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CellクラスのValueプロパティに、コンボボックスの値を設定します。

実行例：

C#
Visual Basic

ComboBoxCellType cbCell = new ComboBoxCellType()
{
    Items = new string[] { "要素1", "要素2", "要素3" },
    Values = new string[] { "1", "2", "3" },
    UseValue = true,
    ShowButton = true
};
FpSpread1.Cells[0, 1].CellType = cbCell;
//コンボボックス型セルに値を設定します
FpSpread1.Cells[0, 1].Value = 2;

Dim cbCell As New ComboBoxCellType() With {
    .Items = New String() {"要素1", "要素2", "要素3"},
    .Values = New String() {"1", "2", "3"},
    .UseValue = True
    .ShowButton = True
}
FpSpread1.Cells(0, 1).CellType = cbCell
'コンボボックス型セルに値を設定します
FpSpread1.Cells(0, 1).Value = 2

クライアント側で処理することもできます。

C#
Visual Basic

ComboBoxCellType cbCell = new ComboBoxCellType()
{
    Items = new string[] { "要素1", "要素2", "要素3" },
    Values = new string[] { "1", "2", "3" },
    UseValue = true,
    ShowButton = true
};
FpSpread1.Cells[0, 1].CellType = cbCell;

Dim cbCell As New ComboBoxCellType() With {
    .Items = New String() {"要素1", "要素2", "要素3"},
    .Values = New String() {"1", "2", "3"},
    .UseValue = True
    .ShowButton = True
}
FpSpread1.Cells(0, 1).CellType = cbCell

JavaScript

<script type="text/javascript">
     window.onload = function () {
         var spread = document.getElementById("FpSpread1");
         spread.SetValue(0, 1, "2");
     }
</script>