T-SQL CLR: Can you CREATE AGGREGATE with optional parameter(s)? - sql-server-2008

Question

I'm hoping to be able to allow optional parameters to be specified, so I can overload the Accumulate() method, can it be done?
I'd like to overload to allow a delimiter to specified, I've seen others where it must force a delimiter to be specified but this behaviour doesn't suit.
CREATE AGGREGATE dbo.Concatenate (#input nvarchar(max), <OPTIONAL PARAMETER HERE>)
RETURNS nvarchar(max)
For reference, here is the aggregate class code that contains the Accumulate() method I'm looking to overload:
using System;
using System.Data;
using Microsoft.SqlServer.Server;
using System.Data.SqlTypes;
using System.IO;
using System.Text;
namespace CLR.Utilities {
/// <summary>
/// <list type="references">
/// <reference>
/// <name>How to: Create and Run a CLR SQL Server Aggregate</name>
/// <link>http://msdn.microsoft.com/en-us/library/91e6taax(v=vs.90).aspx</link>
/// </reference>
/// <reference>
/// <name>SqlUserDefinedAggregateAttribute</name>
/// <link>http://msdn.microsoft.com/en-us/library/microsoft.sqlserver.server.sqluserdefinedaggregateattribute(v=vs.90).aspx</link>
/// </reference>
/// <reference>
/// <name>Invoking CLR User-Defined Aggregate Functions (Provides seed code for this function)</name>
/// <link>http://technet.microsoft.com/en-us/library/ms131056.aspx</link>
/// </reference>
/// </list>
/// </summary>
[Serializable]
[SqlUserDefinedAggregate(
Format.UserDefined, //use clr serialization to serialize the intermediate result
IsInvariantToNulls = true, //optimizer property
IsInvariantToDuplicates = false, //optimizer property
IsInvariantToOrder = false, //optimizer property
MaxByteSize = -1) //no maximum size in bytes of persisted value
]
public class Concatenate : IBinarySerialize {
/// <summary>
/// The variable that holds the intermediate result of the concatenation
/// </summary>
private StringBuilder intermediateResult;
/// <summary>
/// Initialize the internal data structures
/// </summary>
public void Init() {
this.intermediateResult = new StringBuilder();
}
/// <summary>
/// Accumulate the next value, not if the value is null
/// </summary>
/// <param name="value"></param>
public void Accumulate([SqlFacet(MaxSize = -1)] SqlString value) {
if (value.IsNull) {
return;
}
this.intermediateResult.Append(value.Value.Trim()).Append(',');
}
/// <summary>
/// Merge the partially computed aggregate with this aggregate.
/// </summary>
/// <param name="other"></param>
public void Merge(Concatenate other) {
this.intermediateResult.Append(other.intermediateResult);
}
/// <summary>
/// Called at the end of aggregation, to return the results of the aggregation.
/// </summary>
/// <returns></returns>
[return: SqlFacet(MaxSize = -1)]
public SqlString Terminate() {
string output = string.Empty;
//delete the trailing comma, if any
if (this.intermediateResult != null
&& this.intermediateResult.Length > 0) {
output = this.intermediateResult.ToString(0, this.intermediateResult.Length - 1).Trim();
}
return new SqlString(output);
}
public void Read(BinaryReader r) {
intermediateResult = new StringBuilder(r.ReadString());
}
public void Write(BinaryWriter w) {
w.Write(this.intermediateResult.ToString().Trim());
}
}
}
And here is the code for deployment that I'd like to modify also if optional parameters can be set:
IF EXISTS (SELECT * FROM sys.objects WHERE object_id = OBJECT_ID(N'[dbo].[Concatenate]') AND type = N'AF')
DROP AGGREGATE [dbo].[Concatenate]
GO
IF EXISTS (SELECT * FROM sys.assemblies asms WHERE asms.name = N'CLR.Utilities' and is_user_defined = 1)
DROP ASSEMBLY [CLR.Utilities]
GO
ALTER DATABASE [DatabaseName] SET TRUSTWORTHY ON
GO
CREATE ASSEMBLY [CLR.Utilities] FROM 'C:\Path\To\File\CLR.Utilities.dll' WITH PERMISSION_SET = UNSAFE
GO
CREATE AGGREGATE [dbo].[Concatenate] (#input nvarchar(max)) RETURNS nvarchar(max)
EXTERNAL NAME [CLR.Utilities].[CLR.Utilities.Concatenate]
GO
GRANT EXECUTE ON [dbo].[Concatenate] TO PUBLIC
GO

Answer 1

You could, however, make the parameter mandatory but optionally null. And, in your C# code test whether the value is null and act accordingly. Something like:
public void Accumulate([SqlFacet(MaxSize = -1)] SqlString value, SqlString delimiter) {
string _delimiter;
if (value.IsNull) {
return;
}
if (delimiter.IsNull) {
_delimiter = string.Empty;
}
else {
_delimiter = Delimiter.Value;
}
this.intermediateResult.Append(value.Value.Trim()).Append(_delimiter);
}

Answer 2

As far as I know, there's no way to make clr function or aggregate with optional parameters and that's sad.

Answer 3

There is a way, though it isn't nice. You simple add the delimiter to the end of the column you're concatenating with a char(0) between them. then in the clr, you can look for char(0) and grab the delimiter to use.
-- concat columns delimited by a comma
select grp,dbo.clrConcat(surname) as names
from people
-- concat column delimited by specified character
select grp,dbo.clrConcat(surname + char(0) + '|') as names
from people
Then in the clr code
''' <summary>
''' Initialize the internal data structures
''' </summary>
Public Sub Init()
Me.intermediateResult = New StringBuilder()
delimiter = ","
End Sub
''' <summary>
''' Accumulate the next value, not if the value is null
''' </summary>
''' <param name="value"></param>
Public Sub Accumulate(ByVal value As SqlString)
Dim Str As String
Dim Pos As Integer
If value.IsNull Then Return
Pos = value.Value.IndexOf(Chr(0))
If Pos >= 0 Then
Str = value.Value.Substring(0, Pos)
If Pos + 1 < value.Value.Length Then
delimiter = value.Value.Substring(Pos + 1, 1)
End If
Else
Str = value.Value
End If
Me.intermediateResult.Append(Str).Append(delimiter)
End Sub