Question

数据库问题。数据是空的。无法在Null值上调用此方法或属性。

Dim query = "Select DISTINCT SUM(bi.[QTY]) as CountQTY From mytable..."
Dim reader1 As SqlDataReader = cmd3.ExecuteReader()
        While reader1.Read()
            If IsDBNull(reader1) Then
                CartCountL.Text = 0
            Else
                CartCountL.Text = reader1.GetInt32(0)
            End If

        End While

Answer 1

因为即使它是Row，您总是至少有一个NULL，您应该检查该值。你可以这样做：

Dim query As SqlCommand = "Select DISTINCT SUM(bi.[QTY]) as CountQTY From mytable..."
Dim reader1 As SqlDataReader = query.ExecuteReader()
CartCountL.Text = "0"

While reader1.Read()
    If IsDBNull(reader1.GetInt32(0)) = False Then
        CartCountL.Text = reader1.GetInt32(0).ToString()
    End If
End While

您还可以在数据库级别处理NULL，如下所示：

Dim query As SqlCommand = "Select DISTINCT SUM(COALESCE(bi.[QTY], 0)) as CountQTY From mytable..."

然后所需的代码很简单：

While reader1.Read()
    CartCountL.Text = reader1.GetInt32(0).ToString()
End While

Answer 2

您应该将IsDBNull更改为IsNothing，如注释中所述，SqlDataReader是一个对象。它永远不会是DBNull，但可以是null / Nothing。

Dim query As String = "Select DISTINCT SUM(bi.[QTY]) as CountQTY 
From mytable..."
Using sqlConn As New SqlConnection(ConnectionString)
     Dim cmd3 As New SqlCommand(query, sqlConn)
     sqlConn.Open()
     Dim reader1 As SqlDataReader = cmd3.ExecuteReader()
     If reader1.HasRows Then
        While reader1.Read()
            CartCountL.Text = reader1.GetInt32(0)
        End While
     Else
        CartCountL.Text = 0
     End If
     reader.Close()
 End Using

Answer 3

只是想分享另一种做法的方法。如果你想检查多列

，我认为这很好

 Dim reader1 As SqlDataReader = cmd3.ExecuteReader()
        While reader1.Read()
            CartCountL.Text = IIf(IsDBNull(reader1("CountQTY")), "0", reader1("CountQTY"))
        End While

vb数据为空。无法在Null值上调用此方法或属性

3 个答案: