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A custom CQRS library for .NET is worth building only when it solves a recurring, concrete problem in your codebase—not simply because CQRS is popular. The pattern separates command and query responsibilities; it does not require two databases, event sourcing, or a home-grown framework. Whether a library is justified depends on your domain, the conventions you need, and the complexity it adds.
What is CQRS in .NET?
CQRS stands for Command Query Responsibility Segregation. It separates operations that change state (commands) from operations that return information (queries). In .NET, those responsibilities can have distinct handlers and models even when they run in the same application and use the same database.
Microsoft’s eShopOnContainers ordering example demonstrates a simplified form: queries and client-facing ViewModels are separated from commands, the domain model, and transactions, while both sides use one database. Microsoft describes this as a separation of responsibilities and models—not a requirement to split infrastructure. See Applying simplified CQRS and DDD patterns in a microservice.
Do I need separate databases for CQRS?
No. Separate command and query models are a logical design choice; separate read and write stores are an optional, more involved architecture. A single database can serve both command and query paths. Splitting storage may help address particular scaling or workload needs, but it also introduces coordination and consistency work. CQRS does not, by itself, require event sourcing.
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For reads, Microsoft’s guidance describes using Dapper, another micro-ORM, EF Core, or plain ADO.NET. A query can join tables or assemble a response for a particular client without inheriting the transactional constraints of the write-side domain model. That flexibility is one reason to separate query responsibilities; it is not evidence that one persistence technology or topology is universally best. See Using read models in a CQRS microservice.
Should I use MediatR or build my own CQRS library?
There is no universal answer. A library can be useful when a codebase repeatedly needs a particular dispatch convention, handler organization, or other shared behavior. But no specific library, repository, API, test suite, performance result, or author motivation is identified here; therefore, a claim about what this particular library does or why its author built it cannot be substantiated.
For a real build-versus-adopt decision, evaluate the actual project and your application. Check its supported .NET versions, command and query dispatch patterns, dependency assumptions, persistence approach, testing strategy, and limitations. Compare those details with the conventions your team already uses and with a maintained alternative such as MediatR. Do not assume that either a custom library or an established package is the better choice without examining those trade-offs.
When is CQRS worth the complexity?
Use the least complex design that handles the work. Microsoft’s DDD guidance says simpler CRUD responsibilities can use simpler approaches, while complex domains with significant, changing business rules may benefit from richer domain models and DDD patterns. CQRS is more compelling when reads and writes have meaningfully different needs or when separating those responsibilities makes a complex system easier to understand. A custom library is a further decision: it should remove repeated friction without obscuring the domain or adding machinery that simple features do not need.
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In a layered design, the application layer coordinates use cases; domain rules belong in the domain model, which should avoid direct dependencies on infrastructure frameworks. A library can support that boundary, but it cannot create a useful domain model in the absence of domain complexity. See Designing a microservice domain model.
Choose query contracts to fit the API
Query results can be dynamic or represented by explicit DTO classes. Dynamic results can make early changes quicker because a SQL change may not require a matching ViewModel-class change. The trade-off is weaker clarity and client compatibility, as well as less descriptive Swagger documentation. Explicit DTOs make response contracts visible to consumers and improve API documentation, at the cost of updating those types when responses change.
Microsoft’s reference application began with dynamic ViewModels and later moved to explicit DTOs as its API stabilized. That is an example of evolving the contract with the application, not a rule that every project should start dynamically. See Dynamic versus static ViewModels.
Make side effects and service boundaries explicit
Domain events represent something that happened within a domain; they can make the resulting side effects explicit. They are commonly handled in-process and may be synchronous or asynchronous. Microsoft’s reference application uses MediatR to propagate synchronous domain events within one transaction.
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Independent reader supportYour contribution helps us test, update, and keep practical guides available for everyone.Decide how much persistence to abstract
Repositories are a design choice, not a CQRS requirement. Microsoft’s persistence guidance describes repositories around aggregate roots for transactional updates, while allowing separate query paths. The same guidance quotes Jimmy Bogard favoring MediatR and direct use of persistence capabilities rather than repositories that hide persistence details. That is Bogard’s stated preference, not a consensus that repositories are always wrong.
For a custom library, decide whether it should depend on aggregate-oriented repositories, expose persistence details to handlers, or leave persistence largely to the application. Make the choice based on the domain and the tests you need; explain it clearly so the library’s abstraction does not conceal important transaction behavior. See Designing the infrastructure persistence layer.
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