获取图层字段的唯一值集合（ArcEngine）

       有时我们需要读取图层中某个字段的所有值的唯一值集合，或做统计用途，或作为其他功能的输入。

       一般通过IDataStatistics接口来获取唯一值集合，代码如下：

''' <summary>

    ''' 通过IDataStatistic接口获取图层指定字段的唯一值

    ''' </summary>

    ''' <param name='pFeatureLayer'>目标图层</param>

    ''' <param name='strField'>目标字段名</param>

    ''' <returns>目标字段的所有值的唯一值集合</returns>

    ''' <remarks></remarks>

    Public Function GetUVByDataStatisticsFunction GetUVByDataStatisticsByVal pFeatureLayer As IFeatureLayer, ByVal strField As String As IListOf String

        Dim uvList As IListOf String = New ListOf String

        Dim pQueryFilter As IQueryFilter = New QueryFilter

        Dim pFeatureCur As IFeatureCursor

        pQueryFilter.SubFields = strField

        pFeatureCur = pFeatureLayer.FeatureClass.SearchpQueryFilter, True

        Dim pDataStatic As IDataStatistics = New DataStatistics

        pDataStatic.Field = strField

        pDataStatic.Cursor = pFeatureCur

        Dim pEnumvar As System.Collections.IEnumerator = pDataStatic.UniqueValues

        pEnumvar.Reset

        While pEnumvar.MoveNext

            Dim pObj As Object = pEnumvar.Current

            uvList.AddpObj.ToString

        End While

        Return uvList

    End Function

但当图层的数据量很大时，以上这种方法执行起来速度较慢，性能影响比较明显，得考虑使用另外一种方法。通过IQueryDef接口使用类似SQL的查询语句来获取图层字段的唯一值集合，执行速度等同于执行SQL查询语句。

Public Function GetUVByQueryDefFunction GetUVByQueryDefByVal pFeatureLayer As IFeatureLayer, ByVal strField As String As IListOf String

        Dim uvList As IListOf String = New ListOf String

        Dim pQueryDef As IQueryDef

        Dim pRow As IRow

        Dim pCursor As ICursor

        Dim pFeatureWorkspace As IFeatureWorkspace

        Dim pDataset As IDataset

        pDataset = pFeatureLayer.FeatureClass

        pFeatureWorkspace = pDataset.Workspace

        pQueryDef = pFeatureWorkspace.CreateQueryDef

        With pQueryDef

            .Tables = pDataset.Name ' Fully qualified table name

            .SubFields = 'DISTINCT' & strField & ''

            pCursor = .Evaluate

        End With

        pRow = pCursor.NextRow

        While Not pRow Is Nothing

            Dim pObj As Object = pRow.Value0

            uvList.AddpObj.ToString

            pRow = pCursor.NextRow

        End While

        Return uvList

    End Function