ASP.NET Core Middleware Breaks Physical File Controller Method

Question

I use the following middleware to replace the response content:

app.Use(async (context, next) => {
    var body = context.Response.Body;

    using (var updatedBody = new MemoryStream()) {
        context.Response.Body = updatedBody;

        await next();

        context.Response.Body = body;

        updatedBody.Seek(0, SeekOrigin.Begin);

        var newContent = new StreamReader(updatedBody).ReadToEnd();

        // Replace content here ...

        await context.Response.WriteAsync(newContent);
    }
});

This works fine. However now say I have the following action method:

public IActionResult Image() {
    return PhysicalFile(@"C:\myimage.jpg", "image/jpeg");
}

When it tries to display this image it won't load, but if I remove the middleware then it does.

Please note that I am using ASP.NET Core 3.

Answer 1

Here's the solution I have come up with:

app.Use(async (context, next) => {
    var body = context.Response.Body;

    using (var updatedBody = new MemoryStream()) {
        context.Response.Body = updatedBody;

        try {
            await next();
        } catch {
            throw;
        } finally {
            context.Response.Body = body;
        }

        if (context.Response.StatusCode == 200 && context.Response.ContentType != null && context.Response.ContentType.Contains("text/html", StringComparison.InvariantCultureIgnoreCase)) {
            updatedBody.Seek(0, SeekOrigin.Begin);

            using (var reader = new StreamReader(updatedBody)) {
                var newContent = reader.ReadToEnd();

                // Replace content here

                await context.Response.WriteAsync(newContent);
            }
        } else {
            if (updatedBody.Length > 0)
                await context.Response.Body.WriteAsync(updatedBody.ToArray());
        }
    }
});

This article helped thanks to @CodeCaster's suggestion. As an added bonus I have fixed an issue where it breaks the developer exception page by wrapping a try/catch around the call to the delegate.