ASP.NET Core Minimal API Filter

In this tutorial, you'll learn about ASP.NET Core Minimal API Filter. We cover key concepts, practical examples, and best practices to help you understand and apply this topic effectively.

Your minimal API endpoints share validation, logging, or authorization logic. You duplicate the code in every handler.

Wrong

app.MapPost("/api/users", (User user) =>
{
    if (string.IsNullOrEmpty(user.Name))
        return Results.BadRequest("Name is required");
    // ...
});

app.MapPut("/api/users/{id}", (int id, User user) =>
{
    if (string.IsNullOrEmpty(user.Name))
        return Results.BadRequest("Name is required");
    // ...
});

Right

var adminGroup = app.MapGroup("/api/admin")
    .AddEndpointFilter(async (ctx, next) =>
    {
        // Before
        var logger = ctx.HttpContext.RequestServices
            .GetRequiredService<ILogger<Program>>();
        logger.LogInformation("Admin request started");
        
        var result = await next(ctx);
        
        // After
        logger.LogInformation("Admin request completed");
        return result;
    });

adminGroup.MapPost("/users", (User user) =>
{
    return Results.Ok(user);
});

Validation filter:

app.MapPost("/users", (User user) => { ... })
    .AddEndpointFilter(async (ctx, next) =>
    {
        var user = ctx.GetArgument<User>(0);
        if (string.IsNullOrEmpty(user.Name))
            return Results.BadRequest("Name is required");
        return await next(ctx);
    });

Prevention

• Use AddEndpointFilter on individual endpoints or route groups.
• Filters can be async and access DI via ctx.HttpContext.RequestServices.
• Use the filter context to inspect and modify arguments.
• Multiple filters execute in registration order.
• Chain filters for cross-cutting concerns.

Common Mistakes with core minimal filter

1. Forgetting that lazy evaluation defers computation until the value is forced, causing space leaks with unevaluated thunks
2. Using return to exit a function early instead of wrapping a pure value in the monad
3. Mixing let bindings with <- bindings in do notation, producing type errors

These mistakes appear frequently in real-world ASPNET code. DodaTech's contributors have identified these patterns through analysis of open-source projects and production systems.

Practice Exercise

Write a pure function that safely divides two integers using Maybe, then test it with edge cases like division by zero and negative numbers.

This exercise reinforces the concepts covered in this guide. Try implementing it before checking online solutions.

FAQ

Can I create reusable filter classes?

For more minimal API patterns, visit DodaTech.