Could not execute because the application was not found or a compatible .NET SDK is not installed

Question

I created a basic Rest API using ASP.NET Core 5 i want to make run with docker. The application works fine on IIS Express.

https://learn.microsoft.com/fr-fr/aspnet/core/tutorials/first-web-api?view=aspnetcore-5.0&tabs=visual-studio

I also want to create a docker container in order to launch the application.

In the project folder i created a Docker folder with several files. Here is my App.dockerfile:

FROM mcr.microsoft.com/dotnet/aspnet:5.0

ARG WEBAPP_VERSION=0.0.1
LABEL maintainer=anymail@email_server.com \
    Name=webapp \
    Version=${WEBAPP_VERSION}
ARG URL_PORT
WORKDIR /app
ENV NUGET_XMLDOC_MODE skip
ENV ASPNETCORE_URLS http://*:${URL_PORT}
ENTRYPOINT [ "dotnet", "WebApplication.dll" ]

I also have a Build.docker file:

FROM mcr.microsoft.com/dotnet/aspnet:5.0
## Can be Debug or Release.
ARG BUILD_CONFIG=Debug
ARG BUILDER_VERSION=0.0.1
LABEL maintainer=some_email@email_server.com \
    Name=webapp-build-${BUILD_CONFIG} \
    Version=${BUILDER_VERSION}
## Will be the path mapped to the external volume.
ARG BUILD_LOCATION=/app/out
ENV NUGET_XMLDOC_MODE skip
WORKDIR /app
COPY *.csproj .
RUN dotnet restore
COPY . /app
RUN dotnet publish --output ${BUILD_LOCATION} --configuration ${BUILD_CONFIG}

Finally I have a docker-compose.yml

version: '3'
services:
  webapp:
    container_name: webapp.test
    image: webapp:${WEBAPP_VERSION}
    build:
      context: ../
      dockerfile: ./Docker/App.dockerfile
      args:
        WEBAPP_VERSION: ${WEBAPP_VERSION}
        URL_PORT: ${URL_PORT}
    ports:
      - "5000:${URL_PORT}"
    volumes:
      - appbuild:/app
    links:
      - mysql
    environment:
      MYSQL_SERVER_NAME: ${MYSQL_SERVER_NAME}
    env_file:
      - secrets.env
    depends_on:
      - builder
  
  builder:
    container_name: builder
    image: webapp:${BUILDER_VERSION}.${BUILD_CONFIG}
    build:
      context: ../
      dockerfile: ./Docker/Build.dockerfile
      args:
        BUILDER_VERSION: ${BUILDER_VERSION}
        BUILD_CONFIG: ${BUILD_CONFIG}
        BUILD_LOCATION: ${BUILD_LOCATION}
    volumes:
      - appbuild:${BUILD_LOCATION}
   
  mysql:
    container_name: ${MYSQL_SERVER_NAME}
    image: mysql/mysql-server:8.0.23
    restart: always
    volumes:
      - dbvol:/var/lib/mysql
    environment:
      MYSQL_RANDOM_ROOT_PASSWORD: root
    env_file:
      - secrets.env

volumes:
  appbuild:
  dbvol:

Finally, I launch the following command line :

docker build -f Docker/App.dockerfile -t webapp:Debug --build-arg URL_PORT=7909 .

the process is rather quick. I also launch this command line :

docker run --name webapp.test -p 5000:7909 -it webapp:Debug

I unfortunatelly obtain an error message.

Could not execute because the application was not found or a compatible .NET SDK is not installed.
Possible reasons for this include:
  * You intended to execute a .NET program:
      The application 'WebApplication.dll' does not exist.
  * You intended to execute a .NET SDK command:
      It was not possible to find any installed .NET SDKs.
      Install a .NET SDK from:
        https://aka.ms/dotnet-download

Answer 1

Your build Dockerfile uses the ASP.NET Core runtime container image:

FROM mcr.microsoft.com/dotnet/aspnet:5.0

This container doesn't have an SDK, just the runtime. When you build it, it fails:

RUN dotnet restore

Could not execute because the application was not found or a compatible .NET SDK is not installed. and The application 'restore' does not exist

Because the dotnet restore command is an SDK command. It's not available if you are just using the runtime.

You should use the SDK container for your build Dockerfile (not your runtime Dockerfile, that's fine):

FROM mcr.microsoft.com/dotnet/sdk:5.0

Answer 2

If you already installed the SDK and you have x64 machine;

Delete the following item from System Variables > Path

C:\Program Files (x86)\dotnet

Answer 3

In my case dot net SDKs were installed in different locations. A simple where.exe dotnet on powershell returned the following

There were 2 entries in system environment variables i.e. The one with x86 was above so it was taking precedence, so I swapped both of them and hurray it started working. Special thanks to https://www.hanselman.com/blog/dotnet-could-not-execute-because-the-application-was-not-found-or-a-compatible-net-sdk-is-not-installed