我想通过www.example.com将数据放入SQL表中，分为Txn_Amount和Post_Amount两列vb.net in two columns which are Txn_Amount and Post_Amount
其中textbox3 = Txn_Amount
过帐金额= Textbox4 - textbox3
但我希望如果textbox4 =""，则发布量应为0
这是我的代码：

Call Get_TxnID()
        
        Dim Txn_Amount As String = TextBox3.Text
          Dim Post_Amount As String = Val(TextBox4.Text) - Val(TextBox3.Text)
       

        Dim query As String = "Insert into Txn_Master values (@Txn_Amount,  @Post_Amount)"
        Using cmd As New SqlCommand(query, Connection)
            cmd.Parameters.AddWithValue("Txn_Amount", Txn_Amount)
            cmd.Parameters.AddWithValue("Post_Amount", Post_Amount)
            
            Connection.Open()
            cmd.ExecuteNonQuery()
            Connection.Close()

        End Using
        MsgBox("Transaction Success", MsgBoxStyle.Information)

当我在两个框中都有值时，它工作得很好。例如：-textbox3.text = 25000和textbox4.text = 50000，则Post_Amount为25000
但如果textbox3.text = 25000且textbox4.text =""，则post_amount中显示-25000，但如果textbox4 =""，则post金额应为""或"0"
我试过了

Dim Txn_Amount As String = TextBox3.Text
      If textbox4.text="" then
          Dim Post_Amount As String = ""
      Else
          Dim Post_Amount As String = Val(TextBox4.Text) - Val(TextBox3.Text)
      endif
       
        Dim query As String = "Insert into Txn_Master values (@Txn_Amount,  @Post_Amount)"
        Using cmd As New SqlCommand(query, Connection)

         
            cmd.Parameters.AddWithValue("Txn_Amount", Txn_Amount)
            cmd.Parameters.AddWithValue("Post_Amount", Post_Amount)
            
            Connection.Open()
            cmd.ExecuteNonQuery()
            Connection.Close()
        End Using
        MsgBox("Transaction Success", MsgBoxStyle.Information)

不过，现在可以用了，请帮我一下