用文件模型绑定接口：IFormFile (小文件上传)
当你使用IFormFile接口来上传文件的时候，一定要注意，IFormFile会将一个Http请求中的所有文件都读取到服务器内存后，才会触发ASP.NET Core MVC的Controller中的Action方法。这种情况下，如果上传一些小文件是没问题的，但是如果上传大文件，势必会造成服务器内存大量被占用甚至溢出，所以IFormFile接口只适合小文件上传。
一个文件上传页面的Html代码一般如下所示：
<form method="post" enctype="multipart/form-data" action="/Upload"> ???<div> ???????<p>Upload one or more files using this form:</p> ???????<input type="file" name="files" /> ???</div> ???<div> ????????<input type="submit" value="Upload" /> ???</div></form>
为了支持文件上传，form标签上一定要记得声明属性enctype="multipart/form-data"，否者你会发现ASP.NET Core MVC的Controller中死活都读不到任何文件。Input type="file"标签在html 5中支持上传多个文件，加上属性multiple即可。
使用IFormFile接口上传文件非常简单，将其声明为Contoller中Action的集合参数即可：
[HttpPost]public async Task<IActionResult> Post(List<IFormFile> files){ ???long size = files.Sum(f => f.Length); ???foreach (var formFile in files) ???{ ???????var filePath = @"D:\UploadingFiles\" + formFile.FileName; ???????if (formFile.Length > 0) ???????{ ???????????using (var stream = new FileStream(filePath, FileMode.Create)) ???????????{ ???????????????await formFile.CopyToAsync(stream); ???????????} ???????} ???} ???????????????return Ok(new { count = files.Count, size });}注意上面Action方法Post的参数名files，必须要和上传页面中的Input type="file"标签的name属性值一样。
用文件流 (大文件上传)
在介绍这个方法之前我们先来看看一个包含上传文件的Http请求是什么样子的：
Content-Type=multipart/form-data; boundary=---------------------------99614912995-----------------------------99614912995Content-Disposition: form-data; name="SOMENAME"Formulaire de Quota-----------------------------99614912995Content-Disposition: form-data; name="OTHERNAME"SOMEDATA-----------------------------99614912995Content-Disposition: form-data; name="files"; filename="Misc 001.jpg"SDFESDSDSDJXCK+DSDSDSSDSFDFDF423232DASDSDSDFDSFJHSIHFSDUIASUI+/==-----------------------------99614912995Content-Disposition: form-data; name="files"; filename="Misc 002.jpg"ASAADSDSDJXCKDSDSDSHAUSAUASAASSDSDFDSFJHSIHFSDUIASUI+/==-----------------------------99614912995Content-Disposition: form-data; name="files"; filename="Misc 003.jpg"TGUHGSDSDJXCK+DSDSDSSDSFDFDSAOJDIOASSAADDASDASDASSADASDSDSDSDFDSFJHSIHFSDUIASUI+/==-----------------------------99614912995--
这就是一个multipart/form-data格式的Http请求，我们可以看到里面第一行信息是Content-Type，这和我们在html页面中form标签上的enctype属性值一致，第一行中接着有一个boundary=---------------------------99614912995，boundary=后面的值是随机生成的，这个其实是在声明Http请求中表单数据的分隔符是什么，其代表的是在Http请求中每读到一行 ---------------------------99614912995，表示一份section数据，一份section有可能是一个表单键值对，也有可能是一个上传文件的文件数据，例如我们上面的例子中，前两个section就是表单键值对，后面三个section是三个上传的图片文件。
那么接下来，我们来看看怎么用文件流来上传大文件，避免一次性将所有上传的文件都加载到服务器内存中。用文件流来上传比较麻烦的地方在于你无法使用ASP.NET Core MVC的模型绑定器来将上传文件反序列化为C#对象（如同前面介绍的IFormFile接口那样）。首先我们需要定义类MultipartRequestHelper，用于识别Http请求中的各个section类型（是表单键值对section，还是上传文件section）
using System;using System.IO;using Microsoft.Net.Http.Headers;namespace AspNetCore.MultipartRequest{ ???public static class MultipartRequestHelper ???{ ???????// Content-Type: multipart/form-data; boundary="----WebKitFormBoundarymx2fSWqWSd0OxQqq" ???????// The spec says 70 characters is a reasonable limit. ???????public static string GetBoundary(MediaTypeHeaderValue contentType, int lengthLimit) ???????{ ???????????//var boundary = Microsoft.Net.Http.Headers.HeaderUtilities.RemoveQuotes(contentType.Boundary);// .NET Core <2.0 ???????????var boundary = Microsoft.Net.Http.Headers.HeaderUtilities.RemoveQuotes(contentType.Boundary).Value; //.NET Core 2.0 ???????????if (string.IsNullOrWhiteSpace(boundary)) ???????????{ ???????????????throw new InvalidDataException("Missing content-type boundary."); ???????????} ???????????if (boundary.Length > lengthLimit) ???????????{ ???????????????throw new InvalidDataException( ???????????????????$"Multipart boundary length limit {lengthLimit} exceeded."); ???????????} ???????????return boundary; ???????} ???????public static bool IsMultipartContentType(string contentType) ???????{ ???????????return !string.IsNullOrEmpty(contentType) ???????????????????&& contentType.IndexOf("multipart/", StringComparison.OrdinalIgnoreCase) >= 0; ???????} ???????public static bool HasFormDataContentDisposition(ContentDispositionHeaderValue contentDisposition) ???????{ ???????????// Content-Disposition: form-data; name="key"; ???????????return contentDisposition != null ???????????????????&& contentDisposition.DispositionType.Equals("form-data") ???????????????????&& string.IsNullOrEmpty(contentDisposition.FileName.Value) // For .NET Core <2.0 remove ".Value" ???????????????????&& string.IsNullOrEmpty(contentDisposition.FileNameStar.Value); // For .NET Core <2.0 remove ".Value" ???????} ???????????????public static bool HasFileContentDisposition(ContentDispositionHeaderValue contentDisposition) ???????{ ???????????// Content-Disposition: form-data; name="myfile1"; filename="Misc 002.jpg" ???????????return contentDisposition != null ???????????????????&& contentDisposition.DispositionType.Equals("form-data") ???????????????????&& (!string.IsNullOrEmpty(contentDisposition.FileName.Value) // For .NET Core <2.0 remove ".Value" ???????????????????????|| !string.IsNullOrEmpty(contentDisposition.FileNameStar.Value)); // For .NET Core <2.0 remove ".Value" ???????} ???????// 如果一个section的Header是： Content-Disposition: form-data; name="files"; filename="Misc 002.jpg" ???????// 那么本方法返回： files ???????public static string GetFileContentInputName(ContentDispositionHeaderValue contentDisposition) ???????{ ???????????return contentDisposition.Name.Value; ???????} ???????// 如果一个section的Header是： Content-Disposition: form-data; name="myfile1"; filename="Misc 002.jpg" ???????// 那么本方法返回： Misc 002.jpg ???????public static string GetFileName(ContentDispositionHeaderValue contentDisposition) ???????{ ???????????return contentDisposition.FileName.Value; ???????} ???}}然后我们需要定义一个扩展类叫FileStreamingHelper，其中的StreamFiles扩展方法用于读取上传文件的文件流数据，并且将数据写入到服务器的硬盘上，其接受一个参数targetDirectory，用于声明将上传文件存储到服务器的哪个文件夹下。
using Microsoft.AspNetCore.Http;using Microsoft.AspNetCore.Http.Features;using Microsoft.AspNetCore.Mvc.ModelBinding;using Microsoft.AspNetCore.WebUtilities;using Microsoft.Net.Http.Headers;using System;using System.Globalization;using System.IO;using System.Text;using System.Threading.Tasks;namespace AspNetCore.MultipartRequest{ ???public static class FileStreamingHelper ???{ ???????private static readonly FormOptions _defaultFormOptions = new FormOptions(); ???????public static async Task<FormValueProvider> StreamFiles(this HttpRequest request,string targetDirectory) ???????{ ???????????if (!MultipartRequestHelper.IsMultipartContentType(request.ContentType)) ???????????{ ???????????????throw new Exception($"Expected a multipart request, but got {request.ContentType}"); ???????????} ???????????// Used to accumulate all the form url encoded key value pairs in the ????????????// request. ???????????var formAccumulator = new KeyValueAccumulator(); ???????????var boundary = MultipartRequestHelper.GetBoundary( ???????????????MediaTypeHeaderValue.Parse(request.ContentType), ???????????????_defaultFormOptions.MultipartBoundaryLengthLimit); ???????????var reader = new MultipartReader(boundary, request.Body); ???????????var section = await reader.ReadNextSectionAsync();//用于读取Http请求中的第一个section数据 ???????????while (section != null) ???????????{ ???????????????ContentDispositionHeaderValue contentDisposition; ???????????????var hasContentDispositionHeader = ContentDispositionHeaderValue.TryParse(section.ContentDisposition, out contentDisposition); ???????????????if (hasContentDispositionHeader) ???????????????{ ???????????????????/* ???????????????????用于处理上传文件类型的的section ???????????????????POSTDATA =-----------------------------99614912995 ???????????????????Content - Disposition: form - data; name = "files"; filename = "Misc 002.jpg" ???????????????????ASAADSDSDJXCKDSDSDSHAUSAUASAASSDSDFDSFJHSIHFSDUIASUI+/== ???????????????????-----------------------------99614912995 ???????????????????*/ ???????????????????if (MultipartRequestHelper.HasFileContentDisposition(contentDisposition)) ???????????????????{ ???????????????????????if(!Directory.Exists(targetDirectory)) ???????????????????????{ ???????????????????????????Directory.CreateDirectory(targetDirectory); ???????????????????????} ???????????????????????var fileName = MultipartRequestHelper.GetFileName(contentDisposition); ???????????????????????var loadBufferBytes = 1024;//这个是每一次从Http请求的section中读出文件数据的大小，单位是Byte即字节，这里设置为1024的意思是，每次从Http请求的section数据流中读取出1024字节的数据，然后写入下面targetFileStream的文件流中，可以根据服务器的内存大小调整这个值。这样就避免了一次加载所有上传文件的数据到服务器内存中，导致服务器崩溃。 ???????????????????????????????????????????????using (var targetFileStream = System.IO.File.Create(targetDirectory+ "\\"+ fileName)) ???????????????????????{ ???????????????????????????//section.Body是System.IO.Stream类型，表示的是Http请求中一个section的数据流，从该数据流中可以读出每一个section的全部数据，所以我们下面也可以不用section.Body.CopyToAsync方法，而是在一个循环中用section.Body.Read方法自己读出数据，再将数据写入到targetFileStream ???????????????????????????await section.Body.CopyToAsync(targetFileStream, loadBufferBytes); ???????????????????????} ???????????????????????????????????????????} ???????????????????/* ???????????????????用于处理表单键值数据的section ???????????????????POSTDATA =-----------------------------99614912995 ???????????????????Content - Disposition: form - data; name = "SOMENAME" ???????????????????Formulaire de Quota ???????????????????-----------------------------99614912995 ???????????????????*/ ???????????????????else if (MultipartRequestHelper.HasFormDataContentDisposition(contentDisposition)) ???????????????????{ ???????????????????????// Content-Disposition: form-data; name="key" ???????????????????????// ???????????????????????// value ???????????????????????// Do not limit the key name length here because the ????????????????????????// multipart headers length limit is already in effect. ???????????????????????var key = HeaderUtilities.RemoveQuotes(contentDisposition.Name); ???????????????????????var encoding = GetEncoding(section); ???????????????????????using (var streamReader = new StreamReader( ???????????????????????????section.Body, ???????????????????????????encoding, ???????????????????????????detectEncodingFromByteOrderMarks: true, ???????????????????????????bufferSize: 1024, ???????????????????????????leaveOpen: true)) ???????????????????????{ ???????????????????????????// The value length limit is enforced by MultipartBodyLengthLimit ???????????????????????????var value = await streamReader.ReadToEndAsync(); ???????????????????????????if (String.Equals(value, "undefined", StringComparison.OrdinalIgnoreCase)) ???????????????????????????{ ???????????????????????????????value = String.Empty; ???????????????????????????} ???????????????????????????formAccumulator.Append(key.Value, value); // For .NET Core <2.0 remove ".Value" from key ???????????????????????????if (formAccumulator.ValueCount > _defaultFormOptions.ValueCountLimit) ???????????????????????????{ ???????????????????????????????throw new InvalidDataException($"Form key count limit {_defaultFormOptions.ValueCountLimit} exceeded."); ???????????????????????????} ???????????????????????} ???????????????????} ???????????????} ???????????????// Drains any remaining section body that has not been consumed and ???????????????// reads the headers for the next section. ???????????????section = await reader.ReadNextSectionAsync();//用于读取Http请求中的下一个section数据 ???????????} ???????????// Bind form data to a model ???????????var formValueProvider = new FormValueProvider( ???????????????BindingSource.Form, ???????????????new FormCollection(formAccumulator.GetResults()), ???????????????CultureInfo.CurrentCulture); ???????????return formValueProvider; ???????} ???????private static Encoding GetEncoding(MultipartSection section) ???????{ ???????????MediaTypeHeaderValue mediaType; ???????????var hasMediaTypeHeader = MediaTypeHeaderValue.TryParse(section.ContentType, out mediaType); ???????????// UTF-7 is insecure and should not be honored. UTF-8 will succeed in ????????????// most cases. ???????????if (!hasMediaTypeHeader || Encoding.UTF7.Equals(mediaType.Encoding)) ???????????{ ???????????????return Encoding.UTF8; ???????????} ???????????return mediaType.Encoding; ???????} ???}}现在我们还需要创建一个ASP.NET Core MVC的自定义拦截器DisableFormValueModelBindingAttribute，该拦截器实现接口IResourceFilter，用来禁用ASP.NET Core MVC的模型绑定器，这样当一个Http请求到达服务器后，ASP.NET Core MVC就不会在将请求的所有上传文件数据都加载到服务器内存后，才执行Contoller的Action方法，而是当Http请求到达服务器时，就立刻执行Contoller的Action方法。
[AttributeUsage(AttributeTargets.Class | AttributeTargets.Method)]public class DisableFormValueModelBindingAttribute : Attribute, IResourceFilter{ ???public void OnResourceExecuting(ResourceExecutingContext context) ???{ ???????var formValueProviderFactory = context.ValueProviderFactories ???????????.OfType<FormValueProviderFactory>() ???????????.FirstOrDefault(); ???????if (formValueProviderFactory != null) ???????{ ???????????context.ValueProviderFactories.Remove(formValueProviderFactory); ???????} ????????var jqueryFormValueProviderFactory = context.ValueProviderFactories ???????????.OfType<JQueryFormValueProviderFactory>() ???????????.FirstOrDefault(); ???????if (jqueryFormValueProviderFactory != null) ???????{ ???????????context.ValueProviderFactories.Remove(jqueryFormValueProviderFactory); ???????} ???} ????public void OnResourceExecuted(ResourceExecutedContext context) ???{ ???}}最后我们在Contoller中定义一个叫Index的Action方法，并注册我们定义的DisableFormValueModelBindingAttribute拦截器，来禁用Action的模型绑定。Index方法会调用我们前面定义的FileStreamingHelper类中的StreamFiles方法，其参数为用来存储上传文件的文件夹路径。StreamFiles方法会返回一个FormValueProvider，用来存储Http请求中的表单键值数据，之后我们会将其绑定到MVC的视图模型viewModel上，然后将viewModel传回给客户端浏览器，来告述客户端浏览器文件上传成功。
[HttpPost][DisableFormValueModelBinding]public async Task<IActionResult> Index(){ ???FormValueProvider formModel; ???formModel = await Request.StreamFiles(@"D:\UploadingFiles"); ???var viewModel = new MyViewModel(); ???var bindingSuccessful = await TryUpdateModelAsync(viewModel, prefix: "", ???????valueProvider: formModel); ???if (!bindingSuccessful) ???{ ???????if (!ModelState.IsValid) ???????{ ???????????return BadRequest(ModelState); ???????} ???} ???return Ok(viewModel);}视图模型viewModel的定义如下：
public class MyViewModel{ ???public string Username { get; set; }}最后我们用于上传文件的html页面和前面几乎一样：
<form method="post" enctype="multipart/form-data" action="/Home/Index"> ???<div> ???????<p>Upload one or more files using this form:</p> ???????<input type="file" name="files" multiple /> ???</div> ???<div> ???????<p>Your Username</p> ???????<input type="text" name="username" /> ???</div> ???<div> ????????<input type="submit" value="Upload" /> ???</div></form>
这就是所有的代码，希望对大家有所帮助！
参考文献:
File uploads in ASP.NET Core
Uploading Files In ASP.net Core
What is the boundary parameter in an HTTP multi-part (POST) Request?
ASP.NET Core MVC如何上传文件及处理大文件上传
原文地址：https://www.cnblogs.com/OpenCoder/p/9785031.html