How to obtain the last row value of a particular column in the sql database using c# & display it in textbox

See more:

Visual-Studio

C#3.5

hello every one ...I've just started C# programming
problem scenario-
I've taken a table MaintenanceInfo in SQL DB. & assigned primary key to 'ID' column. while adding data through C# I want to add the 'current Id value + 1' & display it into Maintenance ID TextBox. that means if there are 4 records stored then while inserting next record the Maintenance ID should be 5 .

p.s. while adding info I've disabled this Textbox to avoid manual entry as a ID

What I have tried:

public void autoID()
{
con.Open();
string query = "SELECT MAX(MaintenanceID) FROM MaintenanceInfo";
SqlCommand cmd = new SqlCommand(query, con);
int ID = (int)cmd.ExecuteScalar();
ID = ID + 1;
tbMaintID.Text = ID.ToString();
con.Close();
//with this code it shows error -connection still open.....

}
private void MaintenanceDetails_Load(object sender, EventArgs e)
{
autoID();
}