Оставляю ссылочку: GitHub 📱

public record User(string Name, string Email);

var users = new List<User>
{
    new("Alex", "alex@gmail.com"),
    new("Maria", "maria@company.com"),
    new("Bob", "bob@gmail.com")
};

var result = new List<string>();

foreach (var user in users)
{
    if (user.Email.EndsWith("@gmail.com"))
    {
        result.Add(user.Email);
    }
}

var result = users
    .Select(user => user.Email)
    .Where(email => email.EndsWith("@gmail.com"))
    .ToList();

alex@gmail.com
bob@gmail.com

email.EndsWith(
    "@gmail.com",
    StringComparison.OrdinalIgnoreCase
)

var createdAt = DateTime.Now;

var createdAt = DateTimeOffset.UtcNow;

public record Order(string Status, decimal Amount);

var orders = new List<Order>
{
    new("Paid", 1200m),
    new("New", 500m),
    new("Paid", 300m),
    new("Cancelled", 100m)
};

var result = new Dictionary<string, int>();

foreach (var order in orders)
{
    if (!result.ContainsKey(order.Status))
    {
        result[order.Status] = 0;
    }

    result[order.Status]++;
}

var result = orders
    .GroupBy(order => order.Status)
    .ToDictionary(g => g.Key, g => g.Count());

Paid      -> 2
New       -> 1
Cancelled -> 1

var user = users.FirstOrDefault(u => u.Id == id);

null

Console.WriteLine(user.Name);

var user = users.FirstOrDefault(u => u.Id == id);

if (user is null)
{
    return;
}

var user = users.FirstOrDefault(u => u.Id == id)
    ?? throw new InvalidOperationException("User not found");

if (role.ToLower() == "admin")
{
    GrantAccess();
}

if (string.Equals(role, "admin", StringComparison.OrdinalIgnoreCase))
{
    GrantAccess();
}

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if (role.ToLower() == "admin")
{
    GrantAccess();
}

if (string.Equals(role, "admin", StringComparison.OrdinalIgnoreCase))
{
    GrantAccess();
}