export interface IUser {
    id: number;
    name: string;
    email: string;
    password: string;
}

<form>
    <div class="form-group">
        <label form="name">Username</label>
        <input id="name" type="text" class="form-control" />
    </div>
    <div class="form-group">
        <label form="email">Email</label>
        <input id="email" type="text" class="form-control" />
    </div>
    <div class="form-group">
        <label form="password">Password</label>
        <input id="password" type="password" class="form-control" />
    </div>
    <button class="btn btn-primary" type="submit">Submit</button>
</form>

import { Component } from '@angular/core';
import { Control, ControlGroup, Validators } from '@angular/common';
 
@Component({
    selector: 'signup-form',
    templateUrl: 'app/users/signup-form.component.html'
})
export class SignupFormComponent {
    form = new ControlGroup({
        name: new Control('', Validators.required),
        email: new Control('', Validators.required),
        password: new Control('', Validators.required)
    });
 
    onSubmit(): void {
        console.log(this.form.value);
    }
}

<form [ngFormModel]="form" (ngSubmit)="onSubmit()">
    <div class="form-group">
        <label form="name">Username</label>
        <input id="name" type="text" class="form-control"
               ngControl="name"/>
        <label class="text-danger" *ngIf="!form.controls['name'].valid">
            Username is required.
        </label>
    </div>
    <div class="form-group">
        <label form="email">Email</label>
        <input id="email" type="text" class="form-control"
               ngControl="email" #email="ngForm"/>
        <label class="text-danger" *ngIf="email.touched && !email.valid">
            Email is required.
        </label>
    </div>
    <div class="form-group">
        <label form="password">Password</label>
        <input id="password" type="password" class="form-control"
               ngControl="password" #password="ngForm"/>
        <label class="text-danger" *ngIf="password.touched && !password.valid">
            Password is required.
        </label>
    </div>
    <button class="btn btn-primary" type="submit">Submit</button>
</form>

form = new ControlGroup({
    name: new Control('', Validators.required),
    email: new Control('', Validators.required),
    password: new Control('', Validators.required)
});

 import { Control, ControlGroup, Validators } from '@angular/common';

<form [ngFormModel]="form" (ngSubmit)="onSubmit()">

 <label class="text-danger" *ngIf="!form.controls['name'].valid">

    onSubmit(): void {
        console.log(this.form.value);
    }

//same as before...
import { SignupFormComponent } from './users/signup-form.component';
 
@Component({
    //same as before…
    template: `
                //same as before…                    
                <li><a [routerLink]="['AddUser']">Add User</a></li>
               //same as before…
    `,
    //same as before…
})
@RouteConfig([
    //same as before…    
    { path: '/adduser', name: 'AddUser', component: SignupFormComponent }
])
//same as before...

import { Control, ControlGroup, Validators, FormBuilder } from '@angular/common';

form: ControlGroup;
 
constructor(formBuilder: FormBuilder) {
    this.form = formBuilder.group({
        name: ['', Validators.required],
        email: ['', Validators.required],
        password: ['', Validators.required]
    });
}

import { Control } from '@angular/common';
 
export class UsernameValidators {
    static cannotContainSpace(control: Control) {
        if (control.value.indexOf(' ') >= 0) {
            return { cannotContainSpace: true };
        }
        return null;
    }
}

 { required:true }

{
  minlength : {
     requiredLength : 3,
     actualLength : 1
  }
}

 import { UsernameValidators } from './usernameValidators';

 name: ['', Validators.compose([Validators.required, UsernameValidators.cannotContainSpace])],

<div class="form-group">
    <label form="name">Username</label>
    <input id="name" type="text" class="form-control"
           ngControl="name"
           #name="ngForm" />
    <div *ngIf="name.touched && name.errors">
        <label class="text-danger" *ngIf="name.errors.required">
            Username is required.
        </label>
        <label class="text-danger" *ngIf="name.errors.cannotContainSpace">
            Username can't contain space.
        </label>
    </div>
</div>

import {Control} from '@angular/common';
 
export class EmailValidators {
    static email(control: Control) {
        var regEx = /^(([^<>()\[\]\\.,;:\s@"]+(\.[^<>()\[\]\\.,;:\s@"]+)*)|(".+"))@((\[[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}\.[0-9]{1,3}])|(([a-zA-Z\-0-9]+\.)+[a-zA-Z]{2,}))$/;
        var valid = regEx.test(control.value);
        return valid ? null : { email: true };
    }
}

 control(value: Object, validator?: ValidatorFn, asyncValidator?: AsyncValidatorFn) : Control

nameShouldBeUnique(control: Control) {
    let name: string = control.value;
    return new Promise((resolve) => {
        this._userService.isUserNameUnique(<IUser>{ "name": name }).subscribe(
            (result: IResult) => {
                resolve(                    
                    result.result ? null : { 'nameShouldBeUnique': true }
                );
            },
            error => {
                resolve(null);
            }
        );
    });
}

this.form = _formBuilder.group({
    name: ['', Validators.compose([
        Validators.required,
        UsernameValidators.cannotContainSpace
    ]),
        (control: Control) => this.nameShouldBeUnique(control)],

import { Injectable } from '@angular/core';
import { Http, Response } from '@angular/http';
import { Observable } from 'rxjs/Observable';
import { Headers, RequestOptions } from '@angular/http';
import { IUser } from  './user';
import { IResult } from './result';
 
@Injectable()
export class UserService {
    private _checkUserUrl = '/home/checkUser';
 
    constructor(private _http: Http) { }
 
    private handleError(error: Response) {
        console.error(error);
        return Observable.throw(error.json().error || 'Server error');
    }
 
    isUserNameUnique(user: IUser): Observable<IResult> {
        let headers = new Headers({ 'Content-Type': 'application/json' }); // for ASP.NET MVC
        let options = new RequestOptions({ headers: headers });
 
        return this._http.post(this._checkUserUrl, JSON.stringify(user), options)
            .map((response: Response) => <IResult>response.json())
            .do(data => console.log("User: " + JSON.stringify(data)))
            .catch(this.handleError);
    }
}

export interface IResult {
    result: boolean;
}

namespace MVC5Angular2.Controllers
{
    public class HomeController : Controller
    {
        [HttpPost]
        public ActionResult CheckUser(User user)
        {
            var isUnique = new { result = true };
            if (user.Name?.Equals("Vahid", StringComparison.OrdinalIgnoreCase) ?? false)
            {
                isUnique = new { result = false };
            }
 
            return new ContentResult
            {
                Content = JsonConvert.SerializeObject(isUnique, new JsonSerializerSettings
                {
                    ContractResolver = new CamelCasePropertyNamesContractResolver()
                }),
                ContentType = "application/json",
                ContentEncoding = Encoding.UTF8
            };
        }
    }
}

@Component({
    selector: 'signup-form',
    templateUrl: 'app/users/signup-form.component.html',
    providers: [ UserService ]
})
export class SignupFormComponent {
 
    constructor(private _formBuilder: FormBuilder, private _userService: UserService) {

<div *ngIf="name.touched && name.errors">
    <label class="text-danger" *ngIf="name.errors.required">
        Username is required.
    </label>
    <label class="text-danger" *ngIf="name.errors.cannotContainSpace">
        Username can't contain space.
    </label>
    <label class="text-danger" *ngIf="name.errors.nameShouldBeUnique">
        This username is already taken.
    </label>
</div>

<input id="name" type="text" class="form-control"
       ngControl="name"
       #name="ngForm" />
<div *ngIf="name.control.pending">
    Checking server, Please wait ...
</div>

onSubmit(): void {
    console.log(this.form.value);
 
    this.form.find('name').setErrors({
        invalidData : true
    }); 
}

<div *ngIf="name.touched && name.errors">
    <label class="text-danger" *ngIf="name.errors.invalidData">
        Check the inputs....
    </label>
</div>

  { name="VahidN", email="email@site.com", password="123"}

import { Injectable } from '@angular/core';
import { Http, Response } from '@angular/http';
import { Observable } from 'rxjs/Observable';
import { Headers, RequestOptions } from '@angular/http';
import { IUser } from  './user';
import { IResult } from './result';
 
@Injectable()
export class UserService {
    private _addUserUrl = '/home/addUser';
 
    constructor(private _http: Http) { }
 
    private handleError(error: Response) {
        console.error(error);
        return Observable.throw(error.json().error || 'Server error');
    }
 
    addUser(user: IUser): Observable<IUser> {
        let headers = new Headers({ 'Content-Type': 'application/json' }); // for ASP.NET MVC
        let options = new RequestOptions({ headers: headers });
 
        return this._http.post(this._addUserUrl, JSON.stringify(user), options)
            .map((response: Response) => <IUser>response.json())
            .do(data => console.log("User: " + JSON.stringify(data)))
            .catch(this.handleError);
    }
}

[HttpPost]
public ActionResult AddUser(User user)
{
    user.Id = 1; //todo: save user and get id from db
 
    return new ContentResult
    {
        Content = JsonConvert.SerializeObject(user, new JsonSerializerSettings
        {
            ContractResolver = new CamelCasePropertyNamesContractResolver()
        }),
        ContentType = "application/json",
        ContentEncoding = Encoding.UTF8
    };
}

onSubmit(): void {
    console.log(this.form.value);
 
    /*this.form.find('name').setErrors({
            invalidData : true
        });*/
 
    this._userService.addUser(<IUser>this.form.value)
        .subscribe((user: IUser) => {
            console.log(`ID: ${user.id}`);
        });
}

// <![CDATA[
(function ($) {
    $.bootstrapModalAjaxForm = function (options) {
        var defaults = {
            renderModalPartialViewUrl: null,
            renderModalPartialViewData: null,
            postUrl: '/',
            loginUrl: '/login',
            beforePostHandler: null,
            completeHandler: null,
            errorHandler: null
        };
        var options = $.extend(defaults, options);

        var validateForm = function (form) {
            //فعال سازی دستی اعتبار سنجی جی‌کوئری
            var val = form.validate();
            val.form();
            return val.valid();
        };

        var enableBootstrapStyleValidation = function () {
            $.validator.setDefaults({
                highlight: function (element, errorClass, validClass) {
                    if (element.type === 'radio') {
                        this.findByName(element.name).addClass(errorClass).removeClass(validClass);
                    } else {
                        $(element).addClass(errorClass).removeClass(validClass);
                        $(element).closest('.control-group').removeClass('success').addClass('error');
                    }
                    $(element).trigger('highlited');
                },
                unhighlight: function (element, errorClass, validClass) {
                    if (element.type === 'radio') {
                        this.findByName(element.name).removeClass(errorClass).addClass(validClass);
                    } else {
                        $(element).removeClass(errorClass).addClass(validClass);
                        $(element).closest('.control-group').removeClass('error').addClass('success');
                    }
                    $(element).trigger('unhighlited');
                }
            });
        }

        var enablePostbackValidation = function () {
            $('form').each(function () {
                $(this).find('div.control-group').each(function () {
                    if ($(this).find('span.field-validation-error').length > 0) {
                        $(this).addClass('error');
                    }
                });
            });
        }

        var processAjaxForm = function (dialog) {
            $('form', dialog).submit(function (e) {
                e.preventDefault();

                if (!validateForm($(this))) {
                    //اگر فرم اعتبار سنجی نشده، اطلاعات آن ارسال نشود
                    return false;
                }

                //در اینجا می‌توان مثلا دکمه‌ای را غیرفعال کرد
                if (options.beforePostHandler)
                    options.beforePostHandler();

                //اطلاعات نباید کش شوند
                $.ajaxSetup({ cache: false });
                $.ajax({
                    url: options.postUrl,
                    type: "POST",
                    data: $(this).serialize(),
                    success: function (result) {
                        if (result.success) {
                            $('#dialogDiv').modal('hide');
                            if (options.completeHandler)
                                options.completeHandler();
                        } else {
                            $('#dialogContent').html(result);
                            if (options.errorHandler)
                                options.errorHandler();
                        }
                    }
                });
                return false;
            });
        };

        var mainContainer = "<div id='dialogDiv' class='modal hide fade in'><div id='dialogContent'></div></div>";
        enableBootstrapStyleValidation(); //اعمال نکات خاص بوت استرپ جهت اعتبارسنجی یکپارچه با آن
        $.ajaxSetup({ cache: false });
        $.ajax({
            type: "POST",
            url: options.renderModalPartialViewUrl,
            data: options.renderModalPartialViewData,
            contentType: "application/json; charset=utf-8",
            dataType: "json",
            complete: function (xhr, status) {
                var data = xhr.responseText;
                var data = xhr.responseText;
                if (xhr.status == 403) {
                    window.location = options.loginUrl; //در حالت لاگین نبودن شخص اجرا می‌شود
                }
                else if (status === 'error' || !data) {
                    if (options.errorHandler)
                        options.errorHandler();
                }
                else {
                    var dialogContainer = "#dialogDiv";
                    $(dialogContainer).remove();
                    $(mainContainer).appendTo('body');

                    $('#dialogContent').html(data); // دریافت پویای اطلاعات مودال دیالوگ
                    $.validator.unobtrusive.parse("#dialogContent"); // فعال سازی اعتبارسنجی فرمی که با ایجکس بارگذاری شده                            
                    enablePostbackValidation();
                    // و سپس نمایش آن به صورت مودال
                    $('#dialogDiv').modal({
                        backdrop: 'static', //با کلیک کاربر روی صفحه، صفحه مودال بسته نمی‌شود
                        keyboard: true
                    }, 'show');
                    // تحت نظر قرار دادن این فرم اضافه شده
                    processAjaxForm('#dialogContent');
                }
            }
        });
    };
})(jQuery);
// ]]>

using System.ComponentModel;
using System.ComponentModel.DataAnnotations;

namespace Mvc4TwitterBootStrapTest.Models
{
    public class User
    {
        public int Id { set; get; }

        [DisplayName("نام")]
        [Required(ErrorMessage="لطفا نام را تکمیل کنید")]
        public string Name { set; get; }

        [DisplayName("نام خانوادگی")]
        [Required(ErrorMessage = "لطفا نام خانوادگی را تکمیل کنید")]
        public string LastName { set; get; }
    }
}

using System.Web.Mvc;
using Mvc4TwitterBootStrapTest.Models;

namespace Mvc4TwitterBootStrapTest.Controllers
{
    public class ModalFormAjaxController : Controller
    {
        [HttpGet]
        public ActionResult Index()
        {
            return View(); //نمایش صفحه اولیه
        }

        [HttpPost] //برای این حالت امن‌تر است
        //[AjaxOnly]
        public ActionResult RenderModalPartialView()
        {
            //رندر پارشال ویوو صفحه مودال به همراه اطلاعات مورد نیاز آن
            return PartialView(viewName: "_ModalPartialView", model: new User { Name = "", LastName = "" });
        }

        [HttpPost]
        //[AjaxOnly]
        public ActionResult Index(User user) //ذخیره سازی اطلاعات
        {
            if (this.ModelState.IsValid)
            {
                //todo: SaveChanges;
                return Json(new { success = true });
            }

            this.ModelState.AddModelError("", "خطایی رخ داده است");
            return PartialView("_ModalPartialView", user);
        }
    }
}

@{
    ViewBag.Title = "Index";
    var renderModalPartialViewUrl = Url.Action("RenderModalPartialView", "ModalFormAjax");
    var postDataUrl = Url.Action("Index", "ModalFormAjax");
}
<h2>
    Index</h2>
<a href="#" class="btn btn-primary" id="btnCreate">ثبت اطلاعات</a>

@section JavaScript
{
    <script type="text/javascript">
        $(function () {           
            $('#btnCreate').click(function (e) {
                e.preventDefault(); //می‌خواهیم لینک به صورت معمول عمل نکند

                $.bootstrapModalAjaxForm({
                    postUrl: '@postDataUrl',
                    renderModalPartialViewUrl: '@renderModalPartialViewUrl',
                    renderModalPartialViewData: {},
                    loginUrl: '/login',
                    beforePostHandler: function () {                       
                    },
                    completeHandler: function () {
                        // Refresh: برای حالتیکه نیاز به به روز رسانی کامل صفحه زیرین باشد
                        // location.reload();
                    },
                    errorHandler: function () {
                    }
                });
            });
        });             
    </script>
}

@model Mvc4TwitterBootStrapTest.Models.User
<div class="modal-header">
    <button type="button" class="close" data-dismiss="modal" aria-hidden="true">
        &times;</button>
    <h5>
        افزودن کاربر جدید</h5>
</div>
@using (Html.BeginForm("Index", " ModalFormAjax", FormMethod.Post, new { @class = "modal-form" }))
{
    <div class="modal-body">
        @Html.ValidationSummary(true, null, new { @class = "alert alert-error alert-block" })
        <fieldset class="form-horizontal">
            <legend>مشخصات کاربر</legend>
            <div class="control-group">
                @Html.LabelFor(model => model.Name, new { @class = "control-label" })
                <div class="controls">
                    @Html.EditorFor(model => model.Name)
                    @Html.ValidationMessageFor(model => model.Name, null, new { @class = "help-inline" })
                </div>
            </div>
            <div class="control-group">
                @Html.LabelFor(model => model.LastName, new { @class = "control-label" })
                <div class="controls">
                    @Html.EditorFor(model => model.LastName)
                    @Html.ValidationMessageFor(model => model.LastName, null, new { @class = "help-inline" })
                </div>
            </div>
        </fieldset>
    </div>
        
    <div class="modal-footer">
        <button class="btn btn-primary" type="submit">
            ارسال</button>
        <button class="btn" data-dismiss="modal" aria-hidden="true">
            انصراف</button>
    </div>
}

  [ForeignKey("UserId")]
  public virtual User User { set; get; }
  public int UserId { set; get; }

services.AddOptions<BearerTokensOptions>()
           .Bind(configuration.GetSection("BearerTokens"))
           .Validate(bearerTokens =>
          {
                 return bearerTokens.AccessTokenExpirationMinutes < bearerTokens.RefreshTokenExpirationMinutes;
          }, "RefreshTokenExpirationMinutes is less than AccessTokenExpirationMinutes. Obtaining new tokens using the refresh token should happen only if the access token has expired.");

{
  "ApiSettings": {
    "AllowedEndpoints": [
      {
        "Name": "Service 1",
        "Timeout": 30,
        "Url": "http://service1.site.com"
      },
      {
        "Name": "Service 2",
        "Timeout": 10,
        "Url": "https://service2.site.com"
      }
    ]
  }
}

using System;
using System.Collections.Generic;

namespace FluentValidationSample.Models
{
    public class AllowedEndpoint
    {
        public string Name { get; set; }
        public int Timeout { get; set; }
        public Uri Url { get; set; }
    }

    public class ApiSettings
    {
        public IEnumerable<AllowedEndpoint> AllowedEndpoints { get; set; }
    }
}

namespace FluentValidationSample.Web
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.Configure<ApiSettings>(Configuration.GetSection(nameof(ApiSettings)));

using System;
using System.Linq;
using FluentValidation;
using FluentValidationSample.Models;

namespace FluentValidationSample.ModelsValidations
{
    public class ApiSettingsValidator : AbstractValidator<ApiSettings>
    {
        public ApiSettingsValidator()
        {
            RuleFor(apiSetting => apiSetting).NotNull()
            .WithMessage("مدخل ApiSettings تعریف نشده‌است.");

            RuleFor(apiSetting => apiSetting.AllowedEndpoints).NotNull().NotEmpty()
            .WithMessage("مدخل AllowedEndpoints تعریف نشده‌است.");

            When(apiSetting => apiSetting.AllowedEndpoints != null,
            () =>
                {
                    RuleFor(apiSetting => apiSetting.AllowedEndpoints)
                        .Must(endpoints => endpoints.GroupBy(endpoint => endpoint.Name).Count() == endpoints.Count())
                        .WithMessage("نام‌های سرویس‌ها باید منحصربفرد باشند.");

                    RuleFor(apiSetting => apiSetting.AllowedEndpoints)
                        .Must(endpoints => !endpoints.Any(endpoint => endpoint.Timeout > 90 || endpoint.Timeout < 1))
                        .WithMessage("مقدار timeout باید بین 1 و 90 باشد");

                    RuleFor(apiSetting => apiSetting.AllowedEndpoints)
                        .Must(endpoints => endpoints.GroupBy(endpoint => endpoint.Url.ToString().ToLower()).Count() == endpoints.Count())
                        .WithMessage("آدرس‌های سرویس‌ها باید منحصربفرد باشند.");

                    RuleFor(apiSetting => apiSetting.AllowedEndpoints)
                        .Must(endpoints => endpoints.All(endpoint => endpoint.Url.Scheme.Equals("https", StringComparison.CurrentCultureIgnoreCase)))
                        .WithMessage("تمام آدرس‌ها باید HTTPS باشند.");
                });
        }
    }
}

services.AddOptions<BearerTokensOptions>()
           .Bind(configuration.GetSection("BearerTokens"))
           .Validate(bearerTokens =>
          {
                 return bearerTokens.AccessTokenExpirationMinutes < bearerTokens.RefreshTokenExpirationMinutes;
          }, "RefreshTokenExpirationMinutes is less than AccessTokenExpirationMinutes. Obtaining new tokens using the refresh token should happen only if the access token has expired.");

namespace Microsoft.Extensions.Options
{
    public interface IValidateOptions<TOptions> where TOptions : class
    {
        ValidateOptionsResult Validate(string name, TOptions options);
    }
}

using System.Linq;
using FluentValidation;
using Microsoft.Extensions.Options;

namespace FluentValidationSample.ModelsValidations
{
    public class AppConfigValidator<TOptions> : IValidateOptions<TOptions> where TOptions : class
    {
        private readonly IValidator<TOptions> _validator;

        public AppConfigValidator(IValidator<TOptions> validator)
        {
            _validator = validator;
        }

        public ValidateOptionsResult Validate(string name, TOptions options)
        {
            if (options is null)
            {
                return ValidateOptionsResult.Fail("Configuration object is null.");
            }

            var validationResult = _validator.Validate(options);
            return validationResult.IsValid
                ? ValidateOptionsResult.Success
                : ValidateOptionsResult.Fail(validationResult.Errors.Select(error => error.ToString()));
        }
    }
}

namespace FluentValidationSample.Web
{
    public class Startup
    {
        public void ConfigureServices(IServiceCollection services)
        {
            services.Configure<ApiSettings>(Configuration.GetSection(nameof(ApiSettings)));
            services.AddTransient<IValidateOptions<ApiSettings>, AppConfigValidator<ApiSettings>>();

namespace FluentValidationSample.Web.Controllers
{
    public class HomeController : Controller
    {
        private readonly IUsersService _usersService;
        private readonly ApiSettings _apiSettings;

        public HomeController(IUsersService usersService, IOptions<ApiSettings> apiSettings)
        {
            _usersService = usersService;
            _apiSettings = apiSettings.Value;
        }

An unhandled exception occurred while processing the request.
OptionsValidationException: تمام آدرس‌ها باید HTTPS باشند.

   //ViewModels

    public class CustomListItem
    {
        public int Id { get; set; }
        public string Text { get; set; }
    }

    public class ProductAddGetViewModel
    {
        public IEnumerable<CustomListItem> Categories { get; set; }
        public IEnumerable<CustomListItem> Groups { get; set; }
    }

    public class ProductAddViewModel
    {
        public string Name { get; set; }
        public bool IsActive { get; set; }

        public int CategoryId { get; set; }
        public int GroupId { get; set; }
    }

    public class ProductEditGetViewModel
    {
        public int Id { get; set; }
        public string Name { get; set; }

        public IEnumerable<CustomListItem> Categories { get; set; }
        public IEnumerable<CustomListItem> Groups { get; set; }
    }

    public class ProductEditViewModel
    {
        public int Id { get; set; }
        public string Name { get; set; }

        public int CategoryId { get; set; }
        public int GroupId { get; set; }
    }

public static Task ForEachAsync<TSource>(IEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask> body)
public static Task ForEachAsync<TSource>(IEnumerable<TSource> source, CancellationToken cancellationToken, Func<TSource, CancellationToken, ValueTask> body)
public static Task ForEachAsync<TSource>(IEnumerable<TSource> source, ParallelOptions parallelOptions, Func<TSource, CancellationToken, ValueTask> body)
public static Task ForEachAsync<TSource>(IAsyncEnumerable<TSource> source, Func<TSource, CancellationToken, ValueTask> body)
public static Task ForEachAsync<TSource>(IAsyncEnumerable<TSource> source, CancellationToken cancellationToken, Func<TSource, CancellationToken, ValueTask> body)
public static Task ForEachAsync<TSource>(IAsyncEnumerable<TSource> source, ParallelOptions parallelOptions, Func<TSource, CancellationToken, ValueTask> body)

customers.ForEach(c => SendEmailAsync(c));

foreach(var c in customers)
{
    SendEmailAsync(c); // the return task is ignored
}

customers.ForEach(async c => await SendEmailAsync(c));

foreach(var c in customers)
{
   await SendEmailAsync(c);
}

foreach(var o in orders)
{
    await ProcessOrderAsync(o);
}

var tasks = orders.Select(o => ProcessOrderAsync(o)).ToList();
await Task.WhenAll(tasks);