Dim strcmd As String
Dim y As Integer = DateTimePicker1.Value.Year
Dim m As Integer = DateTimePicker1.Value.Month
connstring = "Initial Catalog=jihua;Data Source=.;User ID=sa;Password=zj790214;"
Dim sqlcon As SqlConnection = New SqlConnection(connstring)
sqlcon.Open()
strcmd = "Select sum(需求数量)as a From dbo.sck Where DatePart(yyyy, [日期]) ='" & y & " ' and DatePart(m, [日期]) ='" & m & " '"
Dim sqlcmd As SqlCommand = New SqlCommand(strcmd, sqlcon)
Dim dr As SqlDataReader
dr = sqlcmd.ExecuteReader
If dr.Read() Then
TextBox1.Text = Trim(dr("a"))
Else
MsgBox("111")
End If
出错:其他信息: 从类型“DBNull”到类型“String”的转换无效。