How do I fix the null object reference?

Question

1.00/5 (1 vote)

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I get the null reference on this line

C#

if (objfile.files.Length > 0)

so that tell me it isn't passing the file name.I am not sure where to go from here.

C#

using System;
using System.IO;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Hosting;
using Microsoft.AspNetCore.Http;
using Microsoft.AspNetCore.Mvc;
namespace ImageUploadDemo.Controllers
{
    [Route("api/[controller]")]
    public class ImageController : ControllerBase
    {
        public static IWebHostEnvironment _environment;
        public ImageController(IWebHostEnvironment environment)
        {
            _environment = environment;
        }
        public class ImageUploadAPI
        {
            public IFormFile files
            {
                get;
                set;
            }
        }
        [HttpPost]
        public Task<string> Post(ImageUploadAPI objfile)
        {
            if (objfile.files.Length > 0)
            {
                try
                {
                    if (!Directory.Exists(_environment.WebRootPath + "\\uploads\\"))
                    {
                        Directory.CreateDirectory(_environment.WebRootPath + "\\uploads\\");
                    }
                    using (FileStream filestream = System.IO.File.Create(_environment.WebRootPath + "\\uploads\\" + objfile.files.FileName))
                    {
                        objfile.files.CopyTo(filestream);
                        filestream.Flush();
                        return Task.FromResult("\\uploads\\" + objfile.files.FileName);
                    }
                }
                catch (Exception ex)
                {
                    return Task.FromResult(ex.ToString());
                }
            }
            else
            {
                return Task.FromResult("Unsuccessful");
            }

        }
    }
}

What I have tried:

I have re-written a few times with the same error.

Posted 7-Apr-21 9:45am

Member 10352786

Updated 7-Apr-21 10:20am

Chris Maunder772.9K

v2

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2 solutions

Solution 2

To fix a null-ref error you need to know

1. What is actually "null"
2. Why that variable is not being set to something

First: Check objfile.files.Length

objfile is an object so could be null
files is a property of the that's an array. It's possible for this to be null
Length is a scalar (a number) and can't be null

So the code is trying to get the Length of files of objfile. If files is null you're trying to get objfile of "null". If files is null you're trying to get Length of null.

So to protect your code you can do

C#

int length = 0;
if (objfile != && objfile.files != null)
    length = objfile.files.Length;

or

C#

int length = objfile?.files?.Length ?? 0;

Second: stopping the null-ref happening

This you can't do. If the upload didn't work, if there were errors, if there were simply no files uploaded, then you'll see null. Except nulls on every variable that can be null. It'll get tedious. Welcome to being a developer.

Posted 7-Apr-21 10:20am

Chris Maunder772.9K

Comments

Member 10352786 7-Apr-21 17:00pm

thank you for your help
this is all this api does so I am not sure what I am missing to set file to the file name that is being sent.
I am trying to learn rest to create a service that just gets pass a file and I have not used rest

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OriginalGriff · Accepted Answer · 2021-04-07T10:01:00

This is one of the most common problems we get asked, and it's also the one we are least equipped to answer, but you are most equipped to answer yourself.

Let me just explain what the error means: You have tried to use a variable, property, or a method return value but it contains null - which means that there is no instance of a class in the variable.
It's a bit like a pocket: you have a pocket in your shirt, which you use to hold a pen. If you reach into the pocket and find there isn't a pen there, you can't sign your name on a piece of paper - and you will get very funny looks if you try! The empty pocket is giving you a null value (no pen here!) so you can't do anything that you would normally do once you retrieved your pen. Why is it empty? That's the question - it may be that you forgot to pick up your pen when you left the house this morning, or possibly you left the pen in the pocket of yesterday's shirt when you took it off last night.

We can't tell, because we weren't there, and even more importantly, we can't even see your shirt, much less what is in the pocket!

Back to computers, and you have done the same thing, somehow - and we can't see your code, much less run it and find out what contains null when it shouldn't.
But you can - and the debugger will help you here. Run your program in the debugger and when it fails, it will show you the line it found the problem on. You can then start looking at the various parts of it to see what value is null and start looking back through your code to find out why. So put a breakpoint at the beginning of the method containing the error line, and run your program from the start again. This time, the debuggger will stop before the error, and let you examine what is going on by stepping through the code looking at your values.

But we can't do that - we don't have your code, we don't know how to use it if we did have it, we don't have your data. So try it - and see how much information you can find out!