Introduction

The with expression creates a new object with modified properties

Example 1

Add a new Product to a list, then use the CategoryId value when adding a second item to the list with the with expression.

results

Model

public record Product(int Id, string Name, int CategoryId) { public int Id { get; set; } = Id; public string Name { get; set; } = Name; public int CategoryId { get; set; } = CategoryId; } 
List<Product> products = []; Product product = new(1, "Football", 2); products.Add(product); products.Add(product with { Id = 2, Name = "Golf" }); foreach (var (id, name, categoryId) in products) { Console.WriteLine($"{id,-3}: {name,-20}{categoryId}"); }

Example 2

An extension of Example 1 that uses a Dictionary to add data.

results

var (products, categories) = MockedData.GetProductsAndCategories(); Dictionary<int, string> dictionary = new() { [2] = "Golf", [3] = "Formula 1 racing", [4] = "Soccer" }; Product product = new(1, "Football", 2); products.Add(product); foreach (var (key, value) in dictionary) { products.Add(product with { Id = key, Name = value }); } var joined = products.Join( categories, p => p.CategoryId, c => c.Id, (p, c) => new { ProductId = p.Id, ProductName = p.Name, CategoryName = c.Name }); foreach (var item in joined) { Console.WriteLine($"{item.ProductId,-3}: {item.ProductName,-20}{item.CategoryName}"); }

Real life

Imagine a file service that reads files by file name, which determines the product category. This would be a good case for the with expression.

Source code