using System;
using System.Data;
using Microsoft.SqlServer.Server;
using System.Data.SqlTypes;
using System.IO;
using System.Text;

[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 = 8000) //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(SqlString value)
{
    if (value.IsNull)
    {
        return;
    }

    this.intermediateResult.Append(value.Value).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>
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);
    }

    return new SqlString(output);
}

public void Read(BinaryReader r)
{
    intermediateResult = new StringBuilder(r.ReadString());
}

public void Write(BinaryWriter w)
{
    w.Write(this.intermediateResult.ToString());
}
}