Sending file in API request .NET

Question

I have an enpoint that I send request via Postman form-data. Request has 2 keys.

message : string
file: File

this works ok. So, I want to send the same request in C# code. In c# I just need to choose file from local, somehow convert it to proper format, assign it to file property in request Model, I mean I can not change the logic how request is sent. I must not write additional logic to HttpRequest or MultipartFormDataContent etc (I can not even see this part how it is sent). Just need to add Model to this request body. I have model RequestModel

public class RequestModel
{
[JsonProperty("message")]
public string Message {get; set; }

[JsonProperty("file")]
public whateverType File {get; set; }
}

message part works, so I need to assign local file to File property. how can I do that. I tried almost everything but it does not work. Thanks.

Does this answer your question? [How to upload files in asp.net core?](https://stackoverflow.com/questions/35379309/how-to-upload-files-in-asp-net-core) — Neil, Feb 05 '22 at 16:54
This SO thread is similar for asp.net core: https://stackoverflow.com/questions/54411250/how-to-send-multipart-form-data-to-asp-net-core-web-api — Steve Wong, Feb 05 '22 at 16:57
in .net 6. these answers do not solve my problem. simply I need to convert the way postman converts. when i choose file in postman how it converts this file? what is the equivalent of converted type in c#? — sultanriveriere, Feb 05 '22 at 17:09

AliReza Sabouri · Answer 1 · 2022-02-05T19:05:47.440

0

You don't need to keep the file in your model.

this is an example to post a model just by knowing the file name.

var model = new RequestModel();

// ... fill the model with some data
model.Message = "test";
model.File = @"c:\test.jpg";

using var httpClient = new HttpClient();
{
    var form = new MultipartFormDataContent();
    form.Add(new StringContent(model.Message), nameof(RequestModel.Message));

    var bytes = File.ReadAllBytes(model.File);
    form.Add(new ByteArrayContent(bytes, 0, bytes.Length), "profile_pic", "hello1.jpg");
    var response = await httpClient.PostAsync("PostUrl", form);
    response.EnsureSuccessStatusCode();
    var sd = response.Content.ReadAsStringAsync().Result; // you should use await if your method is async ...
}

public class RequestModel
{
    public string Message { get; set; }

    public string File { get; set; } // simply use file-path
}

edited Feb 05 '22 at 19:05

answered Feb 05 '22 at 17:10

AliReza Sabouri

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I choose file from local directory (I do not need [FromForm]) and the main part is the body of this method. – sultanriveriere Feb 05 '22 at 17:17
i have tried this: error:Could not create an instance of type Microsoft.AspNetCore.Http.IFormFile. – sultanriveriere Feb 05 '22 at 17:32
ContentType = '((Microsoft.AspNetCore.Http.FormFile)model.attachment).ContentType' threw an exception of type 'System.NullReferenceException' – sultanriveriere Feb 05 '22 at 17:33
You should create an instance of `FormFile` not `IFormFile` – AliReza Sabouri Feb 05 '22 at 17:33
yep, i'm creating an instance of FormFile – sultanriveriere Feb 05 '22 at 17:37
You can set the content type manually. I've updated the answer – AliReza Sabouri Feb 05 '22 at 17:49
still getting "NullReferenceException" for IFormFile – sultanriveriere Feb 05 '22 at 18:10
This is how you can create the request model with a file manually. obviously, here `model.File` is not null. try to debug your code and find the null value, or ask another question on StackOverflow about it. – AliReza Sabouri Feb 05 '22 at 18:19
https://stackoverflow.com/questions/63889929/why-do-i-get-an-could-not-create-an-instance-of-type-microsoft-aspnetcore-http - looking to this problem, but i think this is not my case – sultanriveriere Feb 05 '22 at 18:21
I think your new problem is related to sending this model to the API. You have to use `FormUrlEncodedContent` for this. – AliReza Sabouri Feb 05 '22 at 18:23
Now I realized your question was misleading! You don't need to include the file in the request model, You should've asked how to make a post request with a file. this is different since here you can not use JSON serializer for this end-point – AliReza Sabouri Feb 05 '22 at 18:35
I can not reach Request this is the problem, I just need to create request model – sultanriveriere Feb 05 '22 at 18:35
when I send the same request with Postman it works, I send request from form-data, and send "message":string and "attachment": file – sultanriveriere Feb 05 '22 at 18:39
so, in c# i can assign message property, now need to assign file property. file property is required, without it message works – sultanriveriere Feb 05 '22 at 18:40
maybe the property should not be IFormFile – sultanriveriere Feb 05 '22 at 18:42
No, it's not that simple, You should update the question and add your CreateRequest method, your problem is related to that method, not the model. I'm 99 percent sure you are trying to use JSON serializers that is not possible for this specific end-point – AliReza Sabouri Feb 05 '22 at 18:43
Simply you can not post a JSON with a file together. – AliReza Sabouri Feb 05 '22 at 18:44
You must do something like this https://stackoverflow.com/a/19983672/786376 This is what postman is doing to post your request. – AliReza Sabouri Feb 05 '22 at 18:47
yes, thanks. i can not change method to yours as you have mentioned, but i think i can add multipart/form-data to support this request – sultanriveriere Feb 05 '22 at 19:59

Sending file in API request .NET

1 Answers1