What are C# attributes and how to create a custom attribute?

To create your own C# custom properties, you first need to define a class inherited from System.Attribute, then add the constructor and attributes, specify the scope of application through AttributeUsage, and finally read and use them through reflection. For example, define the [CustomAuthor("John")] attribute to mark the code author, use the [CustomAuthor("Alice")] to modify the class or method when applying, and then obtain the attribute information at runtime through the Attribute.GetCustomAttribute method. Common uses include verification, serialization control, dependency injection, and

C#'s TPL simplifies parallel task processing through the Task class. 1. Use Task.Run() or Task.Factory.StartNew() to start the task, and recommend the former; 2. Get the result through Task and wait for completion with await or .Result; 3. Use Task.WhenAll() to execute multiple tasks in parallel, pay attention to resource competition; 4. Use AggregateException to handle exceptions, and traverse specific errors after catching; 5. Use CancellationTokenSource to cancel the task, which is suitable for timeout or user cancellation scenarios; at the same time, pay attention to avoid mixing synchronous and asynchronous code to prevent deadlock problems.

The three service life cycles of dependency injection in C# are Singleton, Scoped and Transient. Their respective features and applicable scenarios are as follows: 1. Singleton is globally unique instance, suitable for logging, global configuration and other objects that do not need to be rebuilt, but it is necessary to avoid injecting Scoped or Transient services; 2. Scoped requests one instance per instance, suitable for database context and session-related services, and cannot be used in Singleton; 3. Transient uses a new instance each time, suitable for stateless lightweight services, but attention should be paid to the impact of creation costs on performance.

When using var, it should be determined based on whether the type is clear and whether the readability is affected. 1. When the type is clear on the right side of the assignment, such as varlist=newList(); can improve the code simplicity; 2. When the type is fuzzy or returns to object or interface type, var should be avoided, such as IEnumerableresult=SomeMethod(); to improve readability; 3. Use var reasonably in anonymous types and LINQ queries, such as receiving anonymous objects, but subsequent processing is recommended to encapsulate it as a specific type; 4. In team projects, coding style should be unified, and var should be used reasonably through .editorconfig or code review to avoid abuse and affect maintenance.

Lambda expressions are used in C# to write inline, anonymous functions that can be used anywhere you need to delegate. They are simple and flexible, especially suitable for LINQ or asynchronous code. Lambda expressions use the => operator, on the left are input parameters, on the right are expressions or statement blocks; for example, x=>xx represents a lambda that accepts x and returns its squared. If there are multiple parameters, it needs to be wrapped in brackets, such as (intx,inty)=>x y, the type can be omitted and inferred by the compiler. Common uses include LINQ query, event processing, asynchronous programming, etc., such as numbers.Where(n=>n%2==0) filtering even numbers. Unlike the conventional method, lambda

Comparing strings in C# should be based on the scene selection method. The == operator is case-sensitive by default and compared based on the current culture, but is not suitable for complex scenarios. 1. Using the == operator is suitable for quick comparison, but may not meet the expected results due to culture or case; 2. Using String.Equals() and passing in StringComparison enumeration can achieve more precise control, such as Ordinal, OrdinalIgnoreCase, InvariantCulture, etc.; 3. Pay attention to handling null or empty strings when comparing. It is recommended to use the string.Equals() static method or use string.IsNullOrEmpt first.

There are two ways to read CSV files in C#: one is to use StreamReader to read line by line, which is suitable for simple scenarios; the other is to use the CsvHelper library to process structured data. 1. When using StreamReader, read line by line through ReadLine() method and split fields by delimiter with Split(), but be careful not to correctly parse quoted fields and no type conversion function. 2. CsvHelper supports automatic mapping of class attributes, type conversion, custom mapping, ignoring columns, processing of quoted fields and multiple separators. Notes include: Ensure that the file is UTF-8 encoding; correctly processing the title line; judging null values; it is recommended to read large files line by line to optimize performance.

To connect to a SQLServer database, the most common way to C# is to use the SqlConnection class in ADO.NET. 1. You need to install the Microsoft.Data.SqlClient package and reference the namespace; 2. Write a connection string containing the server address, database name, and authentication information; 3. Use using and try-catch to establish a connection and execute queries, while ensuring resource release and exception handling; 4. Common problems include remote connection settings, firewall configuration, port release and login permissions, etc., and need to be checked one by one.