In the following example you'll see how to create a UDT that represents a gender code: either M for male or F for female. While you could store this data in a standard one-byte character field, using a UDT ensures that the field will accept only these two values with no additional need for triggers, constraints, or other data validation techniques.
To create a UDT using Visual Studio 2005, select the New | Project option, give your project a name, and click OK to create the project. For this project I used the name of Gender for the new UDT. After naming the project and clicking OK, I filled out the New Database Reference dialog using the required connection values to deploy the project to the appropriate SQL Server system and database. Next, I used the Project | Add User-Defined Type option to display the Add New Item dialog that you can see in Figure 3-11.
Figure 3-11: Creating a .NET SQL Server UDT
Table 3-1: Required UDT Methods
Select User-Defined Type from the list of SQL Server templates. Enter the name that you want to assign to the class and then click Open to have Visual Studio generate a starter project file for the UDT. The starter
Imports System Imports System.Data Imports System.Data.Sql Imports System.Data.SqlTypes Imports Microsoft.SqlServer.Server Imports System.IO
_ _ Public Structure Gender Implements INullable, IBinarySerialize Public Sub Read(ByVal r As BinaryReader) _ Implements IBinarySerialize.Read m_value = r.ReadString.ToString() End Sub Public Overrides Function ToString() As String If m_value.IsNull = False Then Return m_value.Value Else Return Nothing End If End Function Public ReadOnly Property IsNull() As Boolean _ Implements INullable.IsNull Get If m_value.IsNull = True Then Return True Else Return False End If End Get End Property Public Shared ReadOnly Property Null() As Gender Get Dim h As Gender = New Gender h.m_Null = True Return h End Get End Property Public Shared Function Parse(ByVal s As SqlString) As Gender If s.IsNull Then Return Null End If Dim u As Gender = New Gender u.Value = s Return u End Function ' Create a Value Property Public Property Value() As SqlString Get Return m_value End Get Set(ByVal value As SqlString) If (value = "M" Or value = "F") Then m_value = value Else Throw New ArgumentException _ ("Gender data type must be M or F") End If End Set End Property ' Private members Private m_Null As Boolean Private m_value As SqlString End Structure
To create a UDT, the code must adhere to certain conventions. The class's attributes must be serializable, the class must implement the INullable interface, and the class name must be set to the name of the UDT. You can optionally add the IComparable interface. In this example, Gender is the class name. Near the bottom of the listing you can see where a private string variable named m_value is declared to hold the value of the data type.
Like the other CLR database objects, the Attribute plays an important part in the construction of the CLR UDT. The SQL Server UDT Attribute accepts the property values shown in Table 3-2.
The first thing to notice in the code is the use of the INullable and IBinarySerialize interfaces. The INullable interface is required for all UDTs. The IBinarySerialize interface is required for UDTs that use the Format.UserDefined attribute. Because this example uses a String data type, the Format.UserDefined attribute is required, which means that this UDT also needs code to handle the serialization of the UDT. In practical terms, this means that the class must implement the IBinarySerialize Read and Write methods, which you can see in the following section of code.
At first it may seem a bit intimidating to use the IBinarySerialize interfaces, but as you can see in the Read and Write subroutines, it's actually pretty simple. The Read subroutine simply uses the ReadString method to assign a value to the UDT's m_value variable (which contains the UDT's value). Likewise, the Write subroutine uses the Write method to serialize the contents of the m_value variable.
Table 3-2: UDT Attribute Properties
The ToString method checks to see if the contents of the m_value variable are null. If so, then the string "null" is returned. Otherwise, the m_value's ToString method returns the string value of the contents.
The next section of code defines the IsNull property. This property's get method checks the contents of the m_value variable and returns the value of true if m_value is null. Otherwise, the get method returns the value of false. Next, you can see the Null method, which was generated by the template to fulfill the UDT's requirement for nullability.
The Parse method accepts a string argument, which it stores in the object's Value property. You can see the definition for the Value property a bit lower down in the code. The Parse method must be declared as static, or if you're using VB.NET, it must be a Shared property.
The Value property is specific to this implementation. In this example, the Value property is used to store and retrieve the value of the UDT. It's also responsible for editing the allowable values. In the set method, you can see that only the values of M or F are permitted. Attempting to use any other values causes an exception to be thrown that informs the caller that the "Gender data type must be M or F".
Deploying the UDT
Very much like a CLR stored procedure or function, the UDT is compiled into a DLL after the code is completed. That DLL is then imported as a SQL Server assembly using the CREATE ASSEMBLY and CREATE TYPE statements or by simply using the Visual Studio 2005 Deploy option. You can see the T-SQL code to manually create the CLR UDT in the following listing:
create assembly Gender from 'C:tempGender.dll' go CREATE TYPE Gender EXTERNAL NAME Gender.[Gender.Gender] go
This listing assumes that gender.dll has been copied into the c:temp that's on the SQL Server system. One thing to notice in the CREATE TYPE statement is the class parameter. As in the earlier CLR examples, the first part of the External Name clause specifies the assembly that will be used. In the case of a UDT, the second part of the name identifies the namespace and class. In the Gender example, the Namespace was Gender and the UDT's class was also named Gender.
Using the UDT
Once the UDT is created, you can use it in T-SQL much like SQL Server's native data types. However, since UDTs contain methods and properties, there are differences. The following example shows how the Gender UDT can be used as a variable and how its Value property can be accessed:
DECLARE @mf Gender SET @mf='N' PRINT @mf.Value
In this listing the UDT variable is declared using the standard T-SQL DECLARE statement, and the SET statement is used to attempt to assign the value of N to the UDT's Value property. Because N isn't a valid value, the following error is generated:
.Net SqlClient Data Provider: Msg 6522, Level 16, State 1, Line 2 A CLR error occurred during execution of 'Gender': System.ArgumentException: Gender data type must be M or F at Gender.set_Value(SqlString value)
Just as UDTs can be used as variables, they can also be used to create columns. The following listing illustrates creating a table that uses the Gender UDT:
CREATE TABLE MyContacts (ContactID int, FirstName varchar(25), LastName varchar(25), MaleFemale Gender)
While creating columns with the UDT type is the same as when using a native data type, assigning values to the UDT is a bit different than the standard column assignment. Complex UDTs can contain multiple values. In that case you need to assign the values to the UDT's members. You can access the UDT's members by prefixing them with the (.) symbol. In this case, since the UDT uses a simple value, you can assign values to it exactly as you can any of the built-in data types. This example shows how to insert a row into the example MyContacts table that contains the Gender UDT:
INSERT INTO MyContacts VALUES(1, 'Michael', 'Otey', 'M')
To retrieve the contents of the UDT using the SELECT statement, you need to use the UDT.Member notation as shown here when referencing a UDT column:
SELECT ContactID, LastName, MaleFemale.Value FROM MyContacts
To see the UDTs that have been created for a database, you can query the sys.Types view as shown here:
SELECT * FROM sys.Types
The previous tip is from "Developing CLR database objects: 10 tips in 10 minutes," excerpted from Chapter 3 of the book "Microsoft SQL Server 2005: A Developer's Guide" written by Michael Otey and Denielle Otey, courtesy of McGraw-Hill Publishing.
Developing CLR database objects
Tip 1: CLR architecture
Tip 2: CLR assemblies in SQL Server 2005
Tip 3: Creating CLR database objects
Tip 4: CLR stored procedures
Tip 5: User-Defined Functions
Tip 6: CLR triggers
Tip 7: User-Defined Types
Tip 8: CLR aggregates
Tip 9: Debugging CLR database objects
Tip 10: .NET database object security
This was first published in May 2007