The core concepts of .NET asynchronous programming, LINQ and EFCore are: 1. Asynchronous programming improves application responsiveness through async and await; 2. LINQ simplifies data query through unified syntax; 3. EFCore simplifies database operations through ORM.

C#.NET provides powerful tools for concurrent, parallel and multithreaded programming. 1) Use the Thread class to create and manage threads, 2) The Task class provides more advanced abstraction, using thread pools to improve resource utilization, 3) implement parallel computing through Parallel.ForEach, 4) async/await and Task.WhenAll are used to obtain and process data in parallel, 5) avoid deadlocks, race conditions and thread leakage, 6) use thread pools and asynchronous programming to optimize performance.

In C, the char type is used in strings: 1. Store a single character; 2. Use an array to represent a string and end with a null terminator; 3. Operate through a string operation function; 4. Read or output a string from the keyboard.

In C language, special characters are processed through escape sequences, such as: \n represents line breaks. \t means tab character. Use escape sequences or character constants to represent special characters, such as char c = '\n'. Note that the backslash needs to be escaped twice. Different platforms and compilers may have different escape sequences, please consult the documentation.

The char array stores character sequences in C language and is declared as char array_name[size]. The access element is passed through the subscript operator, and the element ends with the null terminator '\0', which represents the end point of the string. The C language provides a variety of string manipulation functions, such as strlen(), strcpy(), strcat() and strcmp().

In C language, char type conversion can be directly converted to another type by: casting: using casting characters. Automatic type conversion: When one type of data can accommodate another type of value, the compiler automatically converts it.

A strategy to avoid errors caused by default in C switch statements: use enums instead of constants, limiting the value of the case statement to a valid member of the enum. Use fallthrough in the last case statement to let the program continue to execute the following code. For switch statements without fallthrough, always add a default statement for error handling or provide default behavior.

The usage methods of symbols in C language cover arithmetic, assignment, conditions, logic, bit operators, etc. Arithmetic operators are used for basic mathematical operations, assignment operators are used for assignment and addition, subtraction, multiplication and division assignment, condition operators are used for different operations according to conditions, logical operators are used for logical operations, bit operators are used for bit-level operations, and special constants are used to represent null pointers, end-of-file markers, and non-numeric values.