Engine中如何对设置了Definition的图层实现缩放到图层？

IMap map = axMapControl1.ActiveView.FocusMap; 

IFeatureLayer featureLayer = map.get_Layer(0) as IFeatureLayer; 

IFeatureLayerDefinition def = featureLayer as IFeatureLayerDefinition; 

string whereclause = FID <5; 

def.DefinitionExpression = whereclause; 

IQueryFilter queryFilter = new QueryFilterClass(); 

queryFilter.WhereClause = whereclause; 

IFeatureCursor cursor = featureLayer.Search(queryFilter, true); 

IFeature fea = cursor.NextFeature(); 

IEnvelope envelope = new EnvelopeClass(); 

while (fea!=null) 

{ 

envelope.Union(fea.ShapeCopy.Envelope); 

fea = cursor.NextFeature(); 

 } 

axMapControl1.Extent = envelope; 

axMapControl1.Refresh();