expandoObj.cs

// From Achiral.Make.Dictionary (modified)
/// <see>http://blogs.wankuma.com/mayuki/archive/2007/12/06/111901.aspx</see>
public static Dictionary<String, Object> Dictionary(params Expression<Func<Object, Object>>[] exprs)
{
    return exprs.ToDictionary(expr => expr.Parameters[0].Name, expr => expr.Compile().Invoke(null));
}

var o = (IDictionary<String,Object>) new ExpandoObject();
Dictionary(a => 123, b => "foo").ForEach(_ => o.Add(_.Key, _.Value));