CustomAttributes are mechanisms used in C# to attach metadata to code elements. Its core function is to inherit the System.Attribute class and read through reflection at runtime to implement functions such as logging, permission control, etc. Specifically, it includes: 1. CustomAttributes are declarative information, which exists in the form of feature classes, and are often used to mark classes, methods, etc.; 2. When creating, you need to define a class inherited from Attribute, and use AttributeUsage to specify the application target; 3. After application, you can obtain feature information through reflection, such as using Attribute.GetCustomAttribute();

The core of designing immutable objects and data structures in C# is to ensure that the state of the object is not modified after creation, thereby improving thread safety and reducing bugs caused by state changes. 1. Use readonly fields and cooperate with constructor initialization to ensure that the fields are assigned only during construction, as shown in the Person class; 2. Encapsulate the collection type, use immutable collection interfaces such as ReadOnlyCollection or ImmutableList to prevent external modification of internal collections; 3. Use record to simplify the definition of immutable model, and generate read-only attributes and constructors by default, suitable for data modeling; 4. It is recommended to use System.Collections.Imm when creating immutable collection operations.

The key to writing C# code well is maintainability and testability. Reasonably divide responsibilities, follow the single responsibility principle (SRP), and take data access, business logic and request processing by Repository, Service and Controller respectively to improve structural clarity and testing efficiency. Multi-purpose interface and dependency injection (DI) facilitate replacement implementation, extension of functions and simulation testing. Unit testing should isolate external dependencies and use Mock tools to verify logic to ensure fast and stable execution. Standardize naming and splitting small functions to improve readability and maintenance efficiency. Adhering to the principles of clear structure, clear responsibilities and test-friendly can significantly improve development efficiency and code quality.

Create custom middleware in ASP.NETCore, which can be implemented by writing classes and registering. 1. Create a class containing the InvokeAsync method, handle HttpContext and RequestDelegatenext; 2. Register with UseMiddleware in Program.cs. Middleware is suitable for general operations such as logging, performance monitoring, exception handling, etc. Unlike MVC filters, it acts on the entire application and does not rely on the controller. Rational use of middleware can improve structural flexibility, but should avoid affecting performance.

Generic constraints are used to restrict type parameters to ensure specific behavior or inheritance relationships, while covariation allows subtype conversion. For example, whereT:IComparable ensures that T is comparable; covariation such as IEnumerable allows IEnumerable to be converted to IEnumerable, but it is only read and cannot be modified. Common constraints include class, struct, new(), base class and interface, and multiple constraints are separated by commas; covariation requires the out keyword and is only applicable to interfaces and delegates, which is different from inverter (in keyword). Note that covariance does not support classes, cannot be converted at will, and constraints affect flexibility.

The following points should be followed when using LINQ: 1. Priority is given to LINQ when using declarative data operations such as filtering, converting or aggregating data to avoid forced use in scenarios with side effects or performance-critical scenarios; 2. Understand the characteristics of delayed execution, source set modifications may lead to unexpected results, and delays or execution should be selected according to requirements; 3. Pay attention to performance and memory overhead, chain calls may generate intermediate objects, and performance-sensitive codes can be replaced by loops or spans; 4. Keep the query concise and easy to read, and split complex logic into multiple steps to avoid excessive nesting and mixing of multiple operations.

Common problems with async and await in C# include: 1. Incorrect use of .Result or .Wait() causes deadlock; 2. Ignoring ConfigureAwait(false) causes context dependencies; 3. Abuse of asyncvoid causes control missing; 4. Serial await affects concurrency performance. The correct way is: 1. The asynchronous method should be asynchronous all the way to avoid synchronization blocking; 2. The use of ConfigureAwait(false) in the class library is used to deviate from the context; 3. Only use asyncvoid in event processing; 4. Concurrent tasks need to be started first and then await to improve efficiency. Understanding the mechanism and standardizing the use of asynchronous code that avoids writing substantial blockage.

Fluent interface is a design method that improves code readability and expressivity through chain calls. The core of it is that each method returns the current object, so that multiple operations can be called continuously, such as varresult=newStringBuilder().Append("Hello").Append("").Append("World"). When implementing, you need to combine the extension method and the design pattern that returns this, such as defining the FluentString class and returning this in its method, and creating an initial instance through the extension method. Common application scenarios include building configurators (such as verification rules), checking