A permission filter in .NET is a type of filter that can be used to restrict access to resources based on a user's permissions. Permission filters are implemented as attribute classes that can be applied to controllers, actions, or razor pages.

To use a permission filter, you first need to create a custom attribute class that implements the IAuthorizationFilter interface. This interface defines a single method, OnAuthorization(), which is called by the ASP.NET Core framework to determine whether the user is authorized to access the resource.

In the OnAuthorization() method, you can check the user's permissions to determine whether they are allowed to access the resource. If the user is not authorized, you can short-circuit the filter pipeline by calling the Context.Result property and setting it to a ForbidResult object.

Here is an example of a custom permission filter attribute:

C#

public class PermissionFilterAttribute : Attribute, IAuthorizationFilter
{
    private readonly string _permissionName;

    public PermissionFilterAttribute(string permissionName)
    {
        _permissionName = permissionName;
    }

    public void OnAuthorization(AuthorizationFilterContext context)
    {
        if (!context.User.HasPermission(_permissionName))
        {
            context.Result = new ForbidResult();
        }
    }
}

To use the PermissionFilterAttribute attribute, you can simply apply it to the controller, action, or razor page that you want to protect. For example:

C#

[PermissionFilter("CanManageUsers")]
public class UsersController : Controller
{
    // ...
}

Conclusion

This will ensure that only users who have the CanManageUsers permission will be able to access the UsersController class.

You can also use permission filters to create more complex authorization policies. For example, you could create a permission filter that checks whether the user is a member of a specific role. Or, you could create a permission filter that checks whether the user has a specific claim in their identity token.

Permission filters are a powerful way to implement role-based access control (RBAC) in your ASP.NET Core applications. By using permission filters, you can ensure that only authorized users have access to your resources.