Helping linqtosql datacontext use implicit conversion between varchar column in the database and tab
- by user213256
I am creating an mssql database table, "Orders", that will contain a varchar(50) field, "Value" containing a string that represents a slightly complex data type, "OrderValue".
I am using a linqtosql datacontext class, which automatically types the "Value" column as a string.
I gave the "OrderValue" class implicit conversion operators to and from a string, so I can easily use implicit conversion with the linqtosql classes like this:
// get an order from the orders table
MyDataContext db = new MyDataContext();
Order order = db.Orders(o => o.id == 1);
// use implicit converstion to turn the string representation of the order
// value into the complex data type.
OrderValue value = order.Value;
// adjust one of the fields in the complex data type
value.Shipping += 10;
// use implicit conversion to store the string representation of the complex
// data type back in the linqtosql order object
order.Value = value;
// save changes
db.SubmitChanges();
However, I would really like to be able to tell the linqtosql class to type this field as "OrderValue" rather than as "string". Then I would be able to avoid complex code and re-write the above as:
// get an order from the orders table
MyDataContext db = new MyDataContext();
Order order = db.Orders(o => o.id == 1);
// The Value field is already typed as the "OrderValue" type rather than as string.
// When a string value was read from the database table, it was implicity converted
// to "OrderValue" type.
order.Value.Shipping += 10;
// save changes
db.SubmitChanges();
In order to achieve this desired goal, I looked at the datacontext designer and selected the "Value" field of the "Order" table.
Then, in properties, I changed "Type" to "global::MyApplication.OrderValue".
The "Server Data Type" property was left as "VarChar(50) NOT NULL"
The project built without errors.
However, when reading from the database table, I was presented with the following error message:
Could not convert from type 'System.String' to type 'MyApplication.OrderValue'.
at System.Data.Linq.DBConvert.ChangeType(Object value, Type type)
at Read_Order(ObjectMaterializer1 )
at System.Data.Linq.SqlClient.ObjectReaderCompiler.ObjectReader2.MoveNext()
at System.Linq.Buffer1..ctor(IEnumerable1 source)
at System.Linq.Enumerable.ToArray[TSource](IEnumerable`1 source)
at Example.OrdersProvider.GetOrders()
at ... etc
From the stack trace, I believe this error is happening while reading the data from the table. When presented with converting a string to my custom data type, even though the implicit conversion operators are present, the DBConvert class gets confused and throws an error.
Is there anything I can do to help it not get confused and do the implicit conversion?
Thanks in advance, and apologies if I have posted in the wrong forum.
cheers / Ben
