Sets the ShowValueAs properties of a PivotTable.

public SetShowValueAs(PivotTableField.ShowValueAsType showValueAs, SourceField baseField, Int32 baseItemIndex)

public Function SetShowValueAs(ByVal showValueAs As ShowValueAs.ShowValueAsType, ByVal baseField As SourceField, ByVal baseItemIndex As Integer)

The type of ShowValueAs described by ShowValueAsType.

The base field for comparisons.

The index of the base item for comparisons

ExcelApplication xla = new ExcelApplication();
          Workbook wb = xla.Open(@"C:\MySpreadsheet.xlsx");
          Worksheet ws = wb.Worksheets[0];
          PivotTable pt = ws.PivotTables[0];
          DataField df = pt.DataFields[0];
          df.SetShowValueAs(ShowValueAs.ShowValueAsType.Difference, pt.SourceFields[0], 0);

Dim xla As New ExcelApplication()
          Dim wb As Workbook = xla.Open("C:\MySpreadsheet.xlsx")
          Dim ws As Worksheet = wb.Worksheets(0)
          Dim pt As PivotTable = ws.PivotTables(0);
          Dim df As DataField = pt.DataFields(0)
          df.SetShowValueAs(ShowValueAs.ShowValueAsType.Difference, pt.SourceFields(0), 0);

This property cannot be set on ValuesLabels. You can check if a PivotField is calculated field with the IsValuesLabel Property

This is the same as going to Value Field Settings > Show values as.

When calling SetShowValueAs(), certain ShowValueAsTypes use a source field as a base field and an index for one of the values in the data source column associated with the source field. The base field and value specified by the base item index are used to compute the values shown in the PivotTable.

Use ShowValueAs.BaseItemNext and ShowValueAs.BaseItemPrevious to specify Next or Previous.