How to manage NullReferenceException?

Question

I want to make a form that user have to fill out it. But I'm getting an error.

Model:

public UserReport Input;

[Key]
//[Required]
public string ReportID { get; set; }


[Required]
public string Description { get; set; }

[Required]
[DataType(DataType.DateTime)]
public string Date { get; set; }

Controller:

private ApplicationDbContext _userReport;
public UserReport Input;

public PageController(ApplicationDbContext userReport)
{
    _userReport = userReport;
}

[HttpGet]
public IActionResult SendReport()
{
    return View();
}

[Authorize]
[HttpPost]
public async Task<IActionResult> SendReport(UserReport userReport)
{
    var user = new UserReport { Description = Input.Description, Date = Input.Date };
    var r = await _userReport.AddAsync(user);
}

View:

@model UserReport
<div class="col-md-6">
    <form id = "SendReport" method="post">
        <div asp-validation-summary="All" class="text-danger"></div>

        <div class="form-group">
            <label asp-for="Input.Description"></label>
            <input asp-for="Input.Description" class="form-control" />
            <span asp-validation-for="Input.Description" class="text-danger"></span>
        </div>

        <div class="form-group">
            <label asp-for="Input.Date"></label>
            <input asp-for="Input.Date" class="form-control" />
            <span asp-validation-for="Input.Date" class="text-danger"></span>
        </div>


        <button type = "submit" class="btn btn-primary">send report</button>
    </form>
</div>

what is wrong with that? It shows me this Error :System.NullReferenceException - Object reference not set to an instance of an object... who can help me to manage this error. Show me where is my mistake please

Can you add on which line of the controller you posted you get the error? — Davey van Tilburg, Apr 22 '20 at 07:53
on this line: var user = new UserReport { Description = Input.Description, Date = Input.Date;} Did i use correct method? — Mohadese99, Apr 22 '20 at 09:26
`public UserReport Input;` - what's this for, in either class? Your NullReferenceException is because this is null. I think you need to just use the members of the UserReport class without the `Input.`, e.g. in the SendReport method you want `userReport.Description` etc. not `Input.` — Rup, Apr 22 '20 at 11:03
If I don't use Input then It won't accepted my View.then I put Input in both class.Do you have any suggestion for that? — Mohadese99, Apr 22 '20 at 17:18

score 3 · Answer 1 · answered Apr 22 '20 at 12:33

I think you need to go back through the tutorials in the docs again. It doesn't seem like you really know what you're doing here. You've got a field named Input, which seems to be pulled from a Razor Page, but you're working in an MVC controller here. However, even that is off, because you'd typically see that as something like:

[BindProperty]
public UserReport Input { get; set; }

In a Razor Page. Here, it's not even a property, so even if this would normally do something, it wouldn't as a field, regardless.

The NullReferenceException comes because you reference this field, but never actually initialize it. Again, in a Razor Page (and if it was a property rather than a field), this would get filled with the post data (via BindProperty), but it doesn't work that way in a controller.

In your controller action, you've got a userReport param, so that is where the post data will go. However, since all the asp-for attributes in your view reference Input.Something, nothing will actually get bound to this param. This too seems to be pulled from a Razor Page, without considering that it only works this way in a Razor Page.

Long and short, it looks like you just copied code from other places, without actually understanding any of it or what it does, and cobbled it together into a controller and view. The whole thing is fundamentally broken top-down.

score 0 · Accepted Answer · answered Apr 23 '20 at 06:54

You have mixed with MVC and Razor Pages.

For MVC:

1.Model:

public class UserReport
{
    [Key]
    //[Required]
    public string ReportID { get; set; }

    [Required]
    public string Description { get; set; }

    [Required]
    [DataType(DataType.DateTime)]
    public string Date { get; set; }
}

2.SendReport.cshtml:

@model UserReport
<div class="col-md-6">
    <form asp-action="SendReport" method="post">
        <div asp-validation-summary="All" class="text-danger"></div>

        <div class="form-group">
            <label asp-for="Description"></label>
            <input asp-for="Description" class="form-control" />
            <span asp-validation-for="Description" class="text-danger"></span>
        </div>

        <div class="form-group">
            <label asp-for="Date"></label>
            <input asp-for="Date" class="form-control" />
            <span asp-validation-for="Date" class="text-danger"></span>
        </div>


        <button type="submit" class="btn btn-primary">send report</button>
    </form>
</div>

3.Controller:

public class UserReportsController : Controller
{
    private readonly YourDbContext _context;

    public UserReportsController(YourDbContext context)
    {
        _context = context;
    }
    public IActionResult SendReport()
    {
        return View();
    }

    [HttpPost]
    public async Task<IActionResult> SendReport([Bind("ReportID,Description,Date")] UserReport userReport)
    {
        if (ModelState.IsValid)
        {
            _context.Add(userReport);
            await _context.SaveChangesAsync();
            return RedirectToAction(nameof(Index));
        }
        return View(userReport);
    }
}

For Razor Pages:

1.Model:

public class UserReport
{
    [Key]
    //[Required]
    public string ReportID { get; set; }


    [Required]
    public string Description { get; set; }

    [Required]
    [DataType(DataType.DateTime)]
    public string Date { get; set; }
}

2.SendReport.cshtml:

@page
@model SendReportModel

<div class="col-md-6">
    <form id="SendReport" method="post">
        <div asp-validation-summary="All" class="text-danger"></div>

        <div class="form-group">
            <label asp-for="Input.Description"></label>
            <input asp-for="Input.Description" class="form-control" />
            <span asp-validation-for="Input.Description" class="text-danger"></span>
        </div>

        <div class="form-group">
            <label asp-for="Input.Date"></label>
            <input asp-for="Input.Date" class="form-control" />
            <span asp-validation-for="Input.Date" class="text-danger"></span>
        </div>


        <button type="submit" class="btn btn-primary">send report</button>
    </form>
</div>

3.SendReport.cshtml.cs:

public class SendReportModel : PageModel
{
    private readonly YourDbContext _context;

    public SendReportModel(YourDbContext context)
    {
        _context = context;
    }

    public IActionResult OnGet()
    {
        return Page();
    }

    [BindProperty]
    public UserReport Input { get; set; }

    public async Task<IActionResult> OnPostAsync()
    {
        if (!ModelState.IsValid)
        {
            return Page();
        }

        _context.UserReport.Add(Input);
        await _context.SaveChangesAsync();

        return RedirectToPage("./Index");
    }
}

Reference:

How to manage NullReferenceException?

2 Answers2