How to get/set a property of an interface that is not always implemented

class bla : Ibla {
    public int Blargh { get; protected internal set; }
}

interface Ibla {
    int Blargh { get; }
}

interface IMyInterface
{ 
   int property { get; }
}

class MyClass : IMyInterface
{
  int property { get; protected set; }
}

    public static void WithKeywords<TEntityType>(this EntityTypeConfiguration<TEntityType> entityTypeConfiguration)
        where TEntityType : EntityBase, IKeywordedEntity
    {
        var rootExpression = Expression.Parameter(typeof (TEntityType));
        var expression = Expression.Property(rootExpression, "Keywords");

        entityTypeConfiguration.HasMany(Expression.Lambda<Func<TEntityType, ICollection<Keyword>>>(expression, rootExpression)).WithMany();
    }

var attibutes = instance.GetType().GetInterfaces()
                    .SelectMany(i => i.GetProperties())
                    .SelectMany(
                        propertyInfo =>
                        propertyInfo.GetCustomAttributes(typeof (BarAttribute), false)
                    );

if (((IEntityModifier)this).AutoDetachOnFinished)

bool AutoDetachOnFinished { get; set; }
bool Finished { get { return this.mFinished; } }