我从数据库中获取一个值。它出现了一个错误，说“从DBNULL转换为Double无效。”任何人，请帮助我？
Public Function 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
答案 0 :(得分：4)
你不能将DbNull强制转换为double。你可以，把它投射到一个可以为空的双（c＃double？）。
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。一个C＃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
}
}
希望这有帮助！
答案 1 :(得分：1)
尝试将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
答案 2 :(得分：1)
你需要检查一下
在设置之前ds.Tables("ShopCartItem").Rows(0))("Total") = DbNull.Value。
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
答案 3 :(得分：0)
如果值可能为null，则返回类型需要加倍？ （可以双倍）。
答案 4 :(得分：0)
你可以在VB端使用Nullable（Of double）作为数据类型吗？
答案 5 :(得分：0)
您必须检查Nullable(of Double)。HasValue。
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