What is model binding in ASP.NET Core, and where can it cause bugs?

What is model binding in ASP.NET Core APIs?

Explains how route values, query strings, headers, and request bodies become typed endpoint input.

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TL;DR

Model binding is how ASP.NET Core reads data from the HTTP request and maps it into action parameters or DTOs. The data can come from route values, query strings, headers, forms, or the request body. Binding gives the endpoint typed input, such as an id, a filter object, or a create request. A bound object can still be invalid, so validation is a separate step.

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Binding sources, request DTOs, input boundaries, validation separation, malformed clients, and default-value pitfalls.

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Why this matters
Model binding looks simple, but it is the first place user input enters typed application code. Good answers separate binding from validation and mention real bugs from defaults, nullability, and ambiguous sources.
Model binding is how ASP.NET Core reads data from the HTTP request and maps it into action parameters or DTOs. The data can come from route values, query strings, headers, forms, or the request body. Binding gives the endpoint typed input, such as an id, a filter object, or a create request. A bound object can still be invalid, so validation is a separate step.
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In practice, I try to make binding sources obvious. For GET /orders/{id}?includeLines=true, the ID comes from the route and comes from the query string. For , the request DTO comes from the body. The trade-off is convenience versus hidden behavior: automatic binding is helpful, but default values or nullable fields can hide missing client input unless validation and tests catch it.

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Details

Assuming a bound object is valid.
Binding directly to EF entities.
Letting default values hide missing input.
Mixing route, query, and body data without clear rules.
Not testing malformed requests.

What is model binding in ASP.NET Core, and where can it cause bugs?

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