Dependency injection: Captive dependency — scoped service injected into singleton

Injecting a scoped service (e.g., DbContext) into a singleton causes it to be captured for the lifetime of the singleton. In a web app this means a single DbContext instance is shared across all requests, leading to thread safety violations and stale data.

Never inject scoped services into singletons directly. Instead inject IServiceScopeFactory and create a scope when needed:

public class MySingleton
{
    private readonly IServiceScopeFactory _scopeFactory;

    public MySingleton(IServiceScopeFactory scopeFactory)
        => _scopeFactory = scopeFactory;

    public async Task DoWorkAsync()
    {
        using var scope = _scopeFactory.CreateScope();
        var db = scope.ServiceProvider.GetRequiredService<AppDbContext>();
        await db.Items.ToListAsync();
    }
}

Enable runtime validation in development:

builder.Host.UseDefaultServiceProvider(options =>
{
    options.ValidateScopes = true;       // throw on captive dependency
    options.ValidateOnBuild = true;      // validate graph at startup
});

DI container resolves scoped instances per scope (usually per HTTP request). A singleton holds a reference to the scope that was active at construction time, which may have been disposed, or extends that scope indefinitely instead of creating a fresh one per request.