Fix EmptyBodyBehavior with empty content-type

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

@@ -8,6 +8,7 @@
 using System.IO;
 using System.Text;
 using System.Threading.Tasks;
+using Microsoft.AspNetCore.Http.Features;
 using Microsoft.AspNetCore.Mvc.Core;
 using Microsoft.AspNetCore.Mvc.Formatters;
 using Microsoft.AspNetCore.Mvc.Infrastructure;
@@ -143,6 +144,17 @@ public async Task BindModelAsync(ModelBindingContext bindingContext)
 
         if (formatter == null)
         {
+            if (AllowEmptyBody)
+            {
+                var hasBody = httpContext.Features.Get<IHttpRequestBodyDetectionFeature>()?.CanHaveBody;
+                hasBody ??= httpContext.Request.ContentLength is not null && httpContext.Request.ContentLength == 0;
+                if (hasBody == false)
+                {
+                    bindingContext.Result = ModelBindingResult.Success(model: null);
+                    return;
+                }
+            }
+
             _logger.NoInputFormatterSelected(formatterContext);
 
             var message = Resources.FormatUnsupportedContentType(httpContext.Request.ContentType);

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

@@ -195,6 +195,62 @@ public async Task BindModel_PassesAllowEmptyInputOptionViaContext(bool treatEmpt
             Times.Once);
     }
 
+    [Fact]
+    public async Task BindModel_SetsModelIfAllowEmpty()
+    {
+        // Arrange
+        var mockInputFormatter = new Mock<IInputFormatter>();
+        mockInputFormatter.Setup(f => f.CanRead(It.IsAny<InputFormatterContext>()))
+            .Returns(false);
+        var inputFormatter = mockInputFormatter.Object;
+
+        var provider = new TestModelMetadataProvider();
+        provider.ForType<Person>().BindingDetails(d => d.BindingSource = BindingSource.Body);
+
+        var bindingContext = GetBindingContext(
+            typeof(Person),
+            metadataProvider: provider);
+        bindingContext.BinderModelName = "custom";
+
+        var binder = CreateBinder(new[] { inputFormatter }, treatEmptyInputAsDefaultValueOption : true);
+
+        // Act
+        await binder.BindModelAsync(bindingContext);
+
+        // Assert
+        Assert.True(bindingContext.Result.IsModelSet);
+        Assert.Null(bindingContext.Result.Model);
+        Assert.True(bindingContext.ModelState.IsValid);
+    }
+
+    [Fact]
+    public async Task BindModel_FailsIfNotAllowEmpty()
+    {
+        // Arrange
+        var mockInputFormatter = new Mock<IInputFormatter>();
+        mockInputFormatter.Setup(f => f.CanRead(It.IsAny<InputFormatterContext>()))
+            .Returns(false);
+        var inputFormatter = mockInputFormatter.Object;
+
+        var provider = new TestModelMetadataProvider();
+        provider.ForType<Person>().BindingDetails(d => d.BindingSource = BindingSource.Body);
+
+        var bindingContext = GetBindingContext(
+            typeof(Person),
+            metadataProvider: provider);
+        bindingContext.BinderModelName = "custom";
+
+        var binder = CreateBinder(new[] { inputFormatter }, treatEmptyInputAsDefaultValueOption: false);
+
+        // Act
+        await binder.BindModelAsync(bindingContext);
+
+        // Assert
+        Assert.False(bindingContext.ModelState.IsValid);
+        Assert.Single(bindingContext.ModelState[bindingContext.BinderModelName].Errors);
+        Assert.Equal("Unsupported content type ''.", bindingContext.ModelState[bindingContext.BinderModelName].Errors[0].Exception.Message);
+    }
+
     // Throwing InputFormatterException
     [Fact]
     public async Task BindModel_CustomFormatter_ThrowingInputFormatterException_AddsErrorToModelState()

src/Mvc/test/Mvc.FunctionalTests/InputFormatterTests.cs

@@ -188,6 +188,20 @@ public async Task BodyIsRequiredByDefault()
             });
     }
 
+    [Fact]
+    public async Task BodyIsRequiredByDefaultFailsWithEmptyBody()
+    {
+        var content = new ByteArrayContent(Array.Empty<byte>());
+        Assert.Null(content.Headers.ContentType);
+        Assert.Equal(0, content.Headers.ContentLength);
+
+        // Act
+        var response = await Client.PostAsync($"Home/{nameof(HomeController.DefaultBody)}", content);
+
+        // Assert
+        await response.AssertStatusCodeAsync(HttpStatusCode.UnsupportedMediaType);
+    }
+
     [Fact]
     public async Task OptionalFromBodyWorks()
     {
@@ -197,4 +211,19 @@ public async Task OptionalFromBodyWorks()
         // Assert
         await response.AssertStatusCodeAsync(HttpStatusCode.OK);
     }
+
+    [Fact]
+    public async Task OptionalFromBodyWorksWithEmptyRequest()
+    {
+        // Arrange
+        var content = new ByteArrayContent(Array.Empty<byte>());
+        Assert.Null(content.Headers.ContentType);
+        Assert.Equal(0, content.Headers.ContentLength);
+
+        // Act
+        var response = await Client.PostAsync($"Home/{nameof(HomeController.OptionalBody)}", content);
+
+        // Assert
+        await response.AssertStatusCodeAsync(HttpStatusCode.OK);
+    }
 }