topic959 
public decimal? CurrentBestPrice { get; }
знак вопроса после decimal - ошибка?
http://msdn.microsoft.co...ry/1t3y8s4s.aspx?ppud=4
Nullable types have the following characteristics:
Nullable types represent value-type variables that can be assigned the value of null. You cannot create a nullable type based on a reference type. (Reference types already support the null value.)
The syntax T? is shorthand for Nullable(Of T), where T is a value type. The two forms are interchangeable.
Assign a value to a nullable type just as you would for an ordinary value type, for example int? x = 10; or double? d = 4.108. A nullable type can also be assigned the value null: int? x = null.
Use the Nullable(Of T).GetValueOrDefault method to return either the assigned value, or the default value for the underlying type if the value is null, for example int j = x.GetValueOrDefault();
Use the HasValue and Value read-only properties to test for null and retrieve the value, as shown in the following example: if(x.HasValue) j = x.Value;
The HasValue property returns true if the variable contains a value, or false if it is null.
The Value property returns a value if one is assigned. Otherwise, a System.InvalidOperationException is thrown.
The default value for HasValue is false. The Value property has no default value.
You can also use the == and != operators with a nullable type, as shown in the following example: if (x != null) y = x;
Use the ?? operator to assign a default value that will be applied when a nullable type whose current value is null is assigned to a non-nullable type, for example int? x = null; int y = x ?? -1;
Nested nullable types are not allowed. The following line will not compile: Nullable<Nullable<int>> n;