内部静态方法问题

Question

我有一个静态方法，当在多线程环境中调用该方法时，该请求似乎已被其他线程修改。例如，请参见下面的代码。

在我的情况下，如果我将EmployeeService.UpdateEmployees设为非静态，则它会给我Im所期望的结果，但如果它是静态的，则它将以另一种方式出现。

有人能启发我为什么会这样吗？什么时候将方法设为静态？

public class Test
{
    public void DoSomething(EntityRequest entityRequest)
    {
        EmployeeService.UpdateEmployees(entityRequest.Employees);
    }
}

public class EmployeeService
{
    public static void UpdateEmployees(List<Employee> employees)
    {
        EmployeeDBService.Update(employees);
    }

    public static IEnumerable<Employee> GetEmployees(EmployeeFind request) 
    {
        //some logic here
    }
}

public class EmployeeDBService
{
    public static void Update(List<Employee>employees)
    {
        //some logic here build SP request
    }
}

public class Employee
{
    public int Id { get; set; }
    public string Name { get; set; }
    public DateTime BirthDay { get; set; }
    public Guid RowGuid { get; set; }
}

public class EmployeeFind
{
    public EmployeeFind()
    {
        Employees = new List<Employee>();
    }
    public EmployeeSearch SearchType { get; set; }
    public IList<Employee> Employees { get; set; }
}

public enum EmployeeSearch
{
    ByDepartmentId
}