Example: Create visitor group criteria

Shows how to develop a criteria for categorizing visitors according to whether a specific cookie exists in the session.

(Thanks to Ted & Gustaf for this example.)

One usage scenario could be to check for a cookie that says that the visitor has previously completed a purchase on the website and is a returning customer.

Create the settings and criterion classes

Visitor group criteria require a settings class and a criterion class.

The settings class is used to persist any settings available to editors when creating visitor groups.
The criterion class contains the logic to determine whether or not a visitor belongs to the visitor group.

You should create the two classes in a suitable folder structure:

Implement the settings class

The only setting required for this visitor group criterion is the cookie name you should look for.

First, make the CookieExistsCriterionSettings class inherits the CriterionModelBase class:
```
public class CookieExistsCriterionSettings : CriterionModelBase
```
Add a public property to let editors specify the cookie name:
```
[Required]
public string CookieName { 
  get; 
  set; 
}
```
If you need custom validation logic when you save a criterion, you can make your settings class implement the IValidateCriterionModel interface (this is optional). By implementing that interface’s Validate method, you can customize how you validate settings before saving a criterion for a visitor group.

The abstract CriterionModelBase class requires you to implement the Copy() method. Because you are not using complex reference types, you can implement it by returning a shallow copy as shown (see Create custom visitor group criteria):
```
public override ICriterionModel Copy() {
  return ShallowCopy();
}
```
The following code shows the complete CookieExistsCriterionSettings class:
```
public class CookieExistsCriterionSettings: CriterionModelBase {
  [Required]
  public string CookieName {
    get;
    set;
  }

  public override ICriterionModel Copy() {
    return ShallowCopy();
  }
}
```

Implement the criterion class

Make the CookieExistsCriterionclass inherit the abstract CriterionBaseclass with the settings class as the type parameter:
```
public class CookieExistsCriterion : CriterionBase<CookieExistsCriterionSettings>
```
Add a VisitorGroupCriterion attribute to set the category, name, and description of your criterion (for more available VisitorGroupCriterion properties, see Create custom visitor group criteria:
```
[VisitorGroupCriterion(   
   Category = "Technical",
   DisplayName = "Cookie Exists",
   Description = "Checks if a specific cookie exists")]
```

The abstract CriterionBase class requires you to implement an IsMatch() method, which determines whether the current user matches this visitor group criterion. You should check if a specific cookie exists based on the criterion settings:

public override bool IsMatch(IPrincipal principal, HttpContextBase httpContext) {
  return httpContext.Request.Cookies[Model.CookieName] != null;
}

The criterion class displays as follows:

[VisitorGroupCriterion(
  Category = "Technical",
  DisplayName = "Cookie Exists",
  Description = "Checks if a specific cookie exists")]

public class CookieExistsCriterion: CriterionBase<CookieExistsCriterionSettings> {
  public override bool IsMatch(IPrincipal principal, HttpContextBase httpContext) {
    return httpContext.Request.Cookies[Model.CookieName] != null;
  }
}