This is a known dynamic binding issue.

Here are some work arounds.

Use ICollection<dynamic> instead:

void Main()
{
    IFoo foo = new Foo();
    dynamic a = new System.Dynamic.ExpandoObject();
    a.Prop = 10000;
    dynamic b = new System.Dynamic.ExpandoObject();
    b.Prop = "Some Text";
    foo.Bars.Add(a);
    foo.Bars.Add(b); 
}

public interface IFoo
{
    ICollection<dynamic> Bars { get; set; }
}

public class Foo : IFoo
{
    public Foo()
    {
        Bars = new List<dynamic>();
    }

    public ICollection<dynamic> Bars { get; set; }
}

Or straight up List<dynamic>:

public interface IFoo
{
    List<dynamic> Bars { get; set; }
}

public class Foo : IFoo
{
    public Foo()
    {
        Bars = new List<dynamic>();
    }

    public List<dynamic> Bars { get; set; }
}

Or use dynamic foo:

void Main()
{
    dynamic foo = new Foo();
    dynamic a = new System.Dynamic.ExpandoObject();
    a.Prop = 10000;
    dynamic b = new System.Dynamic.ExpandoObject();
    b.Prop = "Some Text";
    foo.Bars.Add(a);
    foo.Bars.Add(b); 
}

Or don't dynamic bind add, by casting to object:

void Main()
{
    IFoo foo = new Foo();
    dynamic a = new System.Dynamic.ExpandoObject();
    a.Prop = 10000;
    dynamic b = new System.Dynamic.ExpandoObject();
    b.Prop = "Some Text";
    foo.Bars.Add((object)a); 
    foo.Bars.Add((object)b); 
}

Or be more expressive using a third party framework like my impromptu interface with ActLike & Prototype Builder Syntax (in nuget).

using ImprmoptuInterface;
using Dynamitey;
void Main()
{
    dynamic New = Builder.New<ExpandoObject>();

    IFoo foo = Impromptu.ActLike(
                   New.Foo(
                       Bars: New.List(
                                 New.Obj(Prop:10000),
                                 New.Obj(Prop:"Some Text")
                             )
                       )
                   );
}

public interface IFoo
{
    IList<dynamic> Bars { get; set; }
}