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To solve the error message "The connection was not closed. The connection's current state is open" in a C# Winform, you need to ensure that you close the connection after using it. Here are some solutions:

1. Use a try-finally block to ensure that the connection is closed even if an exception is thrown:

SqlConnection connection = new SqlConnection(connectionString);
try
{
    // Do something with the connection
}
finally
{
    connection.Close();
}

1. Use the using statement, which automatically disposes of the connection when it's no longer needed:

using(SqlConnection connection = new SqlConnection(connectionString))
{
    // Do something with the connection
}

1. Check the state of the connection before opening or closing it:

if (connection.State == ConnectionState.Open)
{
    connection.Close();
}

By following one of these solutions, you can ensure that the connection is closed properly and avoid the error message.