System.ArgumentNullException: MongoDb connection string is not provided. (Parameter 'connectionString') asp.net Core Docker-compose

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01-10-2024

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"System.ArgumentNullException: MongoDb connection string is not provided" in ASP.NET Core with Docker Compose

Encountering the error "System.ArgumentNullException: MongoDb connection string is not provided" in your ASP.NET Core application running within a Docker Compose environment can be frustrating. This usually signifies that your application is unable to locate the MongoDB connection string it needs to connect to the database. This article will explore common causes of this error and provide solutions to help you get your application up and running smoothly.

Scenario and Original Code:

Let's imagine you have a simple ASP.NET Core application utilizing MongoDB for data storage. You're using Docker Compose to manage your application and MongoDB container. Here's a snippet of your Dockerfile and docker-compose.yml files:

Dockerfile:

FROM mcr.microsoft.com/dotnet/aspnet:6.0 AS base
WORKDIR /app
COPY . .

RUN dotnet restore
RUN dotnet publish -c Release -o /app/publish

FROM base AS final
WORKDIR /app/publish
ENTRYPOINT ["dotnet", "MyApplication.dll"]

docker-compose.yml:

version: '3.7'
services:
  app:
    build: .
    ports:
      - "8080:80"
  mongodb:
    image: mongo
    ports:
      - "27017:27017"
    environment:
      MONGO_INITDB_ROOT_USERNAME: user
      MONGO_INITDB_ROOT_PASSWORD: password

Your application might be using the following code to connect to the MongoDB database:

public class MyDbContext : DbContext
{
    public MyDbContext(DbContextOptions options) : base(options) { }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        if (!optionsBuilder.IsConfigured)
        {
            optionsBuilder.UseMongoClient(Configuration["MongoDbConnectionString"]);
        }
    }

    // ... your DbSets ...
}

Running this setup might result in the dreaded error: "System.ArgumentNullException: MongoDb connection string is not provided."

Analysis and Solutions:

The most common causes of this error include:

• Missing or Incorrect Connection String Configuration: Your application needs to know where to find the MongoDB connection string. This can be provided through:
  • Configuration File: Store your connection string in your appsettings.json or appsettings.Development.json file and access it using Configuration["MongoDbConnectionString"].
  • Environment Variables: Set the connection string as an environment variable accessible within the container (MONGO_CONNECTION_STRING) and access it via Configuration["MongoDbConnectionString"].
• Docker Compose Environment Variable Issues: The environment variables defined in your docker-compose.yml file might not be properly accessible by your application container.

Solutions:

1. Configure Connection String in appsettings.json:

   • Update your appsettings.json with the connection string:

     {
       "ConnectionStrings": {
         "MongoDbConnectionString": "mongodb://user:password@mongodb:27017/databaseName"
       }
     }
     

   • Replace user, password, mongodb, and databaseName with your actual MongoDB credentials and container name.

2. Set Connection String as Environment Variable:

   • In docker-compose.yml:

     version: '3.7'
     services:
       app:
         build: .
         ports:
           - "8080:80"
         environment:
           MONGO_CONNECTION_STRING: "mongodb://user:password@mongodb:27017/databaseName"
       mongodb:
         image: mongo
         ports:
           - "27017:27017"
         environment:
           MONGO_INITDB_ROOT_USERNAME: user
           MONGO_INITDB_ROOT_PASSWORD: password
     

   • In your application:

     optionsBuilder.UseMongoClient(Configuration["MONGO_CONNECTION_STRING"]);
     

3. Docker Compose Volume Mounting:

   • If you prefer storing connection string information separately, you can use volume mounting:

     • Create a connectionstrings.txt file with the connection string.

     • Add a volume to your docker-compose.yml file:

       volumes:
         connectionstrings:
       

     • Mount this volume into your application container:

       app:
         ...
         volumes:
           - connectionstrings:/app/connectionstrings.txt
       

     • Modify your application to read the connection string from connectionstrings.txt.

Additional Tips:

• Verify your Docker Compose setup: Ensure that your container links, ports, and environment variables are correctly configured in your docker-compose.yml file.
• Use debugging tools: Utilize Docker's built-in logging and debugging tools to identify the specific issue causing the error.
• Consider a configuration management tool: If your application requires complex configuration management, consider using a tool like Consul or Vault to manage your sensitive data.

By implementing the solutions outlined above, you should be able to resolve the "System.ArgumentNullException: MongoDb connection string is not provided" error and establish a successful connection between your ASP.NET Core application and your MongoDB database within your Docker Compose environment.