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.

When processing large amounts of data, C# can be efficient through streaming, parallel asynchronous and appropriate data structures. 1. Use streaming processing to read one by one or in batches, such as StreamReader or EFCore's AsAsyncEnumerable to avoid memory overflow; 2. Use parallel (Parallel.ForEach/PLINQ) and asynchronous (async/await Task.Run) reasonably to control the number of concurrency and pay attention to thread safety; 3. Select efficient data structures (such as Dictionary, HashSet) and serialization libraries (such as System.Text.Json, MessagePack) to reduce search time and serialization overhead.

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.

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.

C# code performance optimization requires tools rather than intuition. BenchmarkDotNet is the first choice for benchmarking. 1. Automatically handle JIT warm-up and GC effects by scientifically comparing the execution efficiency of different methods; 2. Profiling using tools such as VisualStudio, dotTrace or PerfView to find the truly time-consuming "hot spot" functions; 3. Pay attention to memory allocation, combine [MemoryDiagnoser], DiagnosticTools and PerfView to analyze GC pressure, reduce object creation in high-frequency paths, and give priority to using structures or pooling technology to reduce GC burden.

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.

Reflection is a function in C# for dynamic analysis and modification of program structures at runtime. Its core functions include obtaining type information, dynamically creating objects, calling methods, and checking assembly. Common application scenarios include: 1. Automatically bind the data model, such as mapping dictionary data to class instances; 2. Implement the plug-in system, loading external DLLs and calling its interface; 3. Supporting automated testing and logging, such as executing specific feature methods or automatically recording logs. When using it, you need to pay attention to performance overhead, encapsulation corruption and debugging difficulties. Optimization methods include caching type information, using delegates to improve call efficiency, and generating IL code. Rational use of reflection can improve the flexibility and versatility of the system.