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