C# 4 and .NET 4

958 ❘ ChaPTer 34 .net prOGrAmminG with sQl server
user-defined TyPes
User-defined types (UDTs) can be used similarly to normal SQL Server data types to define the type of a
column in a table. With older versions of SQL Server, it was already possible to define UDTs. Of course,
these UDTs could be based only on SQL types, such as the ZIP type shown in the following code. The
stored procedure sp_addtype allows you to create user-defined types. Here the user-defined type ZIP is
based on the CHAR data type with a length of 5. NOT NULL specifies that NULL is not allowed with the ZIP
data type. By using ZIP as a data type, it is no longer necessary to remember that it should be 5 char long
and not null:
EXEC sp_addtype ZIP 'CHAR(5)', 'NOT NULL'
With SQL Server 2005 and later, UDTs can be defined with CLR classes. However, this feature is not meant
to add object orientation to the database; for example, to create a Person class to have a Person data type.
SQL Server is a relational data store, and this is still true with UDTs. You cannot create a class hierarchy
of UDTs, and it is not possible to reference fields or properties of a UDT type with a SELECT statement. If
properties of a person (for example, FirstName or LastName) must be accessed or a list of Person objects
must be sorted (for example, by FirstName or LastName), it is still better to define columns for first name
or last name inside a Persons table or to use the XML data type.
UDTs are meant for very simple data types. Before .NET, it was also possible to create custom data types;
for example, the ZIP data type. With UDTs it is not possible to create a class hierarchy, and they are not
meant to get complex data types to the database. One requirement of a UDT is that it must be convertible to
a string, because the string representation is used to display the value.
How the data is stored within SQL Server can be defined: either an automatic mechanism can be used to
store the data in a native format, or you can convert the data to a byte stream to define how the data
should be stored.
Creating udTs
In this example, you create a SqlCoordinate type representing the world coordinates longitude and latitude
for easily defining the location of places, cities, and the like. To create CLR objects with Visual Studio,
create a new Visual C# SQL CLR Database Project in the category Database Projects ➪ SQL Server, and
add a UDT by using the User-Defined Type template from the Solution Explorer. The name of the UDT
should be SqlCoordinate.
With the template, the base functionality of a custom type is already defined:
using System;
using System.Data;
using System.Data.SqlClient;
using System.Data.SqlTypes;
using Microsoft.SqlServer.Server;
[Serializable]
[Microsoft.SqlServer.Server.SqlUserDefinedType(Format.Native)]
public struct SqlCoordinate: INullable
{
public override string ToString()
{
// Replace the following code with your code
return "";
}
public bool IsNull
{
get
{
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