Program.cs

using System;
using System.Diagnostics;
using System.Linq;

public interface IFoo
{
    // NOTICE: without "params" keyword.
    void DoIt(object[] args);
}

public static class IFooExtension
{
    public static void DoIt(this IFoo foo, object arg1) => foo.DoIt(new object[] { arg1 });
    public static void DoIt(this IFoo foo, object arg1, object arg2) => foo.DoIt(new object[] { arg1, arg2 });
}

public class Foo : IFoo
{
    public void DoIt(object[] args)
    {
        Debug.WriteLine($"num of args is {args.Length}");
        args.Select((arg, n) => new { arg, n }).ToList().ForEach(a =>
        {
            Debug.WriteLine($"arg {a.n + 1} is {a.arg}");
        });
    }
}

static class Program
{
    static void Main()
    {
        var foo = new Foo() as IFoo;
        foo.DoIt("A");
        foo.DoIt("B", "C");
        
        // Array of value type matches "IFooExtension.DoIt(object arg1)".
        foo.DoIt(new byte[] { 1, 2, 3 });
        foo.DoIt(new[] { 4, 5, 6 });
        foo.DoIt(new[] { DateTime.Now, DateTime.Now });

        // But, Array of reference type matches "IFoo.DoIt(object[] args)".
        foo.DoIt(new[] { "D", "E" });
        foo.DoIt(new Foo[] { new Foo(), new Foo() });
        
        // The output of this program is:

        // num of args is 1
        // arg 1 is A
        // num of args is 2
        // arg 1 is B
        // arg 2 is C
        
        // num of args is 1
        // arg 1 is System.Byte[]
        // num of args is 1
        // arg 1 is System.Int32[]
        // num of args is 1
        // arg 1 is System.DateTime[]
        
        // num of args is 2 <- I expected "num of args is 1".
        // arg 1 is D   <- and I also expected "arg 1 is System.String[]", but wasn't.
        // arg 2 is E
        // num of args is 2
        // arg 1 is FeelTheFlowOfTime.Foo
        // arg 2 is FeelTheFlowOfTime.Foo
    }
}