Cannot be used in the PIVOT operator because it is not invariant to NULLs

I create an aggregate function for string column in SQL Server 2008.

C# code look like this:

using System;
using System.Collections.Generic;
using System.Data.SqlTypes;
using System.IO;
using Microsoft.SqlServer.Server;

[Serializable]
[SqlUserDefinedAggregate(Format.UserDefined, MaxByteSize = 8000)]
public struct strconcat : IBinarySerialize
{
    private List<String> values;

    public void Init()
    {
        this.values = new List<String>();
    }

    public void Accumulate(SqlString value = new SqlString())
    {
        this.va开发者_如何转开发lues.Add(value.Value);
    }

    public void Merge(strconcat value)
    {
        this.values.AddRange(value.values.ToArray());
    }

    public SqlString Terminate()
    {
        return new SqlString(string.Join(", ", this.values.ToArray()));
    }

    public void Read(BinaryReader r)
    {
        int itemCount = r.ReadInt32();
        this.values = new List<String>(itemCount);
        for (int i = 0; i <= itemCount - 1; i++)
        {
            this.values.Add(r.ReadString());
        }
    }

    public void Write(BinaryWriter w)
    {
        w.Write(this.values.Count);

        foreach (string s in this.values)
        {
            w.Write(s);
        }
    }
}

And query in SQL:

DECLARE @listCol NVARCHAR(2000)

SELECT  @listCol = STUFF(( SELECT '],[' + A.Name
                        FROM Attribute A,Category C
                        WHERE A.CategoryId = C.Id
                        ORDER BY A.DisplayOrder DESC
                        FOR XML PATH('')), 1, 2, '') + ']'
DECLARE @query NVARCHAR(2000)
SET @query =

N'SELECT * FROM (SELECT P.*,A.Name AttributeName,PA.OriginalValue FROM Product P,Product_Attribute PA, Attribute A WHERE P.Id = PA.ProductId AND A.Id = PA.AttributeId) src
PIVOT 
(
    dbo.strconcat(OriginalValue) FOR AttributeName 
    IN ('+@listCol+')) AS pvt'

EXECUTE (@query)

But SQL Server returns an error:

Msg 406, Level 16, State 1, Line 5

dbo.strconcat cannot be used in the PIVOT operator because it is not invariant to NULLs.

I googled it but don't know how to fix it.

Please help me!

If your aggregate is invariant to nulls, you need to mark it as such in the SqlUserDefinedAggregateAttribute, something like:

[SqlUserDefinedAggregate(Format.UserDefined, MaxByteSize = 8000,
   IsInvariantToNulls = true)]

The IsInvariantToNulls property describes the requirement as:

Used by the query processor, this property is true if the aggregate is invariant to nulls. That is, the aggregate of S, {NULL} is the same as aggregate of S. For example, aggregate functions such as MIN and MAX satisfy this property, while COUNT(*) does not.

Looking at your aggregate, I think you might need to do some work in your Add method - if the passed in value is null, maybe don't add it to the list?