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There really isn't a completely reliable way. The checks you can do are these, in rough order of difficulty...
a) Check that the email address is in the correct format.
b) Parse out the server and see if you can do a DNS lookup on it.
c) Connect to the SMTP server on port 25 at the server name given in the mail address. On connection you should receive the following:
"220 smtpservername.com ESMTP"
For a further check send "HELO yourservername.com"
You should receive "250 smtpservername.com"
At this point, you at least know the user wasn't lying about the presence of an SMTP server at that address. Now you actually can ask the server whether the user exists without sending any mail. You would do it like this:
You send "MAIL FROM:<you...yourservename.com>"
And receive back "250 OK"
You send "RCPT TO:<user...smtpservername.com>"
And receive back "250 OK"
You now know that the user address exists on the server, and that the server is ready to receive mail for that user. There are various errors that might occur instead. "550 No such user" indicates the address is bad. "551 User has moved try <anotherservername.com>" indicates that the user used to have an address, but no longer does.
In reality servers behave in different ways, so you really can't infer too much. Either 550 or 551 essentially means "no such user." Additionally servers may return other errors, based on their state at the time, so if you're going to do this in a robust manner you need to handle all the possible responses. Few sites go to the trouble of implementing all this.
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It always give me "250 OK"
on RCPT to:
even if the email is not correct ("fdfdfd@fjdkfjkdfjdk.cklclkdl")
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I am Using the Calibri Fonts in Crystal Report, it is comming correctly in Crystal Viewer But when i export it in PDF. Some characters are comming wrong. How i can fix this problem when i export the Report in PDF.Any kind of suggestion is highly appreciated. for eg party is showing as partty. Please help.
Sebastian
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This is not really a C# question.
See here - maybe this can help you.
My signature "sucks" today
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Hello,
How can i search for an outlook contact using c# ?
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OK,
But how can i find a contact in the exchange server?
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michaelgr1 wrote: But how can i find a contact in the exchange server?
I'd recommend the examples on MSDN[^]. There's a list of common programming tasks in the content-treeview on the left side of the page.
I are Troll 
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Hi Experts
i working With VS 2005.
i am reading Some Article from internet about C#.NET
they use ? in passing Parameter in method can u explain what is the use
of ? inside the method
e.g
private void Abc(int? a,int? b)
{
}
and it also working what i am not understand why use the ? inside the
method
Dinesh Sharma
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These are Nullable types[^], which can be represented by null values (i.e. they have no value) as well as the normal range of valid values for the type. For an int type that represents the range from the smallest negative to the largest positive value and also null; but note that the null value is not the same as zero.
It's time for a new signature.
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Sharma Dinesh Kumar wrote: private void Abc(int? a,int? b)
Declaration int? is equivalent to System.Nullable <int > a.
Nullable types allow to store even null value to a value type but nullable variable.Read MS guidelines about this these types ^.
Life is a stage and we are all actors!
modified on Tuesday, May 25, 2010 4:04 AM
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thank u for answare
Can i also Declare DateTime? as nullable so i can save null in database in sql server 2005 if date is blank.
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Sharma Dinesh Kumar wrote: Can i also Declare DateTime
According to the MS documentation yes.
Sharma Dinesh Kumar wrote: as nullable so i can save null in database in sql server 2005 if date is blank.
I suppose not.SQL NULL value matches to System.DBNull.Value not to C# null reference.So you should replace null with System.DBNull.Value when passing commands to SQL Server like the sample bellow.
void Add(int? value)
{
command.Parameters.Add("@foo").Value = (object)value ?? DBNull.Value;
}
Life is a stage and we are all actors!
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Well, to my knowledge you don't have to. Using System.DBNull.Value is required when you retrieve data from nullable column in database. But if you pass parameter through SqlCommand.Parameters, .NET does some magic and sends data, that SQL server understands as DBNull value.
However, when you retrieve data from database, the value in SqlDataReader is DBNull.Value, and it's of course not the same as null in C#.
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lukasz_nowakowski wrote: .NET does some magic and sends data, that SQL server understands as DBNull value.
Very interesting.Are you sure that it's universal and works with older .NET versions like 1.1 .
Life is a stage and we are all actors!
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Well, I have no idea. I never developed in .NET 1.1. But you can try and pass null to some procedure and see what happens. I don't see why shouldn't it work...
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Official ADO.NET documentation here said:
When you send a null parameter value to the server, the user must specify DBNull, not null.
So automatic conversion from null to DBNull.Value isn't even documented.
Life is a stage and we are all actors!
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That's very very strange. I don't remember using DBNull... and I have procedures (mostly for searching tables), that take some parameters, that can be null, and they work. Don't know why. But when I think about it, I guess maybe .NET just doesn't send parameter, that has a value of null and sends parameter that has DBNull. Well, I wouldn't find out, cause I always interact with database with stored procedures and this feature doesn't affect me. But if you use prepared commands (or what they call them), it can be an issue. Maybe someone has some free time to check it (cause I'm quite busy right now). If not, I'll try to check it later today and write what I found out here.
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? is used for nullable types when used like this.
This means a null or an integer value can be passed into the method.
My signature "sucks" today
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i have a login form in my windows application, i want to persist user name value when user login into the application back.
so How to persist user name value in textbox in windows application like web application?
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There are mainly two ways to do so.
1. You can store user name in app.config file programmatically and then
you can access it from there whenever you required.
2. You can store it in custom user class object and access it from the
object itself whenever you require it.
[Either you make property or variable in custom class to store the value]
HTH
Jinal Desai - LIVE
Experience is mother of sage....
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Hi,
I want to build REST based services and for that would like to use Web Project template in my Visual Studio. How can i add that any options need to be enabled ?
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I have a hierarchy of classes, all of which implement this interface:
public class IRuleObject
{
int Sequence { get; set; }
}
Here's the top level class:
[Serializable]
[XmlRoot("Group")]
public class RuleGroup : IRuleObject
{
[XmlAttribute("ID")]
public int GroupID { get; set; }
[XmlAttribute("Sequence")]
public int Sequence { get; set; }
[XmlAttribute("Name")]
public string GroupName { get; set; }
[XmlAttribute("Active")]
public bool Active { get; set; }
[XmlElement("Rules")]
public List<Rule> Rules { get; set; }
}
You can see where it implements the interface.
When I attempt to serialize I get
Type 'RulesEngine.IRuleObject' in Assembly 'RulesEngine, Version=1.0.0.0, Culture=neutral, PublicKeyToken=null' is not marked as serializable.
If I remove the interface, all works well. How do I set this up so i can use the interface & still serialize the
classes?
Everything makes sense in someone's mind
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I'm just curious as I'm not sure this is the problem or if this will work, but did you try adding the Serializable keyword to the interface?
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