EF Core 6 POST/PUT请求触发System.ArgumentNullException(key参数为空)

使用POST/PUT提交PredefinedElement数据时，遇到异常：

System.ArgumentNullException: Value cannot be null. (Parameter 'key')

触发场景 #

通过Radzen Blazor对话框提交数据，当两个PredefinedElement实体关联同一个Market/Product/ActuationType实体时触发该异常。例如关联表Predefinedelement_Market中存在两条记录：

• PredefinedElementId:1、MarketId:1
• PredefinedElementId:2、MarketId:1

异常发生在API控制器调用_dbContext.Add/Update并执行SaveChangesAsync()时。

相关代码 #

模型定义 #

Market 模型

public class Market
{
    [Key]
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<PredefinedElement> PredefinedElements { get; set; } = new List<PredefinedElement>();
}

PredefinedElement 模型

public class PredefinedElement
{
    [Key]
    public int Id { get; set; }
    public string Name { get; set; }
    // 多对多关联
    public ICollection<Market> Markets { get; set; } = new List<Market>();
    public ICollection<Product> Products { get; set; } = new List<Product>();
    public ICollection<ActuationType> ActuationTypes { get; set; } = new List<ActuationType>();
}

Product 模型

public class Product
{
    [Key]
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<PredefinedElement> PredefinedElements { get; set; } = new List<PredefinedElement>();
}

ActuationType 模型

public class ActuationType
{
    [Key]
    public int Id { get; set; }
    public string Name { get; set; }
    public ICollection<PredefinedElement> PredefinedElements { get; set; } = new List<PredefinedElement>();
}

前端Radzen Blazor提交逻辑 #

Submit 提交方法

async Task Submit()
{
    var predefinedElement = new PredefinedElement
    {
        Id = editModel.Id,
        Name = editModel.Name,
        Markets = selectedMarkets.ToList(),
        Products = selectedProducts.ToList(),
        ActuationTypes = selectedActuationTypes.ToList()
    };

    if (isEditMode)
    {
        await _predefinedElementService.UpdatePredefinedElement(predefinedElement);
    }
    else
    {
        await _predefinedElementService.CreatePredefinedElement(predefinedElement);
    }

    DialogService.Close(true);
}

关联项选择事件

void OnMarketSelected(IEnumerable<Market> selected)
{
    selectedMarkets = selected;
}

void OnProductSelected(IEnumerable<Product> selected)
{
    selectedProducts = selected;
}

void OnActuationTypeSelected(IEnumerable<ActuationType> selected)
{
    selectedActuationTypes = selected;
}

服务层HTTP请求代码 #

public class PredefinedElementService
{
    private readonly HttpClient _httpClient;

    public PredefinedElementService(HttpClient httpClient)
    {
        _httpClient = httpClient;
    }

    public async Task CreatePredefinedElement(PredefinedElement model)
    {
        var response = await _httpClient.PostAsJsonAsync("api/PredefinedElements", model);
        response.EnsureSuccessStatusCode();
    }

    public async Task UpdatePredefinedElement(PredefinedElement model)
    {
        var response = await _httpClient.PutAsJsonAsync($"api/PredefinedElements/{model.Id}", model);
        response.EnsureSuccessStatusCode();
    }
}

API控制器代码 #

[Route("api/[controller]")]
[ApiController]
public class PredefinedElementsController : ControllerBase
{
    private readonly AppDbContext _dbContext;

    public PredefinedElementsController(AppDbContext dbContext)
    {
        _dbContext = dbContext;
    }

    [HttpPost]
    public async Task<IActionResult> Create([FromBody] PredefinedElement model)
    {
        if (!ModelState.IsValid)
        {
            return BadRequest(ModelState);
        }

        _dbContext.PredefinedElements.Add(model);
        await _dbContext.SaveChangesAsync(); // 异常触发点

        return CreatedAtAction(nameof(GetById), new { id = model.Id }, model);
    }

    [HttpPut("{id}")]
    public async Task<IActionResult> Update(int id, [FromBody] PredefinedElement model)
    {
        if (id != model.Id)
        {
            return BadRequest();
        }

        _dbContext.Entry(model).State = EntityState.Modified;

        try
        {
            await _dbContext.SaveChangesAsync(); // 异常触发点
        }
        catch (DbUpdateConcurrencyException)
        {
            if (!PredefinedElementExists(id))
            {
                return NotFound();
            }
            else
            {
                throw;
            }
        }

        return NoContent();
    }

    private bool PredefinedElementExists(int id)
    {
        return _dbContext.PredefinedElements.Any(e => e.Id == id);
    }
}

已尝试的排查动作 #

• 为所有实体主键添加[Key]特性，确认主键配置无误
• 检查数据库关联表结构，外键约束均正常
• 项目由两个项目合并而来，存在另一个关联的数据库上下文，暂未排查上下文冲突

求问如何解决这个多对多关联重复实体时触发的ArgumentNullException问题？

内容的提问来源于stack exchange，提问作者j_mzt