dotnet-collection-best-practices

name	dotnet-collection-best-practices
description	Best practices for collection handling in .NET. Load when creating/initializing collections, choosing collection types for properties/fields/return types, designing public APIs with collections, or modernizing collection syntax.

.NET Collection Best Practices

1. Use Collection Expressions with Type Inference

Use collection expressions [] when the type can be inferred:

// ✅ GOOD
List<string> names = ["Alice", "Bob", "Charlie"];
IImmutableList<int> values = [1, 2, 3, 4, 5];
var links = item.Links ?? [];

// ❌ AVOID
List<string> names = new List<string> { "Alice", "Bob", "Charlie" };
IImmutableList<int> values = ImmutableArray.Create(1, 2, 3, 4, 5);
var links = item.Links ?? Enumerable.Empty<Link>();

Use spread operator .. to combine collections:

IImmutableList<string> combined = [.. collection1, .. collection2, "extra"];

2. Prefer Immutable Collections

Use IImmutableList<T> instead of IReadOnlyCollection<T>:

// ✅ GOOD
public required IImmutableList<string> Permissions { get; init; }
public IImmutableList<Customer> GetActiveCustomers() => this._customers.Where(c => c.IsActive).ToImmutableList();

// ❌ AVOID
public IReadOnlyCollection<Customer> GetActiveCustomers() => this._customers.Where(c => c.IsActive).ToList().AsReadOnly();

Key Patterns

// Empty collections
IImmutableList<string> empty = [];

// Properties
public class Document
{
    // Optional property with default (empty) value
    public IImmutableList<string> Tags { get; init; } = [];

    // Required property
    public required IImmutableList<string> Permissions { get; init; }
}

When to Use Alternatives

• List: Private/internal collections that require mutability and are not exposed as part of the public API
• Arrays: External API interop, performance-critical with fixed size, spans