1、使用自定义包装类MyModelWrapper接收上传的文件及JSON数据

1）IJsonAttribute.cs文件文件代码

public interface IJsonAttribute
{
    object TryConvert(string modelValue, Type targertType, out bool success);
}

2）FromJsonAttribute.cs文件代码

using Newtonsoft.Json;
[AttributeUsage(AttributeTargets.Property, AllowMultiple = false)]
public class FromJsonAttribute : Attribute, IJsonAttribute
{
    public object TryConvert(string modelValue, Type targetType, out bool success)
    {
        var value = JsonConvert.DeserializeObject(modelValue, targetType);
        success = value != null;
        return value;
    }
}

3）JsonModelBinderProvider.cs文件代码

public class JsonModelBinderProvider : IModelBinderProvider
{
    public IModelBinder GetBinder(ModelBinderProviderContext context)
    {
        if (context == null) throw new ArgumentNullException(nameof(context));
        if (context.Metadata.IsComplexType)
        {
            var propName = context.Metadata.PropertyName;
            var propInfo = context.Metadata.ContainerType?.GetProperty(propName);
            if(propName == null || propInfo == null)
                return null;
            // 查找FromJson标签
            var attribute = propInfo.GetCustomAttributes(typeof(FromJsonAttribute), false).FirstOrDefault();
            if (attribute != null) 
                return new JsonModelBinder(context.Metadata.ModelType, attribute as IJsonAttribute);
        }
        return null;
    }
}

4）JsonModelBinder.cs文件代码

public class JsonModelBinder : IModelBinder
{
    private IJsonAttribute _attribute;
    private Type _targetType;
    public JsonModelBinder(Type type, IJsonAttribute attribute)
    {
        if (type == null) throw new ArgumentNullException(nameof(type));
        _attribute = attribute as IJsonAttribute;
        _targetType = type;
    }
    public Task BindModelAsync(ModelBindingContext bindingContext)
    {
        if (bindingContext == null) throw new ArgumentNullException(nameof(bindingContext));
        // Check the value sent in
        var valueProviderResult = bindingContext.ValueProvider.GetValue(bindingContext.ModelName);
        if (valueProviderResult != ValueProviderResult.None)
        {
            bindingContext.ModelState.SetModelValue(bindingContext.ModelName, valueProviderResult);
            // Attempt to convert the input value
            var valueAsString = valueProviderResult.FirstValue;
            bool success;
            var result = _attribute.TryConvert(valueAsString, _targetType, out success);
            if (success)
            {
                bindingContext.Result = ModelBindingResult.Success(result);
                return Task.CompletedTask;
            }
        }
        return Task.CompletedTask;
    }
}

使用示例代码：

public class MyModelWrapper
{
    public IList<IFormFile> Files { get; set; }
    [FromJson]
    public MyModel Model { get; set; } // <-- JSON will be deserialized to this object
}
// Controller action:
public async Task<IActionResult> Upload(MyModelWrapper modelWrapper)
{
}
// 在Startup.cs配置服务中添加自定义绑定器提供程序
services.AddMvc(properties => 
{
    properties.ModelBinderProviders.Insert(0, new JsonModelBinderProvider());
});

2、使用ModelBinderAttribute标签实现

使用更简单的方法，更少的代码，不使用包装类来实现。

1）实现JsonModelBinder类

using System;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Mvc.ModelBinding;
public class JsonModelBinder : IModelBinder {
    public Task BindModelAsync(ModelBindingContext bindingContext) {
        if (bindingContext == null) {
            throw new ArgumentNullException(nameof(bindingContext));
        }
        // Check the value sent in
        var valueProviderResult = bindingContext.ValueProvider.GetValue(bindingContext.ModelName);
        if (valueProviderResult != ValueProviderResult.None) {
            bindingContext.ModelState.SetModelValue(bindingContext.ModelName, valueProviderResult);
            // Attempt to convert the input value
            var valueAsString = valueProviderResult.FirstValue;
            var result = Newtonsoft.Json.JsonConvert.DeserializeObject(valueAsString, bindingContext.ModelType);
            if (result != null) {
                bindingContext.Result = ModelBindingResult.Success(result);
                return Task.CompletedTask;
            }
        }
        return Task.CompletedTask;
    }
}

2）Controller中Upload方法代码

public IActionResult Upload(
    [ModelBinder(BinderType = typeof(JsonModelBinder))] SomeObject value,
    IList<IFormFile> files)
{
    // Use serialized json object 'value'
    // Use uploaded 'files'
}

下面是控制器action Upload接受的原始http请求的一个示例，

multipart/form-data请求被分成多个部分，每个部分由指定的boundary=12345分隔。每个部分都在其Content-Disposition-header中分配了一个名称。这些名称默认为ASP.Net-Core知道哪个部分绑定到controller action中的哪个参数。

绑定到IFormFile的文件还需要像请求的第二部分那样指定filename。不需要Content-Type。

另一件需要注意的事情是json部分需要反序列化为controller操作中定义的参数类型。在本例中，类型SomeObject应该有一个string类型的属性key:

POST http://localhost:5000/home/upload HTTP/1.1
Host: localhost:5000
Content-Type: multipart/form-data; boundary=12345
Content-Length: 218
--12345
Content-Disposition: form-data; name="value"
{"key": "value"}
--12345
Content-Disposition: form-data; name="files"; filename="file.txt"
Content-Type: text/plain
This is a simple text file
--12345--

3、用Postman测试

可以使用Postman调用操作并测试服务器端代码。这非常简单，主要是界面操作。创建一个新请求并在body选项卡中选择form-data。可以为reqeust的每个部分选择文本和文件。