得到Unio后的新的FeatureClass（要求原FeatureClass只能是两两相交最简单的情况，即一个Feature最多只能和另外一个Feature相交）

using System;
using System.Collections.Generic;
using System.ComponentModel;
using System.Data;
using System.Drawing;
using System.Text;
using System.Windows.Forms;

using ESRI.ArcGIS.esriSystem;
using ESRI.ArcGIS.SystemUI;
using ESRI.ArcGIS.Geometry;
using ESRI.ArcGIS.Display;
using ESRI.ArcGIS.Carto;
using ESRI.ArcGIS.Controls;
using ESRI.ArcGIS.DataSourcesFile;
using ESRI.ArcGIS.DataSourcesGDB;
using ESRI.ArcGIS.Geodatabase;

namespace union
{
    public partial class Form1 : Form
    {
        public Form1()
        {
            InitializeComponent();
            ListGeometry = new List<IGeometry>();
        }
        private IFeatureClass pFeatureClass;
        private IFeatureWorkspace pFeatureWorkspace;
        private List<IGeometry> ListGeometry;

        private void button1_Click(object sender, EventArgs e)
        {
            # region one of the method
            //string filePath = @"O:\Experiment Labtory\Data\";
            //string fileName = @"unionFeatures.shp";
            //axMapControl1.AddShapeFile(filePath, fileName);
            # endregion

            #region two of the method

            IWorkspaceFactory pWF = new ShapefileWorkspaceFactoryClass();
            IWorkspace pW = pWF.OpenFromFile("O:\\Experiment Labtory\\Data", 0);
            IFeatureWorkspace pFeatureWorkspace = pW as IFeatureWorkspace;

            this.pFeatureWorkspace = pFeatureWorkspace;

            IFeatureClass pFC = pFeatureWorkspace.OpenFeatureClass("newunionFeatrueClass.shp");
            IFeatureLayer pFeatureLayer = new FeatureLayerClass();
            pFeatureLayer.FeatureClass = pFC;

            this.pFeatureClass = pFC;

            axMapControl1.AddLayer(pFeatureLayer);
           

            #endregion
        }

        private void button2_Click(object sender, EventArgs e)
        {
            int i, j;
            //IFeatureCursor pFeatureCursor = pFeatureClass.Search(null, false);
            int featureCount = pFeatureClass.FeatureCount(null);
            //IFeature pFirstFeature = pFeatureCursor.NextFeature();

            IFeature pFirstFeature;
            IPolygon pPolygon;
            IRelationalOperator pRelationOp;
            ITopologicalOperator4 pToplogicalOp;

            for (i = 0; i < featureCount - 1; i++)
            {
                pFirstFeature = pFeatureClass.GetFeature(i);
                pPolygon = (IPolygon)pFirstFeature.ShapeCopy;
                pRelationOp = (IRelationalOperator)pPolygon;
                pToplogicalOp = (ITopologicalOperator4)pPolygon;
                pToplogicalOp.Simplify();
                for (j = i + 1; j < featureCount; j++)
                {
                    IGeometry pGeometry = pFeatureClass.GetFeature(j).ShapeCopy as IGeometry;
                    if (pRelationOp.Overlaps(pGeometry))
                    {
                        ListGeometry.Add(pToplogicalOp.Union(pGeometry));

                    }

                }

            }
  

        }

        private void button3_Click(object sender, EventArgs e)
        {
            //UID u = null;
            //IFeatureClass pFeatureClass=pFeatureWorkspace.CreateFeatureClass
            //    ("NewUnionFeatureClass", null, u, u, esriFeatureType.esriFTSimple, "NewUnionFeatureClass", "");

             IWorkspaceFactory pWF = new ShapefileWorkspaceFactoryClass();
            IWorkspace pW = pWF.OpenFromFile("O:\\Experiment Labtory\\Data", 0);
            IFeatureWorkspace pFeatureWorkspace = pW as IFeatureWorkspace;

            this.pFeatureWorkspace = pFeatureWorkspace;

            IFeatureClass pFC = pFeatureWorkspace.OpenFeatureClass("newnewFeatureClass.shp");
            if (pFC.ShapeType == esriGeometryType.esriGeometryPolygon)
            {
                IFeature pFeature;
                foreach (IGeometry g in ListGeometry)
                {
                    pFeature = pFC.CreateFeature();
                    pFeature.Shape = g;
                    pFeature.Store();

                }
            }

        }
    }
}