数据表 vb.net 2010 中的 sum 列
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sum column in datatable vb.net 2010
提问by Peps
I am trying to get the sum of the column TotalAmountin my DataTabledtbut I always get this error:
我试图让列的总和TotalAmount我DataTabledt，但我总是得到这样的错误：
Object reference not set to an instance of an object.
你调用的对象是空的。
I am using VB in Visual Studio 2012.
我在 Visual Studio 2012 中使用 VB。
Here's the code:
这是代码：
Dim table As DataTable = CartDataSet.Tables("dt")
Dim result As Integer
result = table.Compute("SUM(TotalAmount)", "")
回答by Mitch Wheat
You are trying to de-reference a NULL object reference, which means tableis NULL.
您正在尝试取消引用 NULL 对象引用，这意味着tableis NULL。
That in turn, means CartDataSetdoes not contain a table named "dt"
反过来，意味着CartDataSet不包含名为的表"dt"
回答by Amit Patel
Dim result As Object
result = table.Compute("SUM(TotalAmount)", "")
then do result.ToString() you will get your output
然后做 result.ToString() 你会得到你的输出
回答by Quethzel Díaz
You can use linq, like this.
您可以像这样使用 linq。
Dim result As Decimal = dt.AsEnumerable().Sum(Function(row) row.Field(Of Decimal)("TotalAmount"))
回答by Abdul
Possible Issues
可能的问题
- Table is null
- No records in the table
- 表为空
- 表中没有记录
Solutions:
解决方案：
1- Better to check the datatable status before computation like
1-最好在计算之前检查数据表状态，例如
If Not table Is Nothing AndAlso table.Rows.Count > 0 Then
...
End If2- Check for DBNULL while computing in case no records
2- 在计算时检查 DBNULL 以防没有记录
IIf(IsDBNull(table.Compute("SUM(TotalAmount)", "")), "0", table.Compute("SUM(TotalAmount)", ""))回答by Yocto Karthi
Dim dt As DataTable = TajDataSet.Tables("MAtab")
Dim sum As Integer = Convert.ToInt32(dt.Compute("SUM(ServiceCharge)", String.Empty))
SCC.Text = sum.ToString
回答by DCraft
For the ".Compute" function to work, you have to declare your variable "result" as "object" instead of "integer". Then later you can convert the value in "result" to an integer using "Convert.ToInt".
要使“.Compute”函数工作，您必须将变量“result”声明为“object”而不是“integer”。然后，您可以使用“Convert.ToInt”将“result”中的值转换为整数。
Dim result as object
