How to choose a class whose method to call

Question

Hi all,

 

I would appreciate a hand in designing my code. So far I haven't found a way to do this.

 

I have a number of classes for different entities (currently 7 but there can be more in the future). I need a method in each of these classes that authorizes user's access to a file resource. Implementation varies but an exception must be thrown if user does not have access to the file in question.

 

Problem is, when a file is being accessed, I need to call the authorization method of the correct class based on the data on the file. Files are stored in a db table and one of the fields in that table determines which entity the file is related to and how to find which user has access to the file.

 

So I must read the file data from db, then see which entity it's related to and then call the authorization method of the correct class.

 

Is there a way in C# to accomplish this without having to make a switch case structure on the file field value?

 

Example of the classes used:

public class FileAttachment
{
    // other FileAttachment class stuff

    protected void AuthorizeFileAccess(FileAttachmentData file)
    {
        // file has a field which tells which entity it relates to
        // here I need to choose which class to use based on the file data
    }
}


// simplified example of a related entity
public class ChatMessage
{
    public int SenderUserId;
    public int RecipientUserId;

    // here I need a method that checks that chat message sender's or recipient's
    // user id matches the user id of the logged in user
    // if not, throw exception
}

And this is what I want to avoid:
switch (file.SourceValue)
{
    case "ChatMessageAttachmentFile":
        var cm = new ChatMessage();
        cm.AuthorizeAttachmentAccess();
    case "OtherAttachmentFile":
        var other = new SomeOtherClass();
        other.AuthorizeAttachmentAccess();
    default:
        throw new Exception("Access denied");
}