PreviewDrop for .NET

Deploy previews for ASP.NET Core apps

Push a branch, get a live preview URL. PreviewDrop publishes your app, runs it on Kestrel, and posts the link to the pull request, usually within a couple of minutes.

TL;DR

Connect your repo, push. PreviewDrop detects .NET, runs dotnet publish then boots your app on Kestrel behind a TLS-terminated subdomain. SignalR hubs, IHostedService background workers and EF Core connection pools all run exactly as in production.

What the build needs to do

You don't write this. PreviewDrop detects .NETfrom your repo and builds the container for you — a Dockerfile in your repo is not used. The reference below shows the install, build and start steps a working .NET image needs. If detection gets one wrong, set the matching Install, Build or Start command in Project Settings. Your app must listen on $PORT.

Reference (not read by PreviewDrop)
FROM mcr.microsoft.com/dotnet/sdk:8.0 AS build
WORKDIR /src
COPY *.csproj ./
RUN dotnet restore
COPY . .
RUN dotnet publish -c Release -o /app

FROM mcr.microsoft.com/dotnet/aspnet:8.0
WORKDIR /app
COPY --from=build /app .
EXPOSE 8080
# Replace YourApp.dll with your project's assembly name
CMD ["sh", "-c", "dotnet YourApp.dll --urls http://0.0.0.0:${PORT:-8080}"]

Environment variables

Set these in the Variables tab of your PreviewDrop project. They're encrypted at rest and injected into every preview container at start.

Variable	Example value	Note
ASPNETCORE_ENVIRONMENT	Development	Or a custom "Preview" environment — enables developer-friendly diagnostics.
ConnectionStrings__Default	Host=host;Database=db;Username=user;Password=pass	Double underscore maps to the nested ConnectionStrings:Default config key.
PORT	8080	PreviewDrop injects this; pass it to Kestrel via --urls or ASPNETCORE_URLS.

Common gotchas

Bind Kestrel to 0.0.0.0 and the injected PORT

Kestrel defaults to localhost:5000/5001, which the proxy can’t reach. Pass --urls http://0.0.0.0:${PORT} (as in the reference Start command) or set ASPNETCORE_URLS. Use plain HTTP — TLS is terminated at the edge.

Start the right assembly

The reference Start command runs dotnet YourApp.dll. That filename is your .csproj name with a .dll extension — e.g. a project named Shop.Web.csproj publishes Shop.Web.dll. If a solution has several projects and the wrong one starts, set the Start command in Project Settings.

Disable HTTPS redirection in previews

The edge already serves HTTPS, so app.UseHttpsRedirection() inside the container redirects to a port that isn’t listening and loops. Gate it behind if (!app.Environment.IsDevelopment()) or skip it for the preview environment.

Multi-project solutions

The reference build above restores from a single *.csproj. For multi-project solutions, set the Build command to dotnet publish your web project (or the .sln) explicitly.

Why not Vercel or Netlify?

Vercel and Netlify have no .NET runtime. Serverless .NET exists on other clouds, but you lose the long-lived Kestrel server, SignalR real-time hubs, and IHostedService background workers the moment you fold the app into a per-request function. PreviewDrop runs Kestrel directly — the same container shape as production, as a per-PR preview.

Preview your .NET app in 5 minutes

Connect a repo, push a branch, get a URL. No credit card, no trial clock.

Start free