mysql - 图表与MySql的连接

Question

我正在连接 mysql 数据库中的图表对象，但收到此错误:

object reference not set to an instance of an object

这是我使用的代码:

Imports MySql.Data.MySqlClient
Public Class Form1
    Dim con As New MySqlConnection
    Dim com As MySqlCommand
    Dim dt As New DataTable

    Private Sub btnlaod_Click(sender As Object, e As EventArgs) Handles btnlaod.Click
        con = New MySqlConnection
        con.ConnectionString = "server=localhost;userid=root;password=;database=noh_mis"
        Dim Reader As MySqlDataReader

        Try
            con.Open()
            Dim query As String
            query = "Select * From database.students_profile"
            Reader = com.ExecuteReader
            While Reader.Read
                Chart1.Series("Male").Points.AddXY(Reader.GetString("Gender"), Reader.GetString("GradeLevel"))
            End While
            con.Close()
        Catch ex As Exception
            MessageBox.Show(ex.Message)
        Finally
            con.Dispose()
        End Try

    End Sub

End Class

Answer 1

在使用com.ExecuteReader之前，您没有发出命令。您创建了一串 sql，但从未将其与数据库连接。

尝试在构建查询之后、Reader = com.ExecuteReader之前添加以下行:

com = New MySqlCommand(query, con)