Modern C# Cheat Sheet: C# 10 to C# 14

Softinbit Technology published a Modern C# cheat sheet condensing every language feature from C# 10 (.NET 6) through C# 14 (.NET 10) into before/after snippets compiled and tested on the .NET 10 SDK version 10.0.300. The guide covers file-scoped namespaces, raw string literals, required properties, primary constructors, collection expressions, the Lock type and field-backed properties, and notes that the compiler selects the default C# version from the target framework so LangVersion overrides are unnecessary. It documents real diagnostics, including CS9260 when the C# 14 field keyword is used under language version 13.0.

Note: this article was drafted with AI assistance and reviewed by Softinbit Technology before publishing. It is part of the series The .NET Field Guide . TL;DR — Every C feature from C 10 .NET 6 through C 14 .NET 10 condensed into single-purpose before/after snippets tested on .NET 10 SDK 10.0.300 . — Fast lookups for top idioms: file-scoped namespaces, raw strings, required properties, primary constructors, collection expressions, Lock, and field-backed properties. — Clear guidance on target framework defaults and compiler diagnostics error version without fragile LangVersion overrides. Level: Intermediate · Prerequisites: Working knowledge of C 9 / .NET 5 or earlier · Reading time: 13 min Your team just moved Store.Api to .NET 10, and a reviewer comments "use field here" on a property you wrote the C 9 way. This sheet is for developers who learned C a few versions ago and want the short version of C 10 .NET 6 through C 14 .NET 10 . Every snippet was compiled with the .NET 10 software development kit SDK , version 10.0.300; outputs and error messages are real. The lookups people make most: The compiler picks the default C version from your target framework the “Default for” column above . You do not need LangVersion in the project file. error CS8304: Compiler version: '5.6.0-2.26230.102 caa81fa4... '. Language version: 14.0. Using a C 14 feature when the language version is 13 for example net9.0 built with the .NET 10 SDK : error CS9260: Feature 'field keyword' is not available in C 13.0. Please use language version 14.0 or greater. Global usings. Import a namespace once for the whole project. // Before: repeated at the top of every fileusing Store.Api.Orders; // After: once, in GlobalUsings.cs or <Using Include="Store.Api.Orders" / in the .csproj global using Store.Api.Orders; File-scoped namespaces. One less indentation level per file. // Beforenamespace Store.Api.Orders{ public sealed class OrderService { }} // Afternamespace Store.Api.Orders; public sealed class OrderService { } Record structs. Value-type records with generated equality, ToString, and with. // Before: a struct with hand-written Equals, GetHashCode, ==, and ToString// Afterpublic readonly record struct Money decimal Amount, string Currency ; js var a = new Money 10m, "TRY" ;var b = a with { Amount = 12m };Console.WriteLine a ; // Money { Amount = 10, Currency = TRY }Console.WriteLine a == new Money 10m, "TRY" ; // True Extended property patterns. Reach into nested members with a dot. // Beforeif order is { Lines: { Count: 0 } } { } // Afterif order is { Lines.Count: 0 } { } Constant interpolated strings. Build const strings, for example route templates, with $"". js // Beforepublic const string Products = Base + "/products"; js // Afterpublic const string Base = "/api";public const string Products = $"{Base}/products"; // "/api/products", usable in Route Products Lambda improvements. Lambdas get a natural type, an optional explicit return type, and attributes. js // BeforeFunc<string, int parse = s = int.Parse s ; js // Aftervar parse = string s = int.Parse s ; // inferred as Func<string, int var pick = object bool b = b ? 1 : "one"; // explicit return typeapp.MapGet "/api/health", AllowAnonymous = Results.Ok ; // Microsoft.AspNetCore.Authorization Raw string literals. Multi-line text with quotes and braces, no escaping. // Beforevar json = "{ \"id\": " + id + ", \"name\": \"Mouse\" }"; bash // After: $$ means {{x}} interpolates and a single { is literalvar json = $$""" { "id": {{id}}, "name": "Mouse" } """;// { "id": 42, "name": "Mouse" } Required members. The compiler forces object initializers to set these properties. public sealed class CreateProductRequest{ public required string Name { get; init; } public required decimal Price { get; init; }} js var request = new CreateProductRequest { Name = "Mouse" };// error CS9035: Required member 'CreateProductRequest.Price' must be set in the object initializer or attribute constructor. Mark a constructor that sets them all with SetsRequiredMembers from System.Diagnostics.CodeAnalysis. List patterns. Match arrays and lists by shape, with .. for "the rest". // Before: if ids.Length == 0 ... else if ids.Length == 1 ...// Afterstatic string Describe int ids = ids switch{ = "empty", var only = $"one: {only}", var first, .., var last = $"{first}..{last}",};// Describe 1, 2, 3 - "1..3" Generic math static abstract interface members . Write one numeric algorithm for every number type. using System.Numerics; // Before: Sum IEnumerable<int , Sum IEnumerable<decimal , ...// Afterstatic T Sum<T IEnumerable<T values where T : INumber<T { T total = T.Zero; foreach var v in values total += v; return total;}// Sum 10.5m, 20m - 30.5 Sum 1, 2, 3 - 6 Your own interfaces can declare static abstract TSelf Zero { get; } the same way. UTF-8 string literals. Get bytes at compile time. // Beforebyte ok = Encoding.UTF8.GetBytes "OK" ; // AfterReadOnlySpan<byte ok = "OK"u8; // 2 bytes, no runtime encoding File-local types. A type visible only inside its own file. // Before: internal helper visible to the whole assembly// Afterfile sealed class SkuFormatter{ public static string Format int id = $"SKU-{id:D5}"; // SKU-00042} scoped. Promise that a span parameter does not escape, so callers can pass stack memory. public ref struct ReceiptWriter{ private Span<char dest; public int Written; public ReceiptWriter Span<char dest { dest = dest; Written = 0; } public void Append scoped ReadOnlySpan<char text { text.CopyTo dest Written.. ; Written += text.Length; }}// Without "scoped", w.Append stackallocBuffer fails with error CS8350. Primary constructors for classes. Constructor parameters in the class header, in scope everywhere. // Beforepublic sealed class PriceCalculator{ private readonly decimal vatRate; public PriceCalculator decimal vatRate = vatRate = vatRate;} // Afterpublic sealed class PriceCalculator decimal vatRate { public decimal WithVat decimal net = net 1 + vatRate ; // 100 - 120.00} Collection expressions. One syntax for arrays, lists, spans, and sets, plus .. spread. js // Beforevar ids = new int { 1, 2, 3 };var tags = new List<string { "new", "sale" };var none = Array.Empty<Product ; // Afterint ids = 1, 2, 3 ;List<string tags = "new", "sale" ;string all = .. tags, "featured" ; // new, sale, featuredProduct none = ; Default lambda parameters. Optional parameters on lambdas. // Before: a local function or two overloads// Aftervar discount = decimal price, decimal rate = 0.10m = price 1 - rate ;discount 100m ; // 90.00discount 100m, 0.25m ; // 75.00 Alias any type. using aliases for tuples, arrays, and pointer types. // Before: only named types could be aliased// Afterusing LineTotal = int ProductId, decimal Total ; LineTotal line = 5, 59.90m ;Console.WriteLine line.Total ; // 59.90 Inline arrays. A fixed-size buffer inside a struct, without unsafe. using System.Runtime.CompilerServices; InlineArray 3 public struct LastThreePrices{ private decimal element0;} js var last = new LastThreePrices ;last 0 = 10m; last 1 = 20m; last 2 = 30m; // indexable and foreach-able Experimental attribute. Mark APIs that callers must opt into. using System.Diagnostics.CodeAnalysis; Experimental "STORE001" public static class Recommendations{ public static string For int userId = "Mouse", "Keyboard" ;}// Calling it from Store.Api.Products without suppression:// error STORE001: 'Store.Api.Products.Recommendations' is for evaluation purposes only and is// subject to change or removal in future updates. Suppress this diagnostic to proceed. Suppress with pragma warning disable STORE001 or <NoWarn STORE001</NoWarn . Also new in C 12: ref readonly parameters, and interceptors as a preview feature. params collections. params works with spans and collection interfaces, not only arrays. // Before: allocates an array on every callpublic static string Tags params string tags = string.Join "|", tags ; js // Afterpublic static string Tags params ReadOnlySpan<string tags = string.Join "|", tags ;public static int CountIds params IEnumerable<int ids = ids.Count ;// Tags "new", "sale" - "new|sale" System.Threading.Lock. A dedicated lock type; lock uses its API instead of Monitor. // Beforeprivate readonly object gate = new ; // Afterprivate readonly Lock gate = new ; lock gate // compiles to gate.EnterScope { reserved += qty;} \e escape. The ESCAPE character U+001B for terminal colors. // BeforeConsole.WriteLine "\u001b 32mPaid\u001b 0m" ; // AfterConsole.WriteLine "\e 32mPaid\e 0m" ; Partial properties and indexers. Declare in one part, implement in another source generators use this . public partial class ProductView{ public partial string Sku { get; set; } // declaring part} public partial class ProductView{ private string sku = ""; public partial string Sku // implementing part { get = sku; set = sku = value.ToUpperInvariant ; }} Implicit index access in object initializers. ^ from the end works inside initializers. public sealed class Shelf { public int Slots { get; } = new int 5 ; } // Before: front indexes only, { Slots = { 4 = 99 } }// Aftervar shelf = new Shelf { Slots = { ^1 = 99, 0 = 1 } }; // 1,0,0,0,99 OverloadResolutionPriority. Library authors steer callers to a better overload without breaking them. using System.Runtime.CompilerServices; js public static class Logging{ public static string Log string message = "string overload"; OverloadResolutionPriority 1 public static string Log ReadOnlySpan<char message = "span overload";}// Logging.Log "hello" - "span overload" ref struct interfaces and allows ref struct. Span-holding types can implement interfaces and flow through generics. public interface ISkuSource { int Count { get; } } public ref struct SpanSkuSource : ISkuSource{ private readonly ReadOnlySpan<string skus; public SpanSkuSource ReadOnlySpan<string skus = skus = skus; public int Count = skus.Length;} js static int CountOf<T T source where T : ISkuSource, allows ref struct = source.Count; Also in C 13: ref locals and unsafe blocks inside async methods and iterators not across await or yield , and smarter method group overload resolution. Extension members. Extension blocks add properties, methods, and static members to types you do not own. js // Before: extension methods onlypublic static bool InStock this Product p = p.Stock 0; // Afterpublic static class ProductExtensions{ extension Product product { public bool InStock = product.Stock 0; public decimal PriceWithVat decimal rate = product.Price 1 + rate ; } js extension Product { public static Product Placeholder = new { Name = " none " }; }}// p.InStock - True p.PriceWithVat 0.20m - 120.00 Product.Placeholder.Name - none The field keyword. Use the compiler-generated backing field inside an accessor. js // Beforeprivate string email = "";public string Email{ get = email; set = email = value.Trim .ToLowerInvariant ;} js // Afterpublic string Email{ get; set = field = value.Trim .ToLowerInvariant ;} = "";// " Ali@Example.COM " - "ali@example.com" Null-conditional assignment. ?. and ? on the left side of = and compound assignments. // Beforeif order is not null order.Status = OrderStatus.Paid; // Afterorder?.Status = OrderStatus.Paid; // right side runs only when order is not nullorder?.Total += 10m;// order?.Total++; - error CS1059 ++ and -- are not allowed here Implicit span conversions. Arrays convert to spans for extension receivers and generic inference. public static decimal Total this ReadOnlySpan<decimal prices { decimal sum = 0; foreach var p in prices sum += p; return sum;} decimal prices = 10m, 20m ;prices.Total ;// C 13: error CS1929: 'decimal ' does not contain a definition for 'Total' ...// C 14: compiles and returns 30 nameof with unbound generics. No dummy type argument needed. // Beforenameof List<int // "List" // Afternameof List< // "List"nameof Dictionary<, // "Dictionary" Simple lambda parameters with modifiers. out, ref, in, scoped without spelling out types params still needs them . public delegate bool TryParser<T string text, out T result ; js // BeforeTryParser<decimal p1 = string text, out decimal result = decimal.TryParse text, out result ; js // AfterTryParser<decimal p2 = text, out result = decimal.TryParse text, out result ; Partial constructors and partial events. One defining and one implementing declaration, like partial methods. public partial class Cart{ public partial Cart int id ; public partial event EventHandler? Changed;} public partial class Cart{ private EventHandler? changed; public int Id { get; } public partial Cart int id { Id = id; } public partial event EventHandler? Changed { add = changed += value; remove = changed -= value; }} User-defined compound assignment. Overload += and ++ as instance operators that mutate in place. // Before: static operator + creates a new object on every +=// Afterpublic sealed class Basket{ public decimal Total { get; private set; } public int Items { get; private set; } public void operator += decimal price { Total += price; Items++; } public void operator ++ = Items++;} js var basket = new Basket ;basket += 50m; basket += 25m; basket++; // Total 75, Items 3 C 14 also adds preprocessor directives for file-based apps dotnet run app.cs . Solution-wide defaults. Put this next to your .sln; keep global usings in Store.Api.csproj so test projects still build. php < -- File: Directory.Build.props -- <Project <PropertyGroup <TargetFramework net10.0</TargetFramework <Nullable enable</Nullable <ImplicitUsings enable</ImplicitUsings < -- No LangVersion: net10.0 gives you C 14 -- </PropertyGroup </Project php < -- File: Store.Api.csproj inside <Project -- <ItemGroup <Using Include="Store.Api.Orders" / <Using Include="Store.Api.Products" / </ItemGroup A request data transfer object DTO with required and field validation. // File: Products/CreateProductRequest.csnamespace Store.Api.Products; public sealed class CreateProductRequest{ public required string Name { get; init = field = value.Trim ; } js public required decimal Price { get; init = field = value = 0m ? value : throw new ArgumentOutOfRangeException nameof Price , "Price cannot be negative." ; } public int Stock { get; init; }} A service with a primary constructor, collection expressions, and patterns. // File: Orders/OrderPricing.csnamespace Store.Api.Orders; public sealed class OrderPricing TimeProvider clock { private static readonly HashSet<DayOfWeek Weekend = DayOfWeek.Saturday, DayOfWeek.Sunday ; js public decimal ShippingFee Order order = order switch { { Lines.Count: 0 } = 0m, { Total: = 500m } = 0m, when Weekend.Contains clock.GetUtcNow .DayOfWeek = 29.90m, = 49.90m }; js public static int ProductIds Order order = .. order.Lines.Select l = l.ProductId .Distinct ;} Register it with builder.Services.AddSingleton TimeProvider.System ; and builder.Services.AddScoped<OrderPricing ;. An HTML email body with a raw interpolated string. // File: Infrastructure/Email/OrderEmailTemplates.csusing System.Globalization;using System.Net;using Store.Api.Orders; namespace Store.Api.Infrastructure.Email; public static class OrderEmailTemplates{ private static readonly CultureInfo Turkish = CultureInfo.GetCultureInfo "tr-TR" ; js public static string Confirmation Order order, string customerName = $$""" <h1 Thank you, {{WebUtility.HtmlEncode customerName }}</h1 <p Order {{order.Id}} is {{order.Status}}.</p <p Total: {{order.Total.ToString "C", Turkish }}</p """;} Output for order 1001 with a total of 1200 and the name Ayşe <VIP : <h1 Thank you, Ayşe &lt;VIP&gt;</h1 <p Order 1001 is Paid.</p <p Total: ₺1.200,00</p Pass the result to IEmailService.SendAsync to, subject, htmlBody, ct . Tags: csharp, dotnet, csharp-14, dotnet-10, cheat-sheet Originally published at www.softinbit.com . New articles by email: subscribe . Modern C Cheat Sheet: C 10 to C 14 https://blog.devgenius.io/modern-c-cheat-sheet-c-10-to-c-14-1793a8e612a8 was originally published in Dev Genius https://blog.devgenius.io on Medium, where people are continuing the conversation by highlighting and responding to this story.