Материал для тех, кто уже на чём-то писать умеет, и кому надо срочно что-то написать на C#. Это не шпаргалка для экзаменов, а набор примеров кода. Приведённых здесь примеров достаточно для 80% всего, что можно написать на C# даже без использования IDE (достаточно блокнота). Поехали.

= c#

== Самая короткая программа на C#, которая ничего не делает
class test {static void Main(){}}

== выход из программы
Quit();
return из Main

== компиляция

=== csc
csc form.cs
на выходе появится экзешник

=== mcs
mcs form.cs -r:System.Windows.Forms.dll

== консоль
Console.Write("Press any key to exit . . . ");
Console.ReadKey(true);

== подключение dll
[DllImport("winmm.DLL", EntryPoint = "PlaySound", SetLastError = true, CharSet = CharSet.Unicode,
ThrowOnUnmappableChar = true)]
private static extern bool PlaySound(string szSound, System.IntPtr hMod, PlaySoundFlags flags);

== перебор массивов циклом

=== объявление

==== многомерный массив
int[] nums1 = new int[] { 0, 1, 2, 3, 4, 5 }; 
int[,] nums2 = { { 0, 1, 2 }, { 3, 4, 5 } };
int[,] nums1;
int[,] nums2 = new int[2, 3];
int[,] nums3 = new int[2, 3] { { 0, 1, 2 }, { 3, 4, 5 } };
int[,] nums4 = new int[,] { { 0, 1, 2 }, { 3, 4, 5 } };
int[,] nums5 = new [,]{ { 0, 1, 2 }, { 3, 4, 5 } };
int[,] nums6 = { { 0, 1, 2 }, { 3, 4, 5 } };

==== массив массивов
int[][] nums = new int[3][];
nums[0] = new int[2] { 1, 2 };          // выделяем память для первого подмассива
nums[1] = new int[3] { 1, 2, 3 };       // выделяем память для второго подмассива
nums[2] = new int[5] { 1, 2, 3, 4, 5 }; // выделяем память для третьего подмассива

=== for
int[] numbers = { 1, 2, 3, 4, 5 };
for (int i = 0; i < numbers.Length; i++)
{
    numbers[i] = numbers[i] * 2;
    Console.WriteLine(numbers[i]);
}

=== foreach
int[] numbers = { 1, 2, 3, 4, 5 };
foreach (int i in numbers)
{
    Console.WriteLine(i);
}

=== длина массива
int[] numbers = { 1, 2, 3, 5 };
Console.WriteLine(numbers.Length);  // 4

=== элементы с конца
int[] numbers = { 1, 2, 3, 5}; 
Console.WriteLine(numbers[^1]);  // 5 - последний элемент с конца
Console.WriteLine(numbers[^2]);  // 3 - предпоследний элемент с конца

== чтение и запись файлов 

=== чтение
string text = System.IO.File.ReadAllText(@"C:\tmp\Text.txt");

=== запись
var file = new System.IO.StreamWriter(this.textBoxOutputDir.Text + "\\log.txt", true);
file.WriteLine(OutText);
file.Close();

== http-запросы
using System.Net;
...
private static string POST(string Url, string Data)
{
    WebRequest req = WebRequest.Create(Url);
    req.Method = "POST";
    req.Timeout = 100000;
    req.ContentType = "application/x-www-form-urlencoded";
    byte[] sentData = Encoding.GetEncoding(1251).GetBytes(Data);
    req.ContentLength = sentData.Length;
    Stream sendStream = req.GetRequestStream();
    sendStream.Write(sentData, 0, sentData.Length);
    sendStream.Close();
    WebResponse res = req.GetResponse();
    Stream ReceiveStream = res.GetResponseStream();
    StreamReader sr = new StreamReader(ReceiveStream, Encoding.UTF8);
    //Кодировка указывается в зависимости от кодировки ответа сервера
    Char[] read = new Char[256];
    int count = sr.Read(read, 0, 256);
    ...
}

== Обработка исключений
static double SafeDivision(double x, double y)
{
  if (y == 0)
    throw new DivideByZeroException();
  return x / y;
}
...
try
{
  result = SafeDivision(a, b);
  Console.WriteLine("{0} divided by {1} = {2}", a, b, result);
}
catch (DivideByZeroException)
{
  Console.WriteLine("Attempted divide by zero.");
}

try
{
file.ReadBlock(buffer, index, buffer.Length);
}
catch (System.IO.IOException e)
{
Console.WriteLine("Error reading from {0}. Message = {1}", path, e.Message);
}
finally
{
if (file != null)
{
file.Close();
}
}
// Do

== обработка аргументов
public static int Main(string[] args) {
    if(args.Length > 1){
        string Data="arg1:" + args[1];
        ...
    }
}            

== окна

=== создание окна
using System;
using System.Windows.Forms;
public class HelloWorld : Form {
    static public void Main () {
        Application.Run (new HelloWorld ());
    }
    public HelloWorld () {
        Text = "Hello Mono World";
    }
}

=== добавление кнопки
this.button1 = new System.Windows.Forms.Button();
this.button1.Location = new System.Drawing.Point(192, 40);
this.button1.Name = "button1";
this.button1.Size = new System.Drawing.Size(88, 24);
this.button1.TabIndex = 0;
this.button1.Text = "Browse";
this.button1.Click += new System.EventHandler(this.button1_Click);
this.Controls.Add(this.button1);
...
private void button1_Click(object sender, System.EventArgs e)
{
  ...
}

=== всё вместе
public class MainWindow : System.Windows.Controls.Window
{
    private Button button1;
    public MainWindow()
    {
        button1 = new Button();
        button1.Text = "Click me!";
        button1.ClickEvent += button1_OnClick;
    }
    protected void button1_OnClick(object sender, MouseEventArgs e)
    {
        MessageBox.Show("Clicked!");
    }
}

== лямбды
(input-parameters) => expression
(input-parameters) => { <sequence-of-statements> }
Func<int, int> square = x => x * x;
Console.WriteLine(square(5));
// Output:
// 25

== xml
using System;  
using System.IO;  
using System.Xml;  
  
public class Sample  
{  
    public static void Main()  
    {  
        // Create the XmlDocument.  
        XmlDocument doc = new XmlDocument();  
        doc.LoadXml("<book genre='novel' ISBN='1-861001-57-5'>" +  
                    "<title>Pride And Prejudice</title>" +  
                    "</book>");  
  
        // Save the document to a file.  
        doc.Save("data.xml");  
    }  
}  

== sql
using System.Data.SqlClient;
string query = @"select 1";
using (
    var connection = new SqlConnection(
        "user id=Vasya;password=P@$$w0rd;server=MyServer;database=MyBase;connection timeout=10;"
    )
) {
    connection.Open();
    using (var command = new SqlCommand(query, connection)){
        command.CommandTimeout = 290;
        using(SqlDataReader reader = command.ExecuteReader()){
            int rowcounter = -1;
            while (reader.Read()) {
                rowcounter++; //счётчик строк в результате запроса
                //заполняем значения в сохранённой строке
                if (reader["My_Column"] is int) {
                    int n = (int)reader["My_Column "];
                }
                //…
            }
            reader.Close();
        }
    }
}
//добавить обработчики исключений

Для подключения к базе не под логином и паролем, а через доменную авторизацию
using (var connection = new SqlConnection("server=MyServer;database=MyBase;connection timeout=10; Integrated Security=True;")) {}

== Linq
using System.Linq;
using System.DirectoryServices; 
...
using (var computerEntry = new DirectoryEntry(path)) 
{ 
    var userNames = from DirectoryEntry childEntry 
        in computerEntry.Children 
        where childEntry.SchemaClassName == "User" 
        select childEntry.Name; 
    foreach (var name in userNames) 
        Console.WriteLine(name); 
} 

var list = new List<int>{ 2, 7, 1, 3, 9 };
var result = from i in list
               where i > 1
               select i;

== операции со строками
// Declare without initializing.
string message1;
string message2 = null;
string message3 = System.String.Empty;
string oldPath = "c:\\Program Files\\Microsoft Visual Studio 8.0";
string newPath = @"c:\Program Files\Microsoft Visual Studio 9.0";
System.String greeting = "Hello World!";
var temp = "I'm still a strongly-typed System.String!";
const string message4 = "You can't get rid of me!";
char[] letters = { 'A', 'B', 'C' };
string alphabet = new string(letters);

=== сложение
s1 += s2;

=== подстроки
string s3 = "Visual C# Express";
System.Console.WriteLine(s3.Substring(7, 2));
// Output: "C#"

=== поиск
int index = s3.IndexOf("C");

=== замена
System.Console.WriteLine(s3.Replace("C#", "Basic"));

=== перебор символов
s5[s5.Length - i - 1]

=== ToLower ToUpper
    if (System.Char.IsLower(sb[j]) == true)
        sb[j] = System.Char.ToUpper(sb[j]);
    else if (System.Char.IsUpper(sb[j]) == true)
        sb[j] = System.Char.ToLower(sb[j]);

=== форматированные строки
var jh = (firstName: "Jupiter", lastName: "Hammon", born: 1711, published: 1761);
Console.WriteLine($"{jh.firstName} {jh.lastName} was an African American poet born in {jh.born}.");
Console.WriteLine($"He was first published in {jh.published} at the age of {jh.published - jh.born}.");
Console.WriteLine($"He'd be over {Math.Round((2018d - jh.born) / 100d) * 100d} years old today.");

string name = "Vynn";
string greetings = string.Format("Hello {0}!", name);

string name = "Vjor";
string greeting = $"Hello, {name}!";

=== многострочные строки
string query = @"
SELECT *
FROM some_table
"
@"C:\tmp\Text.txt"

=== unicode
string title = "\"The \u00C6olean Harp\", by Samuel Taylor Coleridge";

== Словари
var jh = (
  firstName: "Jupiter",
  lastName: "Hammon",
  born: 1711,
  published: 1761
);
Console.WriteLine($"{jh.firstName} {jh.lastName} was an African American poet born in {jh.born}.");

var numbers = new Dictionary<int, string>
{
  [7] = "seven",
  [9] = "nine",
  [13] = "thirteen"
};

foreach (KeyValuePair<string, List<string>> group in rudimentaryMultiValuedDictionary1){
  ...
}

== задержка времмени в программе
System.Threading.Thread.Sleep(50);

== многопоточность
public async Task<int> RetrieveDocsHomePage()
{
var client = new HttpClient();
byte[] content = await client.GetByteArrayAsync("https://docs.microsoft.com/");
Console.WriteLine($"{nameof(RetrieveDocsHomePage)}: Finished downloading.");
return content.Length;
}

Thread synchronization
C# provides the lock statement, which is yet another example of beneficial syntactic sugar. It works by marking a block of code as a critical section by mutual exclusion of access to a provided object. Like the using statement, it works by the compiler generating a try ... finally block in its place.

private static StreamWriter _writer;

public void ConcurrentMethod()
{
    lock (_writer)
    {
        _writer.WriteLine("Line 1.");
        _writer.WriteLine("Followed by line 2.");
    }
}


== асинхронность 
static IEnumerable<int> MyIterator()
{
try
{
for (int i = 0; i < 100; i++)
{
Thread.Sleep(1000);
yield return i;
}
}
finally
{
Thread.Sleep(200);
Console.WriteLine("finally");
}
}
static async IAsyncEnumerable<int> MyIterator()
{
try
{
for (int i = 0; i < 100; i++)
{
await Task.Delay(1000);
yield return i;
}
}
finally
{
await Task.Delay(200);
Console.WriteLine("finally");
}
}
IAsyncEnumerable<string> result = from url in urls
where item % 2 == 0
select SomeAsyncMethod(item);
async ValueTask<int> SomeAsyncMethod(int item)
{
await Task.Yield();
return item * 2;
}


public static class SomeAsyncCode
{
    public static Task<XDocument> GetContentAsync()
    {
        HttpClient httpClient = new HttpClient();
        return httpClient.GetStringAsync("www.contoso.com").ContinueWith((task) => {
            string responseBodyAsText = task.Result;
            return XDocument.Parse(responseBodyAsText);
        });
    }
}
var t = SomeAsyncCode.GetContentAsync().ContinueWith((task) => {
    var xmlDocument = task.Result;
});
t.Start();

Here is the same logic written in the async-await syntax:

public static class SomeAsyncCode
{
    public static async Task<XDocument> GetContentAsync()
    {
        HttpClient httpClient = new HttpClient();
        string responseBodyAsText = await httpClient.GetStringAsync("www.contoso.com");
        return XDocument.Parse(responseBodyAsText);
    }
}
var xmlDocument = await SomeAsyncCode.GetContentAsync();
// The Task will be started on call with await.


== Запуск команд CMD из кода программы
using System.Diagnostics;
Process.Start("calc");
Process.Start("explorer", "d:");
Так же можно запускать программы от имени другого пользователя, см. вамианты перегрузки .Start()

== тесты
[TestMethod]
public void Debit_WithValidAmount_UpdatesBalance()
{
    double beginningBalance = 11.99;
    double debitAmount = 4.55;
    double expected = 7.44;
    BankAccount account = new BankAccount("Mr. Bryan Walton", beginningBalance);
    account.Debit(debitAmount);
    double actual = account.Balance;
    Assert.AreEqual(expected, actual, 0.001, "Account not debited correctly");
}

[TestMethod]
public void Debit_WhenAmountIsMoreThanBalance_ShouldThrowArgumentOutOfRange()
{
    double beginningBalance = 11.99;
    double debitAmount = 20.0;
    BankAccount account = new BankAccount("Mr. Bryan Walton", beginningBalance);
    try {
        account.Debit(debitAmount);
    }
    catch (System.ArgumentOutOfRangeException e) {
        // Assert
        StringAssert.Contains(e.Message, BankAccount.DebitAmountExceedsBalanceMessage);
        return;
    }
    Assert.Fail("The expected exception was not thrown.");
}

== прочее

=== пространства имён
using forwpf = System.Windows;
...
public static forwpf::Point Convert(forwinforms::Point point) => new forwpf::Point(point.X, point.Y);

=== узнать текущее имя пользователя
string userName = System.Security.Principal.WindowsIdentity.GetCurrent().Name;

=== какая вкладка открыта в tabControl
this.tabControl1.SelectedTab – сама выбранная вкладка
this.tabControl1.SelectedTab.Text – её заголовок
tabControl1.SelectTab(1); — выбрать вкладку по номеру

=== get, set
public int Y
{
get { return y; }
set { y = value; }
}

struct Point
{
int X;
int Y {get; set;}
}

=== операторы
System.Index operator ^(int fromEnd);

=== обход файлов и папок
foreach (string s in System.IO.Directory.GetDirectories(
System.Environment.CurrentDirectory))
{
System.Console.WriteLine(s);
}

=== stdin, out, err
System.Console.WriteLine(sb.ToString());
System.Console.ReadLine();

TextWriter errorWriter = Console.Error;
  errorWriter.WriteLine(e.Message);

=== HEX
byte[] array = { 0x64, 0x6f, 0x74, 0x63, 0x65, 0x74 };
string hexValue = Convert.ToHexString(array);
/*Output:
646F74636574
*/

=== списки
System.Collections.Generic.List<string> strings = new System.Collections.Generic.List<string>();

List<Cat> cats = new List<Cat>
{
new Cat{ Name = "Sylvester", Age=8 },
new Cat{ Name = "Whiskers", Age=2 },
new Cat{ Name = "Sasha", Age=14 }
};

.at(...)

=== операторы
Primary expressions 	Primary 	x.y f(x) a[x] x++ x-- new typeof default checked unchecked delegate
Unary operators 	Unary 	+ - ! ~ ++x --x (T)x
Arithmetic operators 	Multiplicative 	* / %
Arithmetic operators 	Additive 	+ -
Shift operators 	Shift 	<< >>
Relational and type-testing operators 	Relational and type testing 	< > <= >= is as
Relational and type-testing operators 	Equality 	== !=
Logical operators 	Logical AND 	&
Logical operators 	Logical XOR 	^
Logical operators 	Logical OR 	|
Conditional logical operators 	Conditional AND 	&&
Conditional logical operators 	Conditional OR 	||
The null coalescing operator 	Null coalescing 	??
Conditional operator 	Conditional 	?:
Assignment operators, Anonymous function expressions 	Assignment and lambda expression 	= *= /= %= += -= <<= >>= &= ^= |= =>

=== колбэки
public static void MethodWithCallback(int param1, int param2, Del callback)
{
  callback("The number is: " + (param1 + param2).ToString());
}

=== if else
int statusCode = condition ? 1 : 2;

=== switch
switch (condition)
{
   case 1:
      //do something
      break;
   case 2:
      //do something
      break;
   case 3:
      //do something
      break;
   default:
      //do something else
     break;
}

=== Tuples
public (string FirstName, string LastName) GetName()
{
    return ("Vincent", "Durano");
}

=== делегаты
class Program
{
    // Delegate type:
    delegate int Operation(int a, int b);

    static int Add(int i1, int i2)
    {
        return i1 + i2;
    }

    static int Sub(int i1, int i2)
    {
        return i1 - i2;
    }

    static void Main()
    {
        // Instantiate the delegate and assign the method to it.
        Operation op = Add;

        // Call the method that the delegate points to.
        int result1 = op(2, 3);  // 5

        op = Sub;
        int result2 = op(10, 2); // 8
    }
}

=== указатели
static void Main(string[] args)
{
    unsafe
    {
        int a = 2;
        int* b = &a;

        Console.WriteLine("Address of a: {0}. Value: {1}", (int)&a, a);
        Console.WriteLine("Address of b: {0}. Value: {1}. Value of *b: {2}", (int)&b, (int)b, *b);

        // Will output something like:
        // Address of a: 71953600. Value: 2
        // Address of b: 71953596. Value: 71953600. Value of *b: 2
    }
}

=== анонимные типы
var carl = new { Name = "Carl", Age = 35 }; // Name of the type is only known by the compiler.

=== Boxing and unboxing
int foo = 42;         // Value type.
object bar = foo;     // foo is boxed to bar.
int foo2 = (int)bar;  // Unboxed back to value type.

=== модификаторы
Class modifiers

    abstract - Specifies that a class only serves as a base class. It must be implemented in an inheriting class.
    sealed - Specifies that a class cannot be inherited.

Class member modifiers

    const - Specifies that a variable is a constant value that has to be initialized when it gets declared.
    event - Declares an event.
    extern - Specifies that a method signature without a body uses a DLL-import.
    override - Specifies that a method or property declaration is an override of a virtual member or an implementation of a member of an abstract class.
    readonly - Declares a field that can only be assigned values as part of the declaration or in a constructor in the same class.
    unsafe - Specifies an unsafe context, which allows the use of pointers.
    virtual - Specifies that a method or property declaration can be overridden by a derived class.
    volatile - Specifies a field which may be modified by an external process and prevents an optimizing compiler from modifying the use of the field.

static modifier

=== интерфейсы
public class Adder : IBinaryOperation
{
    double IBinaryOperation.A { get; set; }
    double IBinaryOperation.B { get; set; }

    double IBinaryOperation.GetResult()
    {
        return ((IBinaryOperation)this).A + ((IBinaryOperation)this).B;
    }
}
In use:

Adder add = new Adder();

// These members are not accessible:
// add.A = 2;
// add.B = 3;
// double result = add.GetResult();

// Cast to the interface type to access them:
IBinaryOperation add2 = add;
add2.A = 2;
add2.B = 3;
double result = add2.GetResult();


class MyClass : IInterfaceA, IInterfaceB
{
    ...
}

Here is an interface that extends two interfaces.

interface IInterfaceC : IInterfaceA, IInterfaceB
{
    ...
}

=== Шаблоны типов
public class GenericList<T>
{
    void Add(T input) { }
}

=== Генераторы
public static IEnumerable<int> GetEven(IEnumerable<int> numbers){
    foreach (int i in numbers){
        if (i%2 == 0)
            yield return i;
    }
}

=== XML documentation system
public class Foo
{
    /** <summary>A summary of the method.</summary>
     *  <param name="firstParam">A description of the parameter.</param>
     *  <remarks>Remarks about the method.</remarks> */
    public static void Bar(int firstParam) {}
}

=== ...
Do use the using statement when working with objects that eat resources or implements IDisposable interface:

using (MemoryStream stream = new MemoryStream())
{
    // do something
}

Try to avoid one-liner method implementation with curly braces like in the following:
public string Greeter(string name)
{
    return $"Hello {name}!";
}
+1 Do use Expression-bodied (=>) implementation instead:
public string Greeter(string name) => $"Hello {name}!";

public struct Point { ... }     // defines a “value” type
public class Pen { ... }        // defines a “reference” type

=== ссылки
https://github.com/justinamiller/DotNet-Coding-Guidelines/blob/main/README.md
https://en.wikipedia.org/wiki/C_Sharp_syntax

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