Mysql 这段代码中我的错误是什么？

您必须使用“新建”创建对象

加上

Imports MySql.Data.MySqlClient
Public Class Form1
    Dim myconn As MySqlConnection
    Dim mycommand As MySqlCommand
    Dim myadapter As MySqlDataAdapter
    Dim mydata As DataTable
    Dim sql As String


    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        myconn = New MySqlConnection

        sql = "select * from inventory"

        myconn.ConnectionString = "server =localhost;" _
            & "user id=root;" _
            & "password=;" _
            & "database=testing"


        Try
            myconn.Open()

            mycommand.Connection = myconn
            mycommand.CommandText = sql

            myadapter.SelectCommand = mycommand
            myadapter.Fill(mydata)

            DataGridView1.DataSource = mydata


        Catch myerror As MySqlException
            MessageBox.Show("Cannot connect to database" & myerror.Message)
        Finally
            myconn.Close()
            myconn.Dispose()


        End Try
    End Sub
    End Class

像这样

mycommand = new MySqlCommand()
myadapter =new MySqlDataAdapter()

---

这是初始化问题更改您的单击事件，如下所示：

Imports MySql.Data.MySqlClient
Public Class Form1
    Dim myconn As MySqlConnection
    Dim mycommand As MySqlCommand
    Dim myadapter As MySqlDataAdapter
    Dim mydata As DataTable
    Dim sql As String


    Private Sub Form1_Load(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles MyBase.Load

    End Sub

    Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
        myconn = New MySqlConnection()
        mycommand = new MySqlCommand()
        myadapter =new MySqlDataAdapter()
        sql = "select * from inventory"

        myconn.ConnectionString = "server =localhost;" _
            & "user id=root;" _
            & "password=;" _
            & "database=testing"


        Try
            myconn.Open()

            mycommand.Connection = myconn
            mycommand.CommandText = sql

            myadapter.SelectCommand = mycommand
            myadapter.Fill(mydata)

            DataGridView1.DataSource = mydata


        Catch myerror As MySqlException
            MessageBox.Show("Cannot connect to database" & myerror.Message)
        Finally
            myconn.Close()
            myconn.Dispose()


        End Try
    End Sub
    End Class

---

我得到了一个空引用豁免的错误。它是编译错误还是运行时错误？错误代码？错误消息？调试断点？预期行为？实际行为？如果你想让你的问题保持开放，你需要比这更明确一点。因此，用户对措词不当的问题反应很快。您在哪一行出现此错误？很抱歉，我是初学者，我可以调试我的程序，但当我单击按钮在datagrid中查看数据时，它会给我一个未经处理的错误，并指向代码mycommand.connection=myconn myconn在myconn下面有一条红线。请帮我完成这篇论文。感谢大家，我刚刚添加了Dim myconn作为MySqlConnection Dim mycommand作为MySqlCommand Dim myadapter作为MySqlDataAdapter Dim mydata作为DataTable Dim sql作为字符串在私有子系统修复后，您的问题现在解决了吗？是的，伙计，谢谢大家。如果对你们有用的话，请将答案标记为参考答案。

Private Sub Button1_Click(ByVal sender As System.Object, ByVal e As System.EventArgs) Handles Button1.Click
    myconn = New MySqlConnection

    sql = "select * from inventory"

    myconn.ConnectionString = "server =localhost;" _
        & "user id=root;" _
        & "password=;" _
        & "database=testing"


    Try
        myconn.Open()

        mycommand = New MySqlCommand(sql,myconn)
        myadapter = New MySqlDataAdapter(mycommand)
        myadapter.Fill(mydata)


        DataGridView1.DataSource = mydata


    Catch myerror As MySqlException
        MessageBox.Show("Cannot connect to database" & myerror.Message)
    Finally
        myconn.Close()
        myconn.Dispose()


    End Try
End Sub