Assuming you are trying to get the associated changeset summaries from the specified build. Then you are looking for InformationNodeConverters.GetAssociatedChangesets Method (IBuildDetail).
Check this case which should help you:
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After a lot of fiddling around I finally found the solution. Changeset's have been renamed to Commit's for TFS 2013's git support. Consequently you need to ask for that data when you get build details like this:
var buildDetailSpec = _buildServer.CreateBuildDetailSpec(buildDefinitionUris);
buildDetailSpec.InformationTypes = new[] { "AssociatedCommit" };
Then you perform the query as usual:
var buildQueryResult = _buildServer.QueryBuilds(new [] { buildDetailSpec });
var buildDetail = buildQueryResult[0].Builds;
Then you retrieve the commit, not the changeset, like this:
var commits = buildDetail.Information.GetNodesByType("AssociatedCommit");
var author = commits.First().Fields["Author"];
var comments = commits.First().Fields["Message"];
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