PostAsJsonAsync<string>无法反序列化响应，求排查与修复方案

你遇到的错误'e' is an invalid start of a value是典型的JSON反序列化失败问题，咱们先梳理下你的代码和错误细节：

API代码 #

public class AuthenticationController : ApiController { 
    public async Task<ActionResult<string>> PostAsync(CreateTokenCommand credentials) { 
        string test= "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9"; 
        return Ok(test); 
    } 
}

客户端调用代码 #

public async Task Login(UserCredentials model) { 
    try { 
        var result = await _httpClient.PostJsonAsync<string>("https://localhost:44373/api/authentication", model); 
    } catch (Exception ex) { 
        throw ex; 
    } 
}

错误信息 #

'e' is an invalid start of a value. Path: $ | LineNumber: 0 | BytePositionInLine: 0.

堆栈跟踪 #

at System.Text.Json.ThrowHelper.ReThrowWithPath(ReadStack& readStack, JsonReaderException ex)
at System.Text.Json.JsonSerializer.ReadCore(JsonSerializerOptions options, Utf8JsonReader& reader, ReadStack& readStack)
at System.Text.Json.JsonSerializer.ReadCore(Type returnType, JsonSerializerOptions options, Utf8JsonReader& reader)
at System.Text.Json.JsonSerializer.Deserialize(string json, Type returnType, JsonSerializerOptions options)
at System.Text.Json.JsonSerializer.Deserialize<TValue>(string json, JsonSerializerOptions options)
at Microsoft.AspNetCore.Components.HttpClientJsonExtensions.d__6`1.MoveNext()
at System.Runtime.ExceptionServices.ExceptionDispatchInfo.Throw()
at System.Runtime.CompilerServices.TaskAwaiter.ThrowForNonSuccess(Task task)
at System.Runtime.CompilerServices.TaskAwaiter.HandleNonSuccessAndDebuggerNotification(Task task)
at System.Runtime.CompilerServices.TaskAwaiter`1.GetResult()
at CheckerApp.WebUI.Services.JWTAuthenticationService.d__3.MoveNext() in D:\MyProjects\CheckerApp\CheckerApp.WebUI\Services\JWTAuthenticationService.cs:line 38

问题根源 #

当你在API里用return Ok(test);返回字符串时，ASP.NET Core会把它以**纯文本（Plain Text）**的形式返回，响应内容就是eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9，没有包裹JSON要求的双引号。

但客户端的PostJsonAsync<string>方法会默认把响应内容当成JSON格式字符串解析，而JSON规范要求字符串必须用双引号包裹。纯文本的token不符合JSON格式，所以序列化器会报错，认为开头的'e'不是有效的JSON值起始字符（JSON有效起始字符为{、[、"、数字、true/false/null）。

修复方案 #

方案一：修改API返回JSON格式的字符串

让API输出符合JSON规范的内容，有两种实现方式：

1. 返回包含token的匿名对象
   ASP.NET Core会自动把对象序列化为JSON格式：

   public async Task<ActionResult> PostAsync(CreateTokenCommand credentials) { 
       string test= "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9"; 
       return Ok(new { Token = test }); 
   }
   

   客户端需要对应调整泛型类型，比如用dynamic或者定义一个带Token属性的类：

   var result = await _httpClient.PostJsonAsync<dynamic>("https://localhost:44373/api/authentication", model);
   string token = result.Token;
   

2. 手动序列化字符串为JSON格式
   直接返回包裹双引号的JSON字符串：

   public async Task<ActionResult> PostAsync(CreateTokenCommand credentials) { 
       string test= "eyJhbGciOiJIUzI1NiIsInR5cCI6IkpXVCJ9"; 
       return Content(JsonSerializer.Serialize(test), "application/json"); 
   }
   

   这样客户端用PostJsonAsync<string>就能正常解析响应了。

方案二：修改客户端读取纯文本响应

如果不想改动API，客户端可以跳过JSON反序列化，直接读取纯文本内容：

public async Task Login(UserCredentials model) { 
    try { 
        var content = JsonContent.Create(model);
        var response = await _httpClient.PostAsync("https://localhost:44373/api/authentication", content);
        response.EnsureSuccessStatusCode(); // 确保请求成功
        var result = await response.Content.ReadAsStringAsync();
    } catch (Exception ex) { 
        throw ex; 
    } 
}

内容的提问来源于stack exchange，提问作者Edge of the World