.Net Query Filters: Implementation and Use Cases

Query filters in .NET Core—especially via Entity Framework Core—are a powerful feature that allows you to apply global conditions to your entities. These filters are defined once and automatically applied to all LINQ queries, improving consistency and reducing code duplication.

.Net Query Filters : why to use them ?

.NET query filters are particularly useful in scenarios such as:

Soft delete: hiding entities marked as deleted (IsDeleted = true) without physically removing them from the database.
Multi-tenancy: filtering data based on the current user or tenant (TenantId).
Security: restricting access to sensitive data depending on roles or context.

They are defined in the OnModelCreating method of your DbContext, ensuring they are applied consistently across your application. This approach centralizes logic and reduces the risk of forgetting filters in individual queries.

.Net Query Filter Implementation

Here’s a basic example using soft delete


public class Article
{
    public int Id { get; set; }
    public string Title { get; set; }
    public bool IsDeleted { get; set; }
}

protected override void OnModelCreating(ModelBuilder modelBuilder)
{
    modelBuilder.Entity<Article>().HasQueryFilter(a => !a.IsDeleted);
}

public class Article

{

public int Id { get; set; }

public string Title { get; set; }

public bool IsDeleted { get; set; }

}

protected override void OnModelCreating(ModelBuilder modelBuilder)

{

modelBuilder.Entity<Article>().HasQueryFilter(a => !a.IsDeleted);

}

From this point on, all queries to DbContext.Articles will automatically exclude deleted articles. No need to repeat Where(a => !a.IsDeleted) in every query. This makes your codebase cleaner and easier to maintain.

Watch Out for Gotchas

Query filters also apply to navigation properties, which can lead to unexpected behavior.
You can temporarily disable them using .IgnoreQueryFilters() when needed.
They may not play well with complex projections or custom joins.
Filters are compiled into expressions, so avoid injecting runtime values that change frequently.

Performance Considerations

While .NET query filters offer elegance and consistency, they can introduce subtle performance costs if not used carefully. Because filters are translated into SQL expressions, overly complex conditions may result in inefficient queries or unexpected execution plans. It’s important to monitor generated SQL and ensure indexes support the filtered columns. In high-traffic applications, even a small inefficiency in a global filter can scale poorly. Profiling and query analysis tools like EF Core logging or SQL Server Profiler can help identify bottlenecks early.

Conclusion

.NET query filters are an essential tool for structuring clean and secure data access in .NET Core LTS applications. They enhance maintainability, enforce business rules, and simplify patterns like soft delete and multi-tenancy.

Always test your filters thoroughly—especially when dealing with joins or lazy loading. A good filter should be invisible but effective.

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