Aggregate polygons in edit session using ArcObjects and c#

Question

I want to aggregate some polygons in edit session using ArcObjects and c# language. In other words, my purpose is to use methaheuristic algorithms for polygon aggregation. So, I want to aggregate some polygons, see the numerical results and if it is not good, don't save it, and aggregate some others. And, in order for that, I want to implement it using edit session.For that purpose, I have implemented "Merge" which is in the editor toolbar. The code that I have used for merging two features is as follows:

Private void MergeFeatures (IFeature mainFeature, IFeature subFeature)
{
ITopologicalOperator union=mainFeature.Shape as ITopologicalOperator;
mainFeature.Shape=union.Union (subFeature.Shape);
mainFeature.Store();
}

But my problem is that it does not bring the polygons together. In other words, it just changes the attribute table of the feature class. The screenshot of part of my data is as follows:

Answer 1

I have found calling Simplify() can resolve unexpected results from various geometry operations.

private IGeometry unionPolygons(IPolygon4 p1, IPolygon4 p2)
{
//simplify the inputs
(p1 as ITopologicalOperator3).IsKnownSimple_2 = false;
(p1 as ITopologicalOperator3).Simplify();

(p2 as ITopologicalOperator3).IsKnownSimple_2 = false;
(p2 as ITopologicalOperator3).Simplify();

//union
IGeometry result = (p1 as ITopologicalOperator3).Union(p2);

//simplify the output
(result as ITopologicalOperator3).IsKnownSimple_2 = false;
(result as ITopologicalOperator3).Simplify();

return result;
}

Alternatively, you can use IBufferConstruction to union geometries (just pass 0 for the buffer if you only want to union):

    private IGeometry unionAndBuffer(IEnumGeometry inputGeometries, double bufferSize)
            {
                IBufferConstruction bufferConstructor = new BufferConstructionClass();
                IBufferConstructionProperties2 bufferOptions = bufferConstructor as IBufferConstructionProperties2;

                bufferOptions.EndOption = esriBufferConstructionEndEnum.esriBufferFlat;
                bufferOptions.ExplodeBuffers = false;
                bufferOptions.GenerateCurves = false;
                bufferOptions.OutsideOnly = false;
                bufferOptions.SideOption = esriBufferConstructionSideEnum.esriBufferFull;
//this does the union and ensures a single geometry in the result
                bufferOptions.UnionOverlappingBuffers = true; 
                bufferOptions.UseGeodesicBuffering = false;

                IGeometryCollection outputBuffers = new GeometryBagClass() as IGeometryCollection;

                bufferConstructor.ConstructBuffers(inputGeometries, bufferSize, outputBuffers);

                if (outputBuffers.GeometryCount <= 0)
                    return null;

                IGeometry g = outputBuffers.get_Geometry(0);
                return g;
            }

It can take some experimentation to get the result you want.