要在ASP.NET Core中保持与ESP32的连接，你可以使用WebSocket来实现。下面是一个示例代码：

首先，在ASP.NET Core项目中创建一个WebSocket中间件。在Startup.cs文件的Configure方法中添加以下代码：

app.UseWebSockets();

app.Use(async (context, next) =>
{
    if (context.Request.Path == "/ws")
    {
        if (context.WebSockets.IsWebSocketRequest)
        {
            WebSocket webSocket = await context.WebSockets.AcceptWebSocketAsync();
            await HandleWebSocket(webSocket);
        }
        else
        {
            context.Response.StatusCode = 400;
        }
    }
    else
    {
        await next();
    }
});

接下来，在HandleWebSocket方法中处理WebSocket连接和消息：

private async Task HandleWebSocket(WebSocket webSocket)
{
    var buffer = new byte[1024];
    WebSocketReceiveResult result = await webSocket.ReceiveAsync(new ArraySegment(buffer), CancellationToken.None);
    
    while (!result.CloseStatus.HasValue)
    {
        string message = Encoding.UTF8.GetString(buffer, 0, result.Count);
        // 处理接收到的消息
        
        result = await webSocket.ReceiveAsync(new ArraySegment(buffer), CancellationToken.None);
    }
    
    await webSocket.CloseAsync(result.CloseStatus.Value, result.CloseStatusDescription, CancellationToken.None);
}

在ESP32设备上，你需要使用Arduino IDE和ESPAsyncWebServer库来创建一个WebSocket客户端。以下是一个示例代码：

#include 
#include 
#include 

const char* ssid = "your_SSID";
const char* password = "your_PASSWORD";
const char* serverIP = "your_ASPNETCore_server_IP";
const int serverPort = 80;
const char* websocketPath = "/ws";

WiFiClient client;
AsyncWebSocketClient *webSocketClient;

void setup() {
  Serial.begin(115200);
  
  WiFi.begin(ssid, password);
  while (WiFi.status() != WL_CONNECTED) {
    delay(1000);
    Serial.println("Connecting to WiFi...");
  }

  if (client.connect(serverIP, serverPort)) {
    Serial.println("Connected to server");
    webSocketClient = new AsyncWebSocketClient(&client);
    webSocketClient->onMessage(onWebSocketMessage);
    webSocketClient->onEvent(onWebSocketEvent);
    
    String handshake = "GET " + String(websocketPath) + " HTTP/1.1\r\n" +
                       "Host: " + String(serverIP) + ":" + String(serverPort) + "\r\n" +
                       "Connection: Upgrade\r\n" +
                       "Upgrade: websocket\r\n" +
                       "Sec-WebSocket-Key: dGhlIHNhbXBsZSBub25jZQ==\r\n" +
                       "Sec-WebSocket-Version: 13\r\n\r\n";
    client.print(handshake);
  } else {
    Serial.println("Failed to connect to server");
  }
}

void loop() {
  // 处理其他任务
}

void onWebSocketMessage(void *arg, uint8_t *data, size_t len) {
  // 处理接收到的消息
}

void onWebSocketEvent(void *arg, asyncWebSocketClientEvent_t *event) {
  if (event->type == WS_EVT_CONNECT) {
    Serial.println("WebSocket connected");
  } else if (event->type == WS_EVT_DISCONNECT) {
    Serial.println("WebSocket disconnected");
  }
}

确保将代码中的"your_SSID"、"your_PASSWORD"、"your_ASPNETCore_server_IP"替换为你的WiFi信息和ASP.NET Core服务器的IP地址。

这样，ASP.NET Core服务器和ESP32设备之间就建立了WebSocket连接，并可以通过发送和接收消息来保持连接。