【20100105-01】IFeatureClass.Search(IQuery Filter,bool Recycling)参数说明
IFeatureClass的Search方法大家经常用到，很多人对Search方法的其中一个参数bool Recycling不是很理解。
目前网上关于这个参数的意义的解释有两个版本：
1.第一版本是IsNotNull大牛的：
http://bbs.esrichina-bj.cn/ESRI/viewthread.php?tid=13461
他提出的观点的是：参数Recycling为True的时候是传引用，为False的时候为传值
2.第二个版本是Echo兄的：
http://gis.cnblogs.com/home.aspx?page=3
他提出的观点是：参数Recycling为True的时候游标对象(Ifeaturecursor)是只包含一条记录,为False的时候游标对象(Ifeaturecursor)为10条记录（假设Featureclass有10条记录）
我经过测试提出个人的观点：
1.pFeatureCursor存储的是所有符合条件的Feature对象的引用，
2.Recycling的意思是回收，参数Recycling为True的时当执行这个方法IFeature pFeature=pFeatureCursor.NextFeature()上一条记录的值在内存中所占的地址就会被销毁回收，为False的时候当执行这个方法IFeature pFeature=pFeatureCursor.NextFeature()上一条记录的值依然存在在内存中。
Ok，接下来以实例证明本观点：
1.参数Recycling设置为True
IFeatureClass pFeatureClass = (this.axMapControl1.get_Layer(0) as IFeatureLayer).FeatureClass;
IFeatureCursor pFeatureCursor = pFeatureClass.Search(null, true);
//IFeatureCursor pFeatureCursor = pFeatureClass.Search(null, false);
List<IFeature> pFeatures = new List<IFeature>();
IFeature pFeature = pFeatureCursor.NextFeature();
while (pFeature != null)
{
pFeatures.Add(pFeature);
pFeature = pFeatureCursor.NextFeature();
}
for (int i = 0; i < pFeatures.Count; i++)
{
MessageBox.Show(((pFeatures.Shape as IPolygon) as IArea).Area.ToString());
}
分析：pFeatures集合存储了指向FeatureClass上所有Feature的引用，但是由于Recycling参数设置为TRUE也就是说每执行一个NextFeautre方法上一条记录的Feature值在内存中被回收，所以到最后遍历pFeatures集合的时候所有的IFeature引用指向的Feature对象都为Null。所以会引发一下错误（如下图所示）：
错误.jpg
2.参数Recycling设置为False
IFeatureClass pFeatureClass = (this.axMapControl1.get_Layer(0) as IFeatureLayer).FeatureClass;
//IFeatureCursor pFeatureCursor = pFeatureClass.Search(null, true);
IFeatureCursor pFeatureCursor = pFeatureClass.Search(null, false);
List<IFeature> pFeatures = new List<IFeature>();
IFeature pFeature = pFeatureCursor.NextFeature();
while (pFeature != null)
{
pFeatures.Add(pFeature);
pFeature = pFeatureCursor.NextFeature();
}
for (int i = 0; i < pFeatures.Count; i++)
{
MessageBox.Show(((pFeatures.Shape as IPolygon) as IArea).Area.ToString());
}
分析：pFeatures集合存储了指向FeatureClass上所有Feature的引用，但是由于Recycling参数设置为False也就是说每执行一个NextFeautre方法上一条记录的Feature值在内存中依然存在，所以到最后遍历pFeatures集合的时候所有的IFeature引用指向的Feature对象都依然存在。所以会执行的很Happy（如下图所示）：
正确.jpg
转自 fxlcoco：http://www.gisall.com/?uid-4359-action-viewspace-itemid-587
因此在性能方面，参见：http://www.cnblogs.com/wall/archive/2008/05/07/1186203.html
帮助中有如此论述，指出recycling参数的主要性：
The Recycling parameter controls feature object allocation behavior. Recycling cursors rehydrate a single feature object on each fetch and can be used to optimize read-only access, for example, when drawing. It is illegal to maintain a reference on a feature object returned by a recycling cursor across multiple calls to NextFeature on the cursor. Feature objects returned by a recycling cursor should not be modified.
Non-recycling cursors return a separate feature object on each fetch. The objects returned by a non-recycling cursor may be modified and stored with polymorphic behavior. The geodatabase guarantees 'unique instance semantics' on non-recycling feature objects fetched during an edit session.
Recycling cursors should be used only for drawing and read-only access to object state. Use non-recycling search cursors to fetch objects that are to be updated.
通过下面的函数可以测试该参数的重要性：
Const N = 100000
Sub TestRecycle()
Dim amap As IMxDocument
Set amap = ThisDocument
Dim lyr As IFeatureLayer
Set lyr = amap.FocusMap.Layer(0)
Dim fc As IFeatureCursor
Dim qr As IQueryFilter
Set qr = New QueryFilter
qr.WhereClause = "objectID < " & N
Dim dt As Date
Debug.Print "Recyle: False"
dt = Now
Set fc = lyr.Search(qr, False)
While Not fc.NextFeature Is Nothing
Wend
Debug.Print "Seconds: " & DateDiff("s", dt, Now)
dt = Now
Debug.Print "Recyle: True"
Set fc = lyr.Search(qr, True)
While Not fc.NextFeature Is Nothing
Wend
Debug.Print "Seconds: " & DateDiff("s", dt, Now)
End Sub
结果如下：
N=100000
Recyle: False
Seconds: 3
Recyle: True
Seconds: 1
N=500000
Recyle: False
Seconds: 12
Recyle: True
Seconds: 8
说明在只读查询中，将Recycling参数设为true可以提高函数效率。
