ASP.NET Core 可以通过使用 WebSocket 的 KeepAliveInterval 属性来自动关闭空闲连接。设置了 KeepAliveInterval 属性后，ASP.NET Core 会定期向客户端发送 Ping 帧，如果客户端在特定时间内没有响应，连接就会被自动关闭。

以下是一个代码示例，演示如何设置 WebSocket 的 KeepAliveInterval 属性：

public async Task Echo()
{
    if (HttpContext.WebSockets.IsWebSocketRequest)
    {
        WebSocket webSocket = await HttpContext.WebSockets.AcceptWebSocketAsync();

        // 设置 KeepAliveInterval 属性为 30 秒
        webSocket.Options.KeepAliveInterval = TimeSpan.FromSeconds(30);

        while (webSocket.State == WebSocketState.Open)
        {
            // 接收客户端发送的消息
            var buffer = new byte[1024];
            var result = await webSocket.ReceiveAsync(new ArraySegment(buffer), CancellationToken.None);

            if (result.MessageType == WebSocketMessageType.Text)
            {
                // 处理客户端发送的消息
                var message = Encoding.UTF8.GetString(buffer, 0, result.Count);

                // 将消息发送回客户端
                buffer = Encoding.UTF8.GetBytes(message);
                await webSocket.SendAsync(new ArraySegment(buffer, 0, result.Count), result.MessageType, result.EndOfMessage, CancellationToken.None);
            }
            else if (result.MessageType == WebSocketMessageType.Close)
            {
                // 客户端关闭连接
                break;
            }
        }

        // 关闭 WebSocket 连接
        await webSocket.CloseAsync(WebSocketCloseStatus.NormalClosure, "Connection closed", CancellationToken.None);
    }
    else
    {
        return new StatusCodeResult(StatusCodes.Status404NotFound);
    }
}