I have a .net middleware that modifies the response and I’m trying to unit test it. I am able to successfully assert the status code and content type are correct, but when I attempt to assert the response body, it is an empty string even though a response body has been set.

I had no issues modifying or asserting the request body in another test. Is it possible to do the same for the response body?

Middleware:

using System.Net;
using System.Net.Mime;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;

namespace Web.API.Middleware
{
    public class SampleMiddleware
    {
        private readonly RequestDelegate _next;

        public SampleMiddleware(RequestDelegate next)
        {
            _next = next;
        }

        public async Task InvokeAsync(HttpContext context)
        {
            bool success = true;

            // do other stuff which has been removed...

            context.Response.ContentType = MediaTypeNames.Text.Plain;
            context.Response.StatusCode = (int)HttpStatusCode.BadRequest;

            await context.Response.WriteAsync("Bad request");

            success = false;

            if (success)
            {
                await _next(context);
            }
        }
    }
}

Unit test:

using System.IO;
using System.Net;
using System.Net.Mime;
using System.Threading.Tasks;
using Microsoft.AspNetCore.Http;
using Microsoft.VisualStudio.TestTools.UnitTesting;
using Moq;
using Web.API.Middleware;

namespace Web.API.UnitTests
{
    [TestClass]
    public class SampleMiddlewareTest
    {
        [TestMethod]
        public async Task BadRequestTest()
        {
            // Arrange
            var expected = "Bad request";

            var httpContext = new DefaultHttpContext();
            var nextMock = new Mock<RequestDelegate>();
            var middleware = new SampleMiddleware(nextMock.Object);

            // Act
            await middleware.InvokeAsync(httpContext);

            // Assert
            Assert.AreEqual((int)HttpStatusCode.BadRequest, httpContext.Response.StatusCode); // works
            Assert.AreEqual(MediaTypeNames.Text.Plain, httpContext.Response.ContentType); // works

            StreamReader reader = new StreamReader(httpContext.Response.Body);
            var responseBody = reader.ReadToEnd();

            Assert.AreEqual(expected, responseBody); // fails
        }
    }
}

Test result:

Assert.AreEqual failed. Expected:<Bad request>. Actual:<>.