asp.net - DBNULLをdoubleに変換する方法は？

Question

データベースから値を取得しています。「DBNULLからDoubleへの変換は無効です」というエラーが発生しました。誰か、plsは私を助けますか？

パブリック関数Total（）As Double Dim Total As Double

    Dim strConn As String
    strConn = ConfigurationManager.ConnectionStrings("***").ToString
    Dim conn As New SqlConnection(strConn)

    Dim strSql As String
    strSql = "SELECT SUM (p.Price * c.Quantity) as 'Total' " & _
    "FROM CartItem sci INNER JOIN Product p ON c.ProductID=p.ProductID " & _
    "WHERE c.CartID=@CartID "

    Dim cmd As New SqlCommand(strSql, conn)

    cmd.Parameters.AddWithValue("@CartID", CartID)

    Dim da As New SqlDataAdapter(cmd)

    Dim ds As New DataSet

    conn.Open()

    da.Fill(ds, "CartItem")

    conn.Close()

    If (ds.Tables("CartItem").Rows.Count) <> 0 Then
        **Total = ds.Tables("ShopCartItem").Rows(0)("Total")**
    Else
        Total = 0.0
    End If
    Return Total
End Function

Answer 1

DbNullをdoubleにキャストすることはできません。ただし、null許容のdouble（c＃double？）にキャストすることはできます。

これに大いに役立つDataRow拡張メソッド（C＃）をたくさん書きました。構文をかなり整理します。使い方は簡単です。AC＃の例：

public class Hormone
{
    public int           ID               { get ; private set ; }
    public HormoneLuType Type             { get ; private set ; }
    public int           AgeStarted       { get ; private set ; }
    public int           AgeStopped       { get ; private set ; }
    public int           DurationInMonths { get ; private set ; }
    public bool          IsCurrentlyUsing { get ; private set ; }
    public DateTime?     DateLastEdited   { get ; private set ; }

    public string Name { get { return Type.ToString() } }

    public Hormone( DataRow dr )
    {
        this.ID               =                    dr.CastAsInt(              "ihormoneid" )        ;
        this.Type             = new HormoneLuType( dr.CastAsIntNullable(      "ihormluid"  ) ?? 0 ) ;
        this.AgeStarted       = (int)              dr.CastAsDecimal(          "nstartage"  )        ;
        this.AgeStopped       = (int)              dr.CastAsDecimal(          "nendage"    )        ;
        this.DurationInMonths = (int)              dr.CastAsDecimal(          "nduration"  )        ;
        this.IsCurrentlyUsing =                    dr.CastAsBool(             "lusingnow"  )        ;
        this.DateLastEdited   =                    dr.CastAsDateTimeNullable( "tedit"      )        ;

        return ;
    }
}

拡張クラスは次のとおりです。

using System;
using System.Data;

namespace DataAccess.Utils
{
    public static class DataRowExtensions
    {

        #region downcast to DateTime

        public static DateTime CastAsDateTime( this DataRow row , int index )
        {
            return toDateTime( row[index] ) ;
        }
        public static DateTime CastAsDateTime( this DataRow row , string columnName )
        {
            return toDateTime( row[columnName] ) ;
        }

        public static DateTime? CastAsDateTimeNullable( this DataRow row , int index )
        {
            return toDateTimeNullable( row[index] );
        }
        public static DateTime? CastAsDateTimeNullable( this DataRow row , string columnName )
        {
            return toDateTimeNullable( row[columnName] ) ;
        }

        #region conversion helpers

        private static DateTime toDateTime( object o )
        {
            DateTime value = (DateTime)o;
            return value;
        }

        private static DateTime? toDateTimeNullable( object o )
        {
            bool  hasValue = !( o is DBNull );
            DateTime? value    = ( hasValue ? (DateTime?) o : (DateTime?) null ) ;
            return value;
        }

        #endregion

        #endregion downcast to DateTime

        #region downcast to byte[]

        public static byte[] CastAsByteArray( this DataRow row , int index )
        {
            return toByteArray( row[index] );
        }
        public static byte[] CastAsByteArray( this DataRow row , string columnName )
        {
            return toByteArray( row[columnName] );
        }

        #region conversion helpers

        private static byte[] toByteArray( object o )
        {
            bool   hasValue = !( o is DBNull );
            byte[] value    = ( hasValue ? (byte[]) o : (byte[]) null ) ;
            return value;
        }

        #endregion

        #endregion downcast to Byte[]

        #region downcast to int

        public static int CastAsInt( this DataRow row , int index )
        {
            return toInt( row[index] ) ;
        }
        public static int CastAsInt( this DataRow row , string columnName )
        {
            return toInt( row[columnName] ) ;
        }

        public static int? CastAsIntNullable( this DataRow row , int index )
        {
            return toIntNullable( row[index] );
        }
        public static int? CastAsIntNullable( this DataRow row , string columnName )
        {
            return toIntNullable( row[columnName] ) ;
        }

        #region conversion helpers

        private static int toInt( object o )
        {
            int value = (int)o;
            return value;
        }

        private static int? toIntNullable( object o )
        {
            bool hasValue = !( o is DBNull );
            int? value    = ( hasValue ? (int?) o : (int?) null ) ;
            return value;
        }

        #endregion

        #endregion downcast to int

        #region downcast to int

        public static decimal CastAsDecimal( this DataRow row , int index )
        {
            return toDecimal( row[index] ) ;
        }
        public static decimal CastAsDecimal( this DataRow row , string columnName )
        {
            return toDecimal( row[columnName] ) ;
        }

        public static decimal? CastAsDecimalNullable( this DataRow row , int index )
        {
            return toDecimalNullable( row[index] );
        }
        public static decimal? CastAsDecimalNullable( this DataRow row , string columnName )
        {
            return toDecimalNullable( row[columnName] ) ;
        }

        #region conversion helpers

        private static decimal toDecimal( object o )
        {
            decimal value = (decimal)o;
            return value;
        }

        private static decimal? toDecimalNullable( object o )
        {
            bool     hasValue = !( o is DBNull );
            decimal? value    = ( hasValue ? (decimal?) o : (decimal?) null ) ;
            return value;
        }

        #endregion

        #endregion downcast to int

        #region downcast to bool

        public static bool CastAsBool( this DataRow row , int index )
        {
            return toBool( row[index] ) ;
        }
        public static bool CastAsBool( this DataRow row , string columnName )
        {
            return toBool( row[columnName] ) ;
        }

        public static bool? CastAsBoolNullable( this DataRow row , int index )
        {
            return toBoolNullable( row[index] );
        }
        public static bool? CastAsBoolNullable( this DataRow row , string columnName )
        {
            return toBoolNullable( row[columnName] ) ;
        }

        #region conversion helpers

        private static bool toBool( object o )
        {
            bool value = (bool)o;
            return value;
        }

        private static bool? toBoolNullable( object o )
        {
            bool  hasValue = !( o is DBNull );
            bool? value    = ( hasValue ? (bool?) o : (bool?) null ) ;
            return value;
        }

        #endregion

        #endregion downcast to bool

        #region downcast to string

        public static string CastAsString( this DataRow row , int index )
        {
            return toString( row[index] );
        }
        public static string CastAsString( this DataRow row , string columnName )
        {
            return toString( row[columnName] );
        }

        #region conversion helpers

        private static string toString( object o )
        {
            bool   hasValue = !( o is DBNull );
            string value    = ( hasValue ? (string) o : (string) null ) ;
            return value;
        }

        #endregion

        #endregion downcast to string

    }
}

お役に立てれば！

Answer 2

SQL を次のように変更してみてください。

SELECT SUM (ISNULL(p.Price,0) * ISNULL(c.Quantity,0) ) as 'Total' " & _
    "FROM CartItem sci INNER JOIN Product p ON c.ProductID=p.ProductID " & _
    "WHERE c.CartID=@CartID 

null を許可しないようにデータベース テーブルを設定することもできます。

Answer 3

ds.Tables("ShopCartItem").Rows(0))("Total") = DbNull.Value設定する前に確認する必要があります 。

If (ds.Tables("CartItem").Rows.Count) <> 0 Then
    If ds.Tables("ShopCartItem").Rows(0))("Total") = DbNull.Value
       Total = 0.0
    else
      Total = ds.Tables("ShopCartItem").Rows(0)("Total")**
Else
    Total = 0.0
End If

Answer 4

かどうかを確認する必要がありNullable(of Double)ます。値を持っています。合計 = 0.0

If (ds.Tables("CartItem").Rows.Count) <> 0 Then
     Dim sciValue As Nullable(Of Double) = DirectCast(ds.Tables("ShopCartItem").Rows(0)("Total"), Nullable(Of Double))
     If sciValue.HasValue Then
         Total = sciValue.Value
     End If
End If

Answer 5

値が null になる可能性がある場合、戻り値の型は double にする必要がありますか? (nullable double)。

Answer 6

代わりに、VB 側のデータ型として Nullable(Of double) を使用できますか?