Why I get null value in input file when return model to view?

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Why I get null value in input file when return model to view?

this my model:

<pre lang="C#">public class BookViewModel
{
    public string Author{ get; set; }
    public HttpPostedFileBase Book { get; set; }
}

this my Controller:

C#

[HttpPost]
public ActionResult UploadBook(BookViewModel model)
{
   // DO Stuff
   return View(model);
}

this my View:

Razor

@using (Html.BeginForm("UploadBook", "Home", FormMethod.Post, new { enctype = "multipart/form-data" }))
{
    @Html.TextBoxFor(m => m.Author)
    <br /><br />

    <input type="file" name="Book" /><br />
    <input type="submit" value="submit me"/>
}

Now, when I submit, the model the file upload but when return to view it return null why. please help me.

Thanks.

What I have tried:

I search a lot but I did not found a solution.

Posted 7-Jul-21 5:13am

TheGH00ST91

Updated 8-Jul-21 0:36am

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OriginalGriff · Answer 1 · 2021-07-07T05:46:00

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 Visual Studio 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 debugger 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!

Richard Deeming · Answer 2 · 2021-07-08T00:36:00

Solution 2

The HTML file input cannot be pre-populated from the server. If you re-show your view, the user must select the file again.

This is a security restriction. There is no way around it.

<input type="file"> - HTML: HyperText Markup Language | MDN[^]

Posted 8-Jul-21 0:36am

Richard Deeming

Comments

TheGH00ST91 8-Jul-21 6:43am

Is there way around it to show name of file in input file and save temporary in server side?

Richard Deeming 8-Jul-21 6:46am

No. A file input cannot be pre-populated. There is no way around that.

You can store and display the name of the file in a different property. But you cannot use it to upload the file again. The only file(s) that will be uploaded are those that the user selects in the file input.

TheGH00ST91 8-Jul-21 23:34pm

Thanks for help.