c# - Getting "Program does not contain a static 'Main' method suitable for an entry point" when building using docker, why?

Question

Im running into an issue using docker and couldn't find a proper solution.

I'm trying to build a docker image using dotnet sdk 2.1.

The thing is that when docker tries to run the build statement, it fails and the error output is

CSC : error CS5001: Program does not contain a static 'Main' method suitable for an entry point

The funny thing is that if i perform the buildd statement on command line locally, it works fine.

I have already checked my LanguageVersion tag on the project and it is 7.3.

here is my docker file

FROM microsoft/dotnet:2.1-sdk AS builder  
WORKDIR /src
COPY ./nuget  ./nuget
COPY ./NuGet.Config ./
COPY Services/AadTracking ./

# Copy all the referenced projects

COPY ./Services/AadTracking/Company/Company.Service.AadTracking/Company.Service.AadTracking.csproj ./AadTracking/Company/Company.Service.AadTracking/Company.Service.AadTracking.csproj
COPY ./Services/AadTracking/Office.Re.Service.AadTracking/Office.Re.Service.AadTracking.csproj ./AadTracking/Office.Re.Service.AadTracking/Office.Re.Service.AadTracking.csproj
COPY ./Services/AadTracking/Company/Office.Re.Service.AadTracking.Company/Office.Re.Service.AadTracking.Company.csproj ./AadTracking/Company/Office.Re.Service.AadTracking.Company/Office.Re.Service.AadTracking.Company.csproj
COPY ./Services/AadTracking/Office.Re.Service.AadTracking.EventStore/Office.Re.Service.AadTracking.EventStore.csproj ./AadTracking/Office.Re.Service.AadTracking.EventStore/Office.Re.Service.AadTracking.EventStore.csproj

# Restore packages
RUN dotnet restore "./AadTracking/Company/Company.Service.AadTracking/Company.Service.AadTracking.csproj"

RUN dotnet build -c Debug --no-restore "./AadTracking/Company/Company.Service.AadTracking/Company.Service.AadTracking.csproj"

# COPY source code 
#aad tracking
COPY ./Services/AadTracking/Company/Company.Service.AadTracking ./AadTracking/Company/Company.Service.AadTracking/
COPY ./Services/AadTracking/Office.Re.Service.AadTracking ./AadTracking/Office.Re.Service.AadTracking/
COPY ./Services/AadTracking/Company/Office.Re.Service.AadTracking.Company ./AadTracking/Company/Office.Re.Service.AadTracking.Company/
COPY ./Services/AadTracking/Office.Re.Service.AadTracking.EventStore ./AadTracking/Office.Re.Service.AadTracking.EventStore/

# Publish
RUN dotnet publish "./AadTracking/Company/Company.Service.AadTracking/Company.Service.AadTracking.csproj" -c Debug -o "../../dist"

# #Build the app image
FROM microsoft/dotnet:2.1-aspnetcore-runtime  
WORKDIR /app  

ENV ASPNETCORE_ENVIRONMENT Switch
ENV REINSURANCE_INSTANCE Docker-dev 

COPY --from=builder /dist .  

ENTRYPOINT ["dotnet", "Company.Service.AadTracking.dll"]

Thanks for your help!

Answer 1

I know this is little bit late to answer. Still VS 2019 has the same issue with netcore 3.1. I took a peek at the examples provided by Microsoft. Turns out the docker file resided in a different place in the solution and docker copy command wasn't working properly.

You have to move your docker file One directory up, so that they are at the same level as the sln file. It will fix the issue.

OR Else you can change the paths like below sample docker file WITHOUT changing the docker file location, IMHO it is better to keep the docker file with other files.

FROM mcr.microsoft.com/dotnet/core/aspnet:3.1-buster-slim-arm64v8 AS base
WORKDIR /app
EXPOSE 80
EXPOSE 443

FROM mcr.microsoft.com/dotnet/core/sdk:3.1-buster AS build
WORKDIR /src
COPY ["WhatzThat.Web.csproj", "WhatzThat.Web/"]
RUN dotnet restore "WhatzThat.Web/WhatzThat.Web.csproj" -r linux-arm64

WORKDIR "/src/WhatzThat.Web"
COPY . .

RUN dotnet build "WhatzThat.Web.csproj" -c Release -o /app/build

FROM build AS publish
RUN dotnet publish "WhatzThat.Web.csproj" -c Release -o /app/publish -r linux-arm64 --self-contained false --no-restore

FROM base AS final
WORKDIR /app
COPY --from=publish /app/publish .
ENTRYPOINT ["dotnet", "WhatzThat.Web.dll"]