How to add MongoDB database context into startup .netcore

Question

I'm using MongoDB in my project, I have created a DB context like this:

public class PortfolioDbContext
{
    public readonly IMongoCollection<PortfolioData> _portfolioCollection;

    public PortfolioDbContext()
    {
        var client = new MongoClient("mongodb://localhost:27017");
        _portfolioCollection = client.GetDatabase("portfolioServiceDb").GetCollection<PortfolioData>("Portfolios");
    }
}

I was to inject it into my repository class:

    public PortfolioDbContext _db;

    public DataService(PortfolioDbContext _db)
    {
        _db = this._db;
    }

but db returns null, I thought I need to register it in my startup:

 services.AddSingleton<PortfolioDbContext>();

but I'm still getting null, any idea why?

Try this._db = _db in the constructor of DataService.

Mathias
– Mathias

2021-10-30 18:12:29 +00:00
Commented Oct 30, 2021 at 18:12 — Mathias
– Mathias, Commented Oct 30, 2021 at 18:12

Mathias · Accepted Answer · 2021-10-30 18:37:13Z

2

The error is in the constructor of DataService. Your are assigning this._db (the variable of the class) to _db (the argument of the constructor), instead of the other way around. As the default of public PortfolioDbContext _db is null and you are never assigning a different value to it, it remains null.

The coding guidelines from Microsoft https://learn.microsoft.com/en-us/dotnet/csharp/fundamentals/coding-style/coding-conventions recommends using camel case with a _ prefix for private variables in a class and method parameters with camel case (without underline). This helps preventing issues like this.

Your DataService could look like this:

public class DataService
{
    private PortfolioDbContext _db;

    public DataService(PortfolioDbContext db)
    {
        _db = db;
    }

    // ... some more methods
}

answered Oct 30, 2021 at 18:37

Mathias

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