public?static?bool?IsEmailAddress(string?email)
{
????if?(string.IsNullOrWhiteSpace(email))
????????return?false;

????var?symbolIndex?=?email.IndexOf('@');
????return?symbolIndex?>?0
????????&&?symbolIndex?<?email.Length?-?1
????????&&?symbolIndex?==?email.LastIndexOf('@');
}

在之前的認知里，一般判斷郵箱格式都是用一個正則表達式，有時候正則表達式還可能會特別復雜，在老的 .NET framework 中 EmailAddress 的判斷使用的是一個很復雜的一個正則表達式

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])))\.?$";

可以參考：https://referencesource.microsoft.com/#System.ComponentModel.DataAnnotations/DataAnnotations/EmailAddressAttribute.cs,54

而在 .NET Core 中就比較簡單了，沒有用到正則，前面的實現(xiàn)也是來自于 .NET Core EmailAddressAttribute 的實現(xiàn)，實現(xiàn)如下：

public?sealed?class?EmailAddressAttribute?:?DataTypeAttribute
{
????public?EmailAddressAttribute()
????????:?base(DataType.EmailAddress)
????????{
????????????//?Set?DefaultErrorMessage?not?ErrrorMessage,?allowing?user?to?set
????????????//?ErrorMessageResourceType?and?ErrorMessageResourceName?to?use?localized?messages.
????????????DefaultErrorMessage?=?SR.EmailAddressAttribute_Invalid;
????????}

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

????????if?(!(value?is?string?valueAsString))
????????{
????????????return?false;
????????}

????????//?only?return?true?if?there?is?only?1?'@'?character
????????//?and?it?is?neither?the?first?nor?the?last?character
????????bool?found?=?false;
????????for?(int?i?=?0;?i?<?valueAsString.Length;?i++)
????????{
????????????if?(valueAsString[i]?==?'@')
????????????{
????????????????if?(found?||?i?==?0?||?i?==?valueAsString.Length?-?1)
????????????????{
????????????????????return?false;
????????????????}
????????????????found?=?true;
????????????}
????????}
????????return?found;
????}
}

通過這種方式，我們可以提高判斷郵箱格式的性能又不必維護正則表達式了?？偨Y(jié)：有且僅有一個@并且前后都有字符

More

有一點需要注意，在上面的 EmailAddressAttribute 的實現(xiàn)中，如果值是 null 也會認為是“合法”的，這里的“合法”并不是說郵箱格式合法而是說驗證可以通過，實際情況下一般我們是會認為這并不是一個合法的郵箱

References

https://github.com/dotnet/runtime/blob/main/src/libraries/System.ComponentModel.Annotations/src/System/ComponentModel/DataAnnotations/EmailAddressAttribute.cs
https://github.com/dotnet/runtime/blob/v5.0.0/src/libraries/System.ComponentModel.Annotations/src/System/ComponentModel/DataAnnotations/EmailAddressAttribute.cs
https://github.com/dotnet/corefx/blob/v3.0.0/src/System.ComponentModel.Annotations/src/System/ComponentModel/DataAnnotations/EmailAddressAttribute.cs
https://github.com/dotnet/corefx/blob/v2.2.0/src/System.ComponentModel.Annotations/src/System/ComponentModel/DataAnnotations/EmailAddressAttribute.cs
https://github.com/dotnet/corefx/blob/v2.0.0/src/System.ComponentModel.Annotations/src/System/ComponentModel/DataAnnotations/EmailAddressAttribute.cs