Build a multi-tenant app with EF Core by adding TenantId to entities, applying global query filters, resolving tenant per request, isolating data via shared or separate databases, and enforcing secure dependency injection configuration.
EF Query Filters
Global Filtering
Automatically apply conditions to all queries for an entity (e.g., soft delete, multi-tenancy).
Defined in OnModelCreating
Configured using HasQueryFilter() inside the DbContext model builder.
Soft Delete Support
Commonly used to filter out records where IsDeleted = true.
Multi-Tenant Isolation
Filters data by TenantId to ensure users access only their tenant’s data.
Automatically Applied
No need to manually add Where clauses in every query.
Can Be Disabled Temporarily
Use IgnoreQueryFilters() when you need to bypass global filters.
**Supports Parameters
**Filters can use context-level variables (e.g., current tenant or user ID).
Improves Security & Consistency
Reduces risk of accidental data exposure and keeps filtering logic centralized.
Example
EF + Dynamic Connection String Resolution
Tenant-Based Database Selection
Dynamically choose a connection string based on the current tenant.
Supports Multi-Tenant Architecture
Enables separate databases per tenant for stronger data isolation.
Resolve Per Request
Determine the connection string from request data (subdomain, header, JWT claim).
Use DbContextOptionsBuilder
Configure the connection string inside OnConfiguring() or during service registration.
Dependency Injection Friendly
Inject a TenantProvider or ConnectionStringResolver service into DbContext.
Improves Security & Scalability
Isolates tenant data and allows independent scaling or migration.
Works with Multiple Providers
Can dynamically switch between SQL Server, PostgreSQL, etc., if required.
Supports Central Configuration Store
Store tenant connection strings in a master database or configuration service.
Requires Careful Lifetime Management
DbContext should be scoped per request to avoid cross-tenant leakage.
Useful for SaaS Applications
Ideal for enterprise SaaS platforms needing strict data separation.
Example
Conclusion
Dynamic connection string resolution in EF Core is a powerful approach for building scalable and secure multi-tenant applications. By resolving the database connection per request, you can ensure proper tenant isolation, improve data security, and enable flexible scaling strategies. When combined with dependency injection and proper DbContext scoping, it becomes a clean and maintainable solution for modern SaaS architectures.
In short, EF Core with dynamic connection string resolution enables true database-level multi-tenancy with flexibility and control.
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