[Schema("93CC69FC-AB6F-451A-B075-A5D9467569C2",
    "ComplexSchema")]
public class ComplexEntity : IRevitEntity
{
    [Field]
    public string SimpleField { get; set; }

    [Field]
    public List<IntEntity> ArrayField { get; set; }

    [Field]
    public Dictionary<int, IntEntity> MapField { get; set; }
}

// the same with Extension
ComplexEntity complexEntity =
    new ComplexEntity();
complexEntity.SimpleField = "Hello, Simple entity";

complexEntity.ArrayField =
    new List<IntEntity>
        {
            new IntEntity() {SomeValue = 7},
            new IntEntity() {SomeValue = 8}
        };

complexEntity.MapField =
    new Dictionary<int, IntEntity>
        {
            {9, new IntEntity(){SomeValue = 9}},
            {10, new IntEntity(){SomeValue = 10}}
        };

element.SetEntity(complexEntity);

//Change value in map field
var complexEntity2 =
    element.GetEntity<ComplexEntity>();
if (complexEntity2!=null)
{
    if (complexEntity.MapField.ContainsKey(9))
    {
        var entityInMapField =
            complexEntity.MapField[9];
        entityInMapField.SomeValue = 9898;

        element.SetEntity(complexEntity2);
    }
}