试图通过 mySQL 数据库将数据输入我的 .NET 程序

Question

嗨，我正在尝试将数据库中的数据导入程序中的变量，但我不断收到错误消息：

System.InvalidCastException：“从“DBNull”类型到“String”类型的转换无效。

       Dim value As Integer
       Dim MySqlConn As MySqlConnection
       Dim sql As New MySqlCommand

       Dim dataAdapter As New MySqlDataAdapter
       Dim dataFromDB As New DataSet
       Dim numrows As Integer

       MySqlConn = New MySqlConnection
       MySqlConn.ConnectionString = "server=localhost;user id=root;database=gamedata;"

       Try
           MySqlConn.Open()

           sql = New MySqlCommand("SELECT Ccost FROM cards WHERE `UserName` = '" & UserName & "' AND `Game` = '" & game & "'", MySqlConn)

           dataAdapter = New MySqlDataAdapter(sql)
           dataAdapter.Fill(dataFromDB)
           numrows = dataFromDB.Tables(0).Rows.Count

           For counter = 1 To numrows - 1
               value = dataFromDB.Tables(0).Rows(counter).Item(0)
           Next

           sql = New MySqlCommand("Select Level,Health,Score,PlayerTime FROM savedata WHERE `UserName` = '" & UserName & "' AND `Game` = '" & game & "'", MySqlConn)

           dataAdapter = New MySqlDataAdapter(sql)
           dataAdapter.Fill(dataFromDB)
           numrows = dataFromDB.Tables(0).Rows.Count
           
           'For counter = 0 To 1
           level = dataFromDB.Tables(0).Rows(0).Item(0)
           Phealth = dataFromDB.Tables(0).Rows(0).Item(1)
           score = dataFromDB.Tables(0).Rows(0).Item(2)
           time = dataFromDB.Tables(0).Rows(0).Item(3)

          

       Catch ex As MySqlException
           MsgBox("Error " & ex.Message)
       End Try

数据库代码：

use `Gamedata`;

create table `SaveData`
(`GameCode` int AUTO_INCREMENT not null,`Game` enum('1','2','3','4') not null,`UserName` varchar(20) not null,`level` int not null, `Health` int not null,`Score` int not null,`PlayerTime` time not null,

foreign key(`UserName`) REFERENCES `player` (`UserName`),
primary key(`GameCode`));

Answer 1

这是一种略有不同的方法。 此代码需要文件顶部的以下 Imports。

Imports System.Data.SqlTypes

您需要Using块声明您的数据库对象，这样即使出现错误，它们也会被关闭和处置。 查询应始终使用参数编写。 它不仅避免了 sql 注入，而且更易于读写。

数据库 integer 可以是 null 但在 .net 中不是这样。 SqlInt32有一个可用于测试的.IsNull属性。 如果您尝试将 null 字符串转换为字符串，例如调用.ToString ，您将收到错误消息。

Private ConStr As String = "server=localhost;user id=root;database=gamedata;"

Private Sub OpCode(UserName As String, Game As String)
    Dim value As Integer

    Dim dt As New DataTable

    Dim strSql = "SELECT Ccost FROM cards WHERE `UserName` = @UserName AND `Game` = @Game"
    Try
        Using Sql As New MySqlCommand(strSql, New MySqlConnection(ConStr))
            Sql.Connection.Open()
            Sql.Parameters.Add("@UserName", MySqlDbType.VarChar).Value = UserName
            Sql.Parameters.Add("@Game", MySqlDbType.VarChar).Value = Game
            Dim obj = Sql.ExecuteScalar
            value = If(obj Is Nothing, 0, CInt(obj))
            Sql.CommandText = "Select Level,Health,Score,PlayerTime FROM savedata WHERE `UserName` = @UserName AND `Game` = @Game;"
            Using reader = Sql.ExecuteReader
                dt.Load(reader)
            End Using
        End Using
        Dim level = If(CType(dt.Rows(0)(0), SqlString).IsNull, "", dt.Rows(0)(0).ToString)
        Dim Phealth = If(CType(dt.Rows(0)(1), SqlString).IsNull, "", dt.Rows(0)(1).ToString)
        Dim score = If(CType(dt.Rows(0)(2), SqlInt32).IsNull, 0, CInt(dt.Rows(0)(2)))
        Dim time = If(CType(dt.Rows(0)(3), SqlInt32).IsNull, 0, CInt(dt.Rows(0)(3)))
    Catch ex As MySqlException
        MsgBox("Error " & ex.Message)
    End Try
End Sub