azure-functions-host: Azure Functions v3 HttpTrigger: request body is empty

HttpTrigger works incorrectly in the following conditions:

• Azure Functions v3
• TargetFramework: netcoreapp3.1
• Microsoft.NET.Sdk.Functions version 3.0.2
• Code:

        [FunctionName("RunNetCore3")]
        public static async Task<string> RunNetCore3Async(
            [HttpTrigger(AuthorizationLevel.Anonymous, "POST", Route = "netcore3/{id}")]
            CustomRouteParameter parameter,
            HttpRequest request,
            CancellationToken cancellationToken)
        {
            using (var reader = new StreamReader(request.Body, Encoding.UTF8))
            {
                return await reader.ReadToEndAsync();
            }
        }

• The request body in the request variable is empty if request contains Content-Type: application/json header (works ok with any other Content-Type values except application/json). So in the example the empty response body will be returned which is unexpected.

Investigative information

I’ve created the repository to demonstrate this issue. The source code of this repository is deployed to the Function App.

• Timestamp: 09:27 AM (UTC Time)
• Function App version: 3.0
• Function App name: NetCore3HttpBindingBug
• Function name(s) (as appropriate): RunNetCore3
• Invocation ID: 32894c31-74ac-4a98-8f94-fe6c76662d7d
• Region: Central US

Repro steps

• Send POST https://netcore3httpbindingbug.azurewebsites.net/api/netcore3/qqq request with some body and Content-Type: application/json header.

Expected behavior

Request’s body is returned in the response.

Actual behavior

Request’s body is not returned in the response.

Known workarounds

Do not send Content-Type: application/json header or send any other value in the Content-Type header.

Related information

The same code works correct in Azure Functions v2 on netcoreapp2.1 with Microsoft.NET.Sdk.Functions version 1.0.29 and in ASP.NET Core 3.1 Web Application.

There are two endpoints to check it (see the repository):

• Send POST /netcore2/some-id request with some body and Content-Type: application/json header
• Send POST /aspnetcore3/some-id request with some body and Content-Type: application/json header

In both cases the request body will be returned in the response (I didn’t deploy it to any Function App but it’s reproducible locally as well).