asp.net - 如何将 DBNULL 转换为双精度值？

Question

我从数据库中获取一个值。它出现了一个错误，说“从 DBNULL 到 Double 的转换无效。” 任何人，请帮助我？

公共函数 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

score 5 · Accepted Answer

您不能将 DbNull 强制转换为双精度。但是，您可以将其强制转换为可为空的双精度（c# 双精度？）。

我写了一堆 DataRow 扩展方法 (C#) 对此有很大帮助。使语法更加整洁。用法很简单。AC# 示例：

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 ;
    }
}

这是扩展类：

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

    }
}

希望这可以帮助！

score 1 · Accepted Answer

尝试将您的 SQL 更改为：

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

您还可以将数据库表设置为不允许空值

score 1 · Accepted Answer

您需要 ds.Tables("ShopCartItem").Rows(0))("Total") = DbNull.Value在设置之前检查它。

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

score 0 · Accepted Answer

如果值可能为 null，那么您的返回类型需要为 double 吗？（可为空的双精度）。

score 0 · Accepted Answer

0

您可以改为使用 Nullable(Of double) 作为 VB 端的数据类型吗？

于 2011-01-21T23:09:06.433 回答

score 0 · Accepted Answer

你必须检查是否Nullable(of Double). 有值。总计 = 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

asp.net - 如何将 DBNULL 转换为双精度值？

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