Custom exception objects in .Net:

In order to make a custom exception we have to derive the System.Exception class. We can

derive the custom exception directly from the System.Exception class or can use an

intermediate exception like SystemException or ApplicationException as base class.

Note: According to .NET framework documentation, SystemException is meant only for those

exceptions defined by the common language runtime whereas ApplicationException is intended

to be used by user code as ApplicationException is thrown by a user program, not by the

common language runtime.

For Example:- Let us create a custom exception for the login mechanism :

The basic structure looks like:-

public class LoginFailedException: System.Exception
{

}

C# doesnt inherit constructors of base classes,So we need to add at least one constructor

which does something useful:

public class LoginFailedException: System.Exception
{

   public LoginFailedException()
   {

   }

   public LoginFailedException(string message): base(message)
   {

   }
}

Now we can throw an instance of LoginFailedException and pass a message which describes

the occurred error.

Incase of Serialization: We need to implement the ISerializable interface (Since our base Exception class already implements the ISerializable interface we can omit it in our base classes),this interface contains a single

method, named GetObjectData, which is responsible for storing object fields during the

serialization process. Now when an object is being deserialized, the previously filled

SerializationInfo object is passed to the constructor of the class in which we are

responsible for restoring the original object fields.

Example:

[Serializable]
public class LoginFailedException: System.Exception
{
   private string fUserName;

   protected LoginFailedException(SerializationInfo info,StreamingContext context): base

(info, context)
   {
      if (info != null)
      {
         this.fUserName = info.GetString("fUserName");
      }
   }

   public override void GetObjectData(SerializationInfo info,StreamingContext context)
   {
      base.GetObjectData(info, context);

      if (info != null)
      {
         info.AddValue("fUserName", this.fUserName);
      }
   }
}