thanks Hristo, but my problem is not about formatting, it's about aggregating strings.
I've solved my problem with a custom aggregator.
01.public class StringAggregateFunction : AggregateFunction
02.{
03. protected override Cloneable CreateInstanceCore()
04. {
05. return new StringAggregateFunction();
06. }
07.
08. protected override void CloneCore(Cloneable source)
09. { }
10.
11. public override string DisplayName
12. {
13. get { return "StringAggregateFunction"; }
14. }
15.
16. protected override AggregateValue CreateAggregate(IAggregateContext context)
17. {
18. return new StringAggregate();
19. }
20.}
21.
22.public class StringAggregate : AggregateValue
23.{
24. private string value = String.Empty;
25.
26. protected override object GetValueOverride()
27. {
28. return value;
29. }
30.
31. protected override void AccumulateOverride(object value)
32. {
33. // only set value the first time
34. if (this.value == String.Empty)
35. {
36. this.value = Convert.ToString(value);
37. }
38.
39. //this.value = this.value + Convert.ToString(value);
40. }
41.
42. protected override void MergeOverride(AggregateValue childAggregate)
43. {
44. var stringAggregate = childAggregate as StringAggregate;
45. if (stringAggregate != null)
46. {
47. if (this.value == String.Empty)
48. {
49. // first aggregation value
50. this.value = stringAggregate.value;
51. }
52. else if (this.value != stringAggregate.value)
53. {
54. // next aggregation differs
55. this.value = "Mixed";
56. }
57. else
58. {
59. // next aggregation value is the same
60. }
61. }
62. else
63. {
64. throw new InvalidOperationException();
65. }
66. }
67.}