.cs

public class CustomersController 
{
  private readonly IQueryable<Customer> customersQuery;
  
  public CustomersController(IQueryable<Customer> customersQuery)
  {
    this.customersQuery = customersQuery;
  }
  
  public Customer Get(int id)
  {
    try
    {
      return customerQuery.Single(customer => customer.Id = id);
    }
    catch(InvalidOperationException e)
    {
      return NotFound();
    }
  }
}