How to fix COM object that has been separated from its underlying RCW cannot be used.

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Hello, I'm having a hard time solving this one I'm working with vs2017 and access as my database. I don't really know what happened here.

I have a COMBOBOX that displays the name of all category name field from my database and from there I can delete a row in my database using the name displayed in the combobox But I encountered that error.

Below is my code:

VB

Private Sub cmbCategory_SelectedIndexChanged(sender As Object, e As EventArgs) Handles cmbCategory.SelectedIndexChanged

Using cons As New OleDb.OleDbConnection(conSTR)
    Dim rdr As OleDb.OleDbDataReader

    With cmd
        .Connection = cons
        .CommandText = "SELECT CategoryName, CatPercent FROM Category WHERE CategoryName = @cn"
        .Parameters.AddWithValue("cn", cmbCategory.Text)
        .Connection.Open()
        rdr = cmd.ExecuteReader

        If rdr.HasRows Then

            While rdr.Read()

                txtCategory.Text = rdr("CategoryName").ToString
                numCategory.Text = rdr("CatPercent")

            End While

            rdr.Close()
            .Connection.Close()

        Else
            MessageBox.Show("NO DATA FOUND", "CATEGORY", MessageBoxButtons.OK, MessageBoxIcon.Exclamation)

        End If


    End With


        End Using


    End Sub

The exception occurs in CONS which holds my connection the one I made bold and underlined above. I don't know what does it mean.

It occurs after I select in my combo box a name and then I delete it and then I select again and try to delete but this error pops up

COM object that has been separated from its underlying RCW cannot be used.

What I have tried:

I thought it's just my connection string that causing multiple connections but it wasn't.

It occurs after I select in my combo box a name and then I delete it and then I select again and try to delete but this error pops up <pre>COM object that has been separated from its underlying RCW cannot be used.

Posted 1-Apr-20 18:10pm

lelouch_vi 2

Updated 2-Apr-20 2:25am

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Comments

Richard MacCutchan 2-Apr-20 5:02am

Why are you trying to delete the same item more than once?

The error message suggests that you may be trying to access cons from a separate thread in your application.

Richard Deeming 2-Apr-20 8:20am

Why are you reusing the OleDbCommand instance cmd? Just create a new instance for each command you want to execute, and wrap it in a Using block.

rdr should also be wrapped in a Using block.

lelouch_vi 2 2-Apr-20 21:12pm

I see.Thank you for pointing that out. The error indeed is gone but can you explain a bit more about creating instances of cmd? I thought if I declare cmd as a local variable it can't be accessed by other functions?

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Richard Deeming · Accepted Answer · 2020-04-02T02:26:00

I suspect the problem is that you are storing your OleDbCommand object in a class-level field, and reusing it across methods.

Change your code to create the command instance when you need it, and wrap it in a Using block, and your error will almost certainly go away.

VB.NET

Private Sub cmbCategory_SelectedIndexChanged(sender As Object, e As EventArgs) Handles cmbCategory.SelectedIndexChanged

    Using cons As New OleDb.OleDbConnection(conSTR)
        Using cmd As New OleDb.OleDbCommand()
            cmd.Connection = cons
            cmd.CommandText = "SELECT CategoryName, CatPercent FROM Category WHERE CategoryName = @cn"
            cmd.Parameters.AddWithValue("cn", cmbCategory.Text)
            
            cons.Open()
            Using rdr As OleDb.OleDbDataReader = cmd.ExecuteReader()
                If rdr.Read() Then
                    txtCategory.Text = rdr("CategoryName").ToString
                    numCategory.Text = rdr("CatPercent")
                Else
                    MessageBox.Show("NO DATA FOUND", "CATEGORY", MessageBoxButtons.OK, MessageBoxIcon.Exclamation)
                End If
            End Using
        End Using
    End Using
End Sub