/*

Challenge: Employee Management System

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Create an Employee Management System that handles employee data using various data structures in C#. The system should include the following features:

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1. Store employee information in a dictionary where the key is the employee ID and the value is another dictionary containing personal details.

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2. Implement a list to store all employees.

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3. Create a method to add new employees to the system.

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4. Develop a function to retrieve employee details by ID.

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5. Implement a method to update employee information.

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6. Create a function to remove an employee from the system.

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7. Add a feature to display all employees in the system.

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8. Implement a method to calculate the average salary of employees in a specific department.

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9. Create a class `Employee` with properties for ID, name, department, position, and salary.

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10. Use LINQ queries to filter and manipulate employee data efficiently.

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11. Implement proper error handling and input validation.

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12. Optimize memory usage by avoiding unnecessary object creation.

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13. Use appropriate data structures (lists, dictionaries) based on the access patterns of your methods.

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14. Implement thread-safe operations where necessary.

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15. Add unit tests to verify the functionality of your implementation.

*/

// ### Example Data Structures:

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public class Employee

{

    public int Id { get; set; }

    public string Name { get; set; }

    public string Department { get; set; }

    public string Position { get; set; }

    public decimal Salary { get; set; }

}

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public class EmployeeSystem

{

    private Dictionary<int, Dictionary<string, object>> _employees;

    private List<Employee> _employeeList;

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    // Implement all required features here

}

​