Roundで四捨五入（.NET Framework 2.0からOK）
Dim ret1 As Decimal = Math.Round(1.5, MidpointRounding.AwayFromZero)
結果：2
Dim ret2 As Decimal = Math.Round(2.5, MidpointRounding.AwayFromZero)
結果：3
Dim ret3 As Decimal = Math.Round(-1.5, MidpointRounding.AwayFromZero)
結果：-2
Dim ret4 As Decimal = Math.Round(-2.5, MidpointRounding.AwayFromZero)
結果：-3
