selectionset.AddList

private void SelectionSetTest()
		{
			this.axMapControl1.AddLayerFromFile(@"C:\Program Files\ArcGIS\Bin\templatedata\usa\states layer.lyr",0);
			IFeatureLayer fl = this.axMapControl1.get_Layer(0) as IFeatureLayer;
			ITable table = fl as ITable;
			IQueryFilter qf = new QueryFilterClass();
			qf.WhereClause = @"SUB_REGION = 'Pacific'";  
			
			IDataset ds = fl.FeatureClass as IDataset;
			IWorkspace workspace = ds.Workspace;
			
			ISelectionSet selectionset = table.Select(qf,esriSelectionType.esriSelectionTypeHybrid,esriSelectionOption.esriSelectionOptionNormal, workspace);


			System.Diagnostics.Debug.WriteLine(selectionset.Count.ToString());

			int[] OIDList = new int[2];
			OIDList[0] = 23;
			OIDList[1] = 32;
			selectionset.AddList(2, ref OIDList[0]);

			System.Diagnostics.Debug.WriteLine(selectionset.Count.ToString());

			selectionset.RemoveList(2,ref OIDList[0]);

			System.Diagnostics.Debug.WriteLine(selectionset.Count.ToString());

		}