Question 1

How do I compile and run a C# program from the command line?

Accepted Answer

Use the .NET CLI to scaffold, build, and run. Example:

Commands:
```
dotnet new console -n HelloApp
cd HelloApp
dotnet run           # quick run
dotnet build -c Release
```
Code (Program.cs, top-level statements):
```csharp
Console.WriteLine("Hello, C#");
```
To produce an optimized binary: `dotnet publish -c Release -r win-x64 --self-contained true /p:PublishSingleFile=true` (adjust RID for linux-x64/osx-x64).

Question 2

What is async/await in C#, and how do I use it correctly?

Accepted Answer

async/await lets you write non-blocking code over Tasks. Prefer cooperative cancellation and aggregate concurrent I/O with Task.WhenAll.

Example:
```csharp
using System;
using System.Linq;
using System.Net.Http;
using System.Threading;
using System.Threading.Tasks;

var urls = new[] { "https://example.com", "https://dotnet.microsoft.com" };
using var http = new HttpClient();
using var cts = new CancellationTokenSource(TimeSpan.FromSeconds(10));

Task<string>[] fetches = urls.Select(u => http.GetStringAsync(u, cts.Token)).ToArray();
string[] pages = await Task.WhenAll(fetches);
Console.WriteLine(string.Join(Environment.NewLine, pages.Select(p => $"Length={p.Length}")));
```
Tips: avoid `async void` (except event handlers), don’t block with `.Result`/`.Wait()`, pass `CancellationToken`, and prefer `ConfigureAwait(false)` only in libraries.

Question 3

How can I query and transform data efficiently with LINQ—and when should I not use it?

Accepted Answer

LINQ is great for expressive querying, grouping, and projections; prefer it for clarity over small/medium datasets and I/O-bound flows. Example: ```csharp using System; using System.Collections.Generic; using System.Linq; record Order(int Id, string Customer, decimal Amount); var orders = new List { new(1, "Ada", 120m), new(2, "Bob", 75m), new(3, "Ada", 30m) }; var totals = orders .GroupBy(o => o.Customer) .Select(g => new { Customer = g.Key, Total = g.Sum(o => o.Amount) }) .OrderByDescending(x => x.Total) .ToList(); totals.ForEach(t => Console.WriteLine($"{t.Customer}: {t.Total:C}")); ``` Avoid overusing LINQ in ultra-hot loops or when you need tight control over allocations—plain `for` loops or Span can be faster. Always measure with BenchmarkDotNet before micro-optimizing.

Question 4

How do I structure a C# app with dependency injection and configuration?

Accepted Answer

Use the generic host, options pattern, and DI container from Microsoft.Extensions.*. ```csharp using Microsoft.Extensions.DependencyInjection; using Microsoft.Extensions.Hosting; using Microsoft.Extensions.Options; var builder = Host.CreateApplicationBuilder(args); builder.Services.Configure(builder.Configuration.GetSection("App")); builder.Services.AddSingleton(); var app = builder.Build(); var greeter = app.Services.GetRequiredService(); greeter.Greet(); record AppOptions { public string Name { get; init; } = "World"; } interface IGreeter { void Greet(); } sealed class Greeter(IOptions opt) : IGreeter { public void Greet() => Console.WriteLine($"Hello, {opt.Value.Name}!"); } ``` Add `appsettings.json` with `{ "App": { "Name": "Ada" } }`. Benefits: testability, separation of concerns, and environment-based config.

Question 5

How do I publish cross-platform, add logging, and test my code?

Accepted Answer

Publishing: choose a Runtime Identifier (RID) and single-file/self-contained options.
```
# Windows
dotnet publish -c Release -r win-x64 /p:PublishSingleFile=true --self-contained true
# Linux/macOS
dotnet publish -c Release -r linux-x64  /p:PublishSingleFile=true --self-contained true
```
Logging with built-ins:
```csharp
using Microsoft.Extensions.Logging;

using var loggerFactory = LoggerFactory.Create(b => b.AddSimpleConsole().SetMinimumLevel(LogLevel.Information));
var log = loggerFactory.CreateLogger("App");
try { log.LogInformation("Starting"); /* work */ }
catch (Exception ex) { log.LogError(ex, "Unhandled error"); }
```
Testing (xUnit):
```csharp
using Xunit;
public class MathTests { [Fact] public void Adds() => Assert.Equal(4, 2 + 2); }
```
Run tests: `dotnet test`.

C# (Csharp)-C# development assistant

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Using C# (Csharp) in 5 steps

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Install .NET SDK (prerequisite)

Choose an IDE and set it up

Create, run, and debug your first app

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C# (Csharp) — Five in-depth Q&A

How do I compile and run a C# program from the command line?

What is async/await in C#, and how do I use it correctly?

How can I query and transform data efficiently with LINQ—and when should I not use it?

How do I structure a C# app with dependency injection and configuration?

How do I publish cross-platform, add logging, and test my code?