ASP.NET Core Blazor创建新记录时反序列化响应为String报错求助

错误场景 #

开发ASP.NET Core Blazor项目时，创建新服务器记录过程中遇到JSON反序列化错误，错误信息如下：

Error: System.Text.Json.JsonException: The JSON value could not be converted to System.String. Path: $ | LineNumber: 0 | BytePositionInLine: 1.
 ---> System.InvalidOperationException: Cannot get the value of a token type 'StartObject' as a string.
   at System.Text.Json.ThrowHelper.ThrowInvalidOperationException_ExpectedString(JsonTokenType tokenType)
   at System.Text.Json.Utf8JsonReader.GetString()

错误出现在以下语句：

JsonSerializer.DeserializeAsync<string>(responsestream)

相关代码 #

Blazor组件中的CreateClick方法 #

private async Task CreateClick()
{
    var deptobj = new ServerNamesClass() { server_Name = server_Name, server_Type = server_Type, operating_System = operating_System, version = version, isActive = isActive };
    var request = new HttpRequestMessage(HttpMethod.Post, config["API_URL"] + "ServerNames");
    request.Content = new StringContent(JsonSerializer.Serialize(deptobj), null, "application/json");
    var client = ClientFactory.CreateClient();
    var response = await client.SendAsync(request);
    using var responsestream = await response.Content.ReadAsStreamAsync();
    string res = await JsonSerializer.DeserializeAsync<string>(responsestream);
    await JS.InvokeVoidAsync("alert", res);
    await RefreshList();
}

数据模型类 #

public class ServerNamesClass
{
    public int serverID { get; set; }
    public string server_Name { get; set; }
    public string server_Type { get; set; }
    public string operating_System { get; set; }
    public string version { get; set; }
    public bool isActive { get; set; }
}

API控制器代码 #

[HttpPost]
public ActionResult AddServer(ServerNames ServersName)
{
    return Ok(_IserverNamesService.Insert(ServersName));
}

问题原因 #

API控制器通过Ok(_IserverNamesService.Insert(ServersName))返回的是序列化后的实体对象（JSON格式的ServerNames对象），但客户端代码试图将这个JSON对象反序列化为string类型，类型不匹配导致反序列化失败。

解决方案 #

方案一：客户端反序列化为对应实体类（推荐） #

将反序列化的目标类型改为与API返回实体匹配的ServerNamesClass，然后提取需要的信息展示：

private async Task CreateClick()
{
    var deptobj = new ServerNamesClass() { server_Name = server_Name, server_Type = server_Type, operating_System = operating_System, version = version, isActive = isActive };
    var request = new HttpRequestMessage(HttpMethod.Post, config["API_URL"] + "ServerNames");
    request.Content = new StringContent(JsonSerializer.Serialize(deptobj), null, "application/json");
    var client = ClientFactory.CreateClient();
    var response = await client.SendAsync(request);
    using var responsestream = await response.Content.ReadAsStreamAsync();
    // 修改反序列化目标类型为ServerNamesClass
    var res = await JsonSerializer.DeserializeAsync<ServerNamesClass>(responsestream);
    // 展示创建成功的信息，比如服务器ID
    await JS.InvokeVoidAsync("alert", $"服务器创建成功，ID：{res.serverID}");
    await RefreshList();
}

方案二：修改API返回字符串 #

如果业务上确实需要API返回字符串，调整控制器代码，直接返回文本信息：

[HttpPost]
public ActionResult AddServer(ServerNames ServersName)
{
    var insertedServer = _IserverNamesService.Insert(ServersName);
    // 返回字符串形式的成功提示
    return Ok($"服务器创建成功，ID：{insertedServer.serverID}");
}

此时客户端原有的JsonSerializer.DeserializeAsync<string>代码即可正常工作。

内容的提问来源于stack exchange，提问作者ahmed abdelaziz