選択フィーチャの範囲を取得する方法

/// <summary>
/// 選択フィーチャの範囲を取得
/// </summary>
/// <param name="FeatureLayer">FeatureLayer</param>
/// <returns>Envelope</returns>
public static IEnvelope getSelectedFeaturesExtent(IFeatureLayer FeatureLayer)
{
    IFeatureSelection pFeatureSelection = (IFeatureSelection)FeatureLayer;

    IEnumGeometryBind pEnumGeometryBind = new EnumFeatureGeometryClass();
    pEnumGeometryBind.BindGeometrySource(null,pFeatureSelection.SelectionSet);

    System.Type t = Type.GetTypeFromProgID("esriGeometry.GeometryEnvironment");
    IGeometryFactory3 pGeometryFactory = (IGeometryFactory3)Activator.CreateInst

    IGeometry pGeometry = pGeometryFactory.CreateGeometryFromEnumerator((IEnumGeEnumGeometryBind);

    return pGeometry.Envelope;
}