C# – Check Email Address Formatting

Uses EmailAddressAttribute (uses RegEx) — see Notes.

Compiled in Visual Studio 2017.

using System;
using System.ComponentModel.DataAnnotations;

class CheckEmails
{
static void Main(string[] args)
{
var objEmail = new EmailAddressAttribute();
bool testEmailAddress;
testEmailAddress = objEmail.IsValid("eddie@aol.com");
Console.Write("eddie@aol.com: " + testEmailAddress);

testEmailAddress = objEmail.IsValid("eddie@aol.org");
Console.Write("\n\neddie@aol.org: " + testEmailAddress);

testEmailAddress = objEmail.IsValid("eddie+isme@aol.net");
Console.Write("\n\neddie+isme@aol.net: " + testEmailAddress);

testEmailAddress = objEmail.IsValid("edward@aol.yooo");
Console.Write("\n\nedward@aol.yooo: " + testEmailAddress);

testEmailAddress = objEmail.IsValid("test");
Console.Write("\n\ntest: " + testEmailAddress);

testEmailAddress = objEmail.IsValid("test@");
Console.Write("\n\ntest@: " + testEmailAddress);

testEmailAddress = objEmail.IsValid("test@abcd");
Console.Write("\n\ntest@abcd: " + testEmailAddress);

testEmailAddress = objEmail.IsValid("test@abcd.");
Console.Write("\n\ntest@abcd.: " + testEmailAddress);

// using an if statement
if (new EmailAddressAttribute().IsValid("\n\neddie@aol.com"))
testEmailAddress = true;
Console.Write("\n\neddie@aol.com: " + testEmailAddress);

// wait
Console.ReadKey();
}
}

 

Notes

EmailAddressAttribute

namespace System.ComponentModel.DataAnnotations {
using System;
using System.ComponentModel.DataAnnotations.Resources;
using System.Text.RegularExpressions;

[AttributeUsage(AttributeTargets.Property | AttributeTargets.Field | AttributeTargets.Parameter, AllowMultiple = false)]
public sealed class EmailAddressAttribute : DataTypeAttribute {

// This attribute provides server-side email validation equivalent to jquery validate,
// and therefore shares the same regular expression.  See unit tests for examples.
private static Regex _regex = CreateRegEx();

public EmailAddressAttribute()
: base(DataType.EmailAddress) {

// DevDiv 468241: set DefaultErrorMessage not ErrorMessage, allowing user to set
// ErrorMessageResourceType and ErrorMessageResourceName to use localized messages.
DefaultErrorMessage = DataAnnotationsResources.EmailAddressAttribute_Invalid;
}

public override bool IsValid(object value) {
if (value == null) {
return true;
}

string valueAsString = value as string;

// Use RegEx implementation if it has been created, otherwise use a non RegEx version.
if (_regex != null) {
return valueAsString != null && _regex.Match(valueAsString).Length > 0;
}
else {
int atCount = 0;

foreach (char c in valueAsString) {
if (c == '@') {
atCount++;
}
}

return (valueAsString != null
&& atCount == 1
&& valueAsString[0] != '@'
&& valueAsString[valueAsString.Length - 1] != '@');
}
}

private static Regex CreateRegEx() {
// We only need to create the RegEx if this switch is enabled.
if (AppSettings.DisableRegEx) {
return null;
}

const string pattern = @"^((([a-z]|\d|[!#\$%&'\*\+\-\/=\?\^_`{\|}~]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])+(\.([a-z]|\d|[!#\$%&'\*\+\-\/=\?\^_`{\|}~]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])+)*)|((\x22)((((\x20|\x09)*(\x0d\x0a))?(\x20|\x09)+)?(([\x01-\x08\x0b\x0c\x0e-\x1f\x7f]|\x21|[\x23-\x5b]|[\x5d-\x7e]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(\\([\x01-\x09\x0b\x0c\x0d-\x7f]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF]))))*(((\x20|\x09)*(\x0d\x0a))?(\x20|\x09)+)?(\x22)))@((([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])*([a-z]|\d|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])))\.)+(([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])|(([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])([a-z]|\d|-|\.|_|~|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])*([a-z]|[\u00A0-\uD7FF\uF900-\uFDCF\uFDF0-\uFFEF])))\.?$";
const RegexOptions options = RegexOptions.Compiled | RegexOptions.IgnoreCase | RegexOptions.ExplicitCapture;

// Set explicit regex match timeout, sufficient enough for email parsing
// Unless the global REGEX_DEFAULT_MATCH_TIMEOUT is already set
TimeSpan matchTimeout = TimeSpan.FromSeconds(2);

try {
if (AppDomain.CurrentDomain.GetData("REGEX_DEFAULT_MATCH_TIMEOUT") == null) {
return new Regex(pattern, options, matchTimeout);
}
}
catch {
// Fallback on error
}

// Legacy fallback (without explicit match timeout)
return new Regex(pattern, options);
}
}
}