Add FromFormOrJsonModelBinder for mixed multipart/form-data and JSON support

src/Mvc/Mvc.Core/src/ModelBinding/Binders/FromFormOrJsonModelBinder.cs

@@ -0,0 +1,83 @@
+using System.Text.Json;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Primitives;
+
+namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders;
+
+/// <summary>
+/// A generic ModelBinder that supports both 'application/json' and 'multipart/form-data' content types.
+/// For multipart requests, it expects a 'payload' field containing a JSON object.
+/// Optionally, the target model can implement <see cref="IFormFileReceiver"/> to receive uploaded files from the form.
+/// </summary>
+public class FromFormOrJsonModelBinder<T> : IModelBinder where T : class
+{
+    private static readonly JsonSerializerOptions _defaultJsonOptions = new() { PropertyNameCaseInsensitive = true };
+
+    public async Task BindModelAsync(ModelBindingContext bindingContext)
+    {
+        var request = bindingContext.HttpContext.Request;
+        var modelName = bindingContext.ModelName;
+
+        if (string.IsNullOrEmpty(request.ContentType))
+        {
+            bindingContext.ModelState.AddModelError(modelName, "Missing Content-Type header.");
+            bindingContext.Result = ModelBindingResult.Failed();
+            return;
+        }
+
+        try
+        {
+            T? model = null;
+
+            if (request.ContentType.StartsWith("multipart/form-data", StringComparison.OrdinalIgnoreCase))
+            {
+                var form = await request.ReadFormAsync(bindingContext.HttpContext.RequestAborted);
+
+                if (form.TryGetValue("payload", out var payloadJson) && !StringValues.IsNullOrEmpty(payloadJson))
+                {
+                    model = JsonSerializer.Deserialize<T>(payloadJson.ToString(), _defaultJsonOptions);
+                }
+
+                if (model is IFormFileReceiver receiver)
+                {
+                    receiver.SetFiles(form.Files);
+                }
+            }
+            else if (request.ContentType.StartsWith("application/json", StringComparison.OrdinalIgnoreCase))
+            {
+                model = await JsonSerializer.DeserializeAsync<T>(
+                    request.Body,
+                    _defaultJsonOptions,
+                    bindingContext.HttpContext.RequestAborted
+                );
+            }
+            else
+            {
+                bindingContext.ModelState.AddModelError(modelName, $"Unsupported Content-Type '{request.ContentType}'.");
+                bindingContext.Result = ModelBindingResult.Failed();
+                return;
+            }
+
+            if (model is null)
+            {
+                bindingContext.ModelState.AddModelError(modelName, "Could not deserialize the request body.");
+                bindingContext.Result = ModelBindingResult.Failed();
+                return;
+            }
+
+            bindingContext.Result = ModelBindingResult.Success(model);
+        }
+        catch (JsonException)
+        {
+            bindingContext.ModelState.AddModelError(bindingContext.ModelName, "Invalid JSON payload.");
+            bindingContext.Result = ModelBindingResult.Failed();
+        }
+        catch (Exception)
+        {
+            bindingContext.ModelState.AddModelError(bindingContext.ModelName, "Unexpected error during model binding.");
+            bindingContext.Result = ModelBindingResult.Failed();
+        }
+    }
+}

src/Mvc/Mvc.Core/src/ModelBinding/Binders/FromFormOrJsonModelBinderProvider.cs

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+using System;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+using System.Linq;
+
+namespace Microsoft.AspNetCore.Mvc.ModelBinding.Binders
+{
+    /// <summary>
+    /// Provides a model binder for parameters annotated with FromFormOrJsonAttribute.
+    /// </summary>
+    public class FromFormOrJsonModelBinderProvider : IModelBinderProvider
+    {
+        public IModelBinder? GetBinder(ModelBinderProviderContext context)
+        {
+            if (context == null)
+            {
+                throw new ArgumentNullException(nameof(context));
+            }
+
+            var parameter = context.Metadata.ParameterInfo;
+            var binderAttribute = parameter?.GetCustomAttributes(typeof(FromFormOrJsonAttribute), true)
+                                        .FirstOrDefault() as FromFormOrJsonAttribute;
+
+            if (binderAttribute != null)
+            {
+                var binderType = typeof(FromFormOrJsonModelBinder<>).MakeGenericType(context.Metadata.ModelType);
+                return (IModelBinder?)Activator.CreateInstance(binderType);
+            }
+
+            return null;
+        }
+    }
+}

src/Mvc/Mvc.Core/src/ModelBinding/FromFormOrJsonAttribute.cs

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+using Microsoft.AspNetCore.Mvc.ModelBinding;
+
+namespace Microsoft.AspNetCore.Mvc;
+
+/// <summary>
+/// Indicates that a parameter should be bound using the FromFormOrJsonModelBinder.
+/// </summary>
+[AttributeUsage(AttributeTargets.Parameter, AllowMultiple = false, Inherited = true)]
+public sealed class FromFormOrJsonAttribute : Attribute, IBindingSourceMetadata
+{
+    public BindingSource BindingSource => BindingSource.Custom;
+}

src/Mvc/Mvc.Core/src/ModelBinding/IFormFileReceiver.cs

@@ -0,0 +1,11 @@
+using Microsoft.AspNetCore.Http;
+
+namespace Microsoft.AspNetCore.Mvc.ModelBinding;
+
+/// <summary>
+/// Optional interface for models that wish to receive uploaded files from multipart/form-data requests.
+/// </summary>
+public interface IFormFileReceiver
+{
+    void SetFiles(IFormFileCollection files);
+}

src/Mvc/Mvc.Core/test/ModelBinding/Binders/FromFormOrJsonModelBinderTest.cs

@@ -0,0 +1,87 @@
+using System.IO;
+using System.Text;
+using System.Text.Json;
+using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http;
+using Microsoft.AspNetCore.Mvc.ModelBinding;
+using Microsoft.AspNetCore.Mvc.ModelBinding.Binders;
+using Microsoft.Extensions.Logging;
+using Microsoft.Extensions.Logging.Abstractions;
+
+using Xunit;
+
+namespace Microsoft.AspNetCore.Mvc.ModelBinding.Tests;
+
+public class FromFormOrJsonModelBinderTest
+{
+    private static DefaultHttpContext CreateHttpContextWithJson<T>(T payload)
+    {
+        var context = new DefaultHttpContext();
+        context.Request.ContentType = "application/json";
+
+        var json = JsonSerializer.Serialize(payload);
+        var bytes = Encoding.UTF8.GetBytes(json);
+        context.Request.Body = new MemoryStream(bytes);
+
+        return context;
+    }
+
+    private static DefaultHttpContext CreateHttpContextWithForm<T>(T payload)
+    {
+        var context = new DefaultHttpContext();
+        context.Request.ContentType = "multipart/form-data; boundary=----WebKitFormBoundary7MA4YWxkTrZu0gW";
+        var form = new FormCollection(new Dictionary<string, Microsoft.Extensions.Primitives.StringValues>
+        {
+            { "payload", JsonSerializer.Serialize(payload) }
+        });
+
+        context.Request.Form = form;
+        return context;
+    }
+
+    public class SampleDto
+    {
+        public string Name { get; set; }
+    }
+
+    [Fact]
+    public async Task BindsFromJsonPayload()
+    {
+        // Arrange
+        var dto = new SampleDto { Name = "Json Test" };
+        var httpContext = CreateHttpContextWithJson(dto);
+        var bindingContext = GetBindingContext<SampleDto>(httpContext);
+
+        var binder = new FromFormOrJsonModelBinder<SampleDto>();
+
+        // Act
+        await binder.BindModelAsync(bindingContext);
+
+        // Assert
+        Assert.True(bindingContext.Result.IsModelSet);
+        var result = Assert.IsType<SampleDto>(bindingContext.Result.Model);
+        Assert.Equal("Json Test", result.Name);
+    }
+
+    private static DefaultModelBindingContext GetBindingContext<T>(HttpContext httpContext)
+    {
+        var metadataProvider = new EmptyModelMetadataProvider();
+        var modelMetadata = metadataProvider.GetMetadataForType(typeof(T));
+        return new DefaultModelBindingContext
+        {
+            ModelMetadata = modelMetadata,
+            ModelName = "model",
+            ModelState = new ModelStateDictionary(),
+            ValueProvider = new SimpleValueProvider(),
+            ActionContext = new ActionContext
+            {
+                HttpContext = httpContext,
+                RouteData = new Microsoft.AspNetCore.Routing.RouteData(),
+                ActionDescriptor = new Microsoft.AspNetCore.Mvc.Abstractions.ActionDescriptor()
+            },
+            FieldName = "model",
+            BindingSource = BindingSource.Custom,
+            HttpContext = httpContext,
+        };
+    }
+}