public class UserService { public async Task<User?> GetUserAsync(string id) { return await _repository.FindByIdAsync(id); } }


## Core Concepts

### Records & Nullable

```csharp
// Records for immutable data
public record User(string Id, string Name, string Email)
{
    public string DisplayName => Name.ToUpperInvariant();
}

// With-expressions for copies
var updated = user with { Name = "New Name" };

// Nullable reference types
public User? FindUser(string id)
{
    return users.FirstOrDefault(u => u.Id == id);
}

Async/Await

public async Task<List<User>> GetUsersAsync()
{
    // Parallel async operations
    var tasks = ids.Select(id => GetUserAsync(id));
    var users = await Task.WhenAll(tasks);
    return users.ToList();
}

// Async streams
public async IAsyncEnumerable<User> StreamUsersAsync()
{
    await foreach (var user in _repository.GetAllAsync())
    {
        yield return user;
    }
}

// Cancellation
public async Task<User> GetUserAsync(string id, CancellationToken ct)
{
    return await _client.GetFromJsonAsync<User>($"/users/{id}", ct);
}

Common Patterns

LINQ

// Query syntax
var adults = from user in users
             where user.Age >= 18
             orderby user.Name
             select user;

// Method syntax (preferred)
var result = users
    .Where(u => u.IsActive)
    .OrderBy(u => u.Name)
    .Select(u => new UserDto(u.Id, u.Name))
    .ToList();

// Grouping
var byCountry = users
    .GroupBy(u => u.Country)
    .ToDictionary(g => g.Key, g => g.ToList());

Pattern Matching

string GetStatus(object obj) => obj switch
{
    User { IsActive: true } => "Active user",
    User { IsActive: false } => "Inactive user",
    null => "No data",
    _ => "Unknown"
};

// List patterns
if (numbers is [var first, _, var last])
{
    Console.WriteLine($"First: {first}, Last: {last}");
}

Best Practices

Do:

Use record for DTOs and value objects
Enable nullable reference types
Use async/await for I/O operations
Use dependency injection

Don't:

Block async code with .Result or .Wait()
Ignore cancellation tokens
Use dynamic when type is known
Create God classes

Troubleshooting

Error	Cause	Solution
`NullReferenceException`	Accessing null	Enable nullable, use `?.`
`ObjectDisposedException`	Using disposed resource	Check lifetime, use `using`
`TaskCanceledException`	Operation cancelled	Handle or propagate

Async/Await

Common Patterns

LINQ

Pattern Matching

Best Practices

Troubleshooting

References

csharp

C#

When to Use

Quick Start