Using foreach with the ICursor

My first post on this blog was about how to loop over an ESRI cursor in Python with the for statement instead of the while statement. The same problem exists in .NET when using the various cursor objects like ICursor and IFeatureCursor. Normally you would code something like this :

ICursor cursor = table.Search(null, false);
IRow row;
while ((row = cursor.NextRow()) != null)
{
    // some code
}

Or this:

ICursor cursor = table.Search(null, false);
IRow row = cursor.NextRow();
while (row != null)
{
    // some code
    row = cursor.NextRow();
}

And I want to replace it with the following.

ITable table; // get table from somewhere
IQueryFilter queryFilter; // create QueryFilter or leave it null
CursorGS cursorGS = new CursorGS(table, queryFilter);
foreach (IRow row in cursorGS)
{
    // ... insert your code here
}

To make a class usable in a foreach statement you need to implement the IEnumerable and IEnumerator interfaces. I found a good introduction to making class usable in a foreach statement in this Microsoft article. So what I did was creating a class that inherited from IEnumerator<IRow> and IEnumerable<IRow> and implement all the needed properties and methods. For IEnumerator these where Current, MoveNext, Reset and Dispose and for IEnumerable only the method GetEnumerator was needed. The biggest problem was the Reset method because an ICursor doesn't have a reset method. I decided to set the cursor null and the recreated at the moment that MoveNext is called. This also makes sure the ICursor is only created when we really need it. Below you can find my full class

using System;
using System.Collections;
using System.Collections.Generic;
using System.Text;
using ESRI.ArcGIS.Geodatabase;

namespace GisSolved.GeoDiff.Esri
{
    public class CursorGS : IEnumerator<IRow>, IEnumerable<IRow>
    {
        ITable _table;
        IQueryFilter _queryFilter;

        ESRI.ArcGIS.ADF.ComReleaser _comReleaser;
        IRow _currentRow = null;
        ICursor _cursor = null;

        public CursorGS(ITable table, IQueryFilter queryFilter)
        {
            _table = table;
            _queryFilter = queryFilter;
            _comReleaser = new ESRI.ArcGIS.ADF.ComReleaser();
        }

        #region IEnumerator<IRow> Members

        public IRow Current
        {
            get { return _currentRow; }
        }

        #endregion

        #region IEnumerator Members

        object System.Collections.IEnumerator.Current
        {
            get { return _currentRow; }
        }

        public bool MoveNext()
        {
            if(_cursor == null) // initialize the cursor
            {
                _cursor = _table.Search(_queryFilter, false);
                _comReleaser.ManageLifetime(_cursor);
            }
            _currentRow = _cursor.NextRow();
            return _currentRow != null;
        }

        public void Reset()
        {
            _cursor = null;
        }

        #endregion

        #region IEnumerable<IRow> Members

        public IEnumerator<IRow> GetEnumerator()
        {
            return (IEnumerator<IRow>)this;
        }

        #endregion

        #region IEnumerable Members

        System.Collections.IEnumerator System.Collections.IEnumerable.GetEnumerator()
        {
            return (System.Collections.IEnumerator)this;
        }

        #endregion

        #region IDisposable Members

        public void Dispose()
        {
            _comReleaser.Dispose();
        }

        #endregion
    }
}

As you can see I used the ESRI ComReleaser object to make sure that the ICursor references get released properly.

So now you know how to create a wrapper around the ICursor you can start creating other wrappers for often used cursors like the IFeatureCursor, IFields, ... and I can go to my bed. Success and feel free to post any comments or your implementation !!!

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