如何拆分文本框值并检查它是c＃中的电子邮件ID还是纯文本

Question

I have a text box(txt_to) which has values seperated by ; How can I split the values, and even if there is only one value the code should not through error.

I tried to split it with the following code but got an error

string[] recipients = Request.Form["txt_to"].Split(';');
Object reference not set to an instance of an object.

The values in the text box are either a email like (abc@aaa.com) or just a plain text like (Ios Group). the values in the text box are like abc@aaa.com; Ios Group. How to split the value and how to check whether it is email id or just a plain text

if(email id)
{
Do this
}
else if (Plain text)
{
Do this
}

Answer 1

RFC 2822 states that you can validate an e-mail with the following regular expression:

[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?

So you can do:

if (Regex.IsMatch(input, **REGEX STRING HERE**))
{
    // E-mail
}
else
{
    // Not an e-mail
}

Answer 2

string.Split doesn't throw an error when you split a string using a character that is not present in the string.

Example:

string test =  "jiberish";

string [] result = test.Split(';');
Console.WriteLine(result[0]);   

Won't blow. If you get a null reference exception is because Request.Form["txt_to"] is null

If you want to validate email addresses, use a Regular expression.

Something like:

if(Regex.IsMatch("\b[A-Z0-9._%-]+@[A-Z0-9.-]+\.[A-Z]{2,4}\b",yourstring))
{
   //valid email
}
else
{
   //not valid
}

Note: above regular expression may not be the most appropriate. I used it to give you an idea.

Answer 3

You should be able to use aa RegularExpressionValidator for this:

<asp:RegularExpressionValidator ID="regEmail" runat="server" Display="Dynamic" ErrorMessage="*" ValidationExpression="^[\w\.\-]+@[a-zA-Z0-9\-]+(\.[a-zA-Z0-9\-]{1,})*(\.[a-zA-Z]{2,3}){1,2}$" ControlToValidate="txtEmail" />                                    

Once implemented, you can verify that the field(s) are valid in code-behind like this:

bool isValid = Page.IsValid;

If you don't want to use validators, you can just use an expression in code-behind to validate the email address like this:

var regex = new Regex(@"[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?");
bool isValid = regex.IsMatch(txtEmail.Text);

Here are a couple of expressions you can try out:

[a-z0-9!#$%&'*+/=?^_`{|}~-]+(?:\.[a-z0-9!#$%&'*+/=?^_`{|}~-]+)*@(?:[a-z0-9](?:[a-z0-9-]*[a-z0-9])?\.)+[a-z0-9](?:[a-z0-9-]*[a-z0-9])?

^[\w\.\-]+@[a-zA-Z0-9\-]+(\.[a-zA-Z0-9\-]{1,})*(\.[a-zA-Z]{2,3}){1,2}$

Answer 4

You can simply do :

if the string contains an "@" than try to build a MailAdress object (in a try/catch) if it doesn't it's just plain text :)

if(!string.IsNullOrEmpty(textToTest){
    try{
       mail = new MailAdress(textToTest);
    }catch(Exception e){
       plainText = textToTest
    }
}

EDIT : Update in order to take into account comments :)