert from 'System.Collections.Generic.IEnumerable' to 'int[]'

This is what I'd expect from the first one actually. Can someone explain what's going on here? Thanks!

Answer 1

Type inference converts your first call to

In<object>(x, p); 

The parameters of this closed generic method are object, params object[].
Therefore, passing p implicitly converts it to an object, which becomes the single item in the params array.

Your second call is inferred (because of the first parameter) to

In<int>(x, p); 

Here, the second parameter can either be an int[] (passing an array directly) or a series of ints (via params).
Since IEnumerable<int> is neither of those, you get an error.