Expression Trees - Accessing Nested Properties
In this wiki, let’s see how we can access nested properties in Expression Trees. If you are new to Expression Trees, go and have a look at Writing a Very Basic Dynamic Query using Expression Trees. There you can have a good understanding on Expression Trees basics and after reading that particular wiki completely, you should be able to write a basic Expression Tree which accesses the first level properties of a class. And in this wiki, let's learn how to write an expression tree which accesses the nested properties of a class.
Let’s consider the following scenario. We have two classes named “Employee” and “Department”. Employee is belonging to a department and we can model the relationship as follows.
public class Employee
{
public int EmployeeId { get; set; }
public string Name { get; set; }
public virtual Department Department { get; set; }
public static List<Employee> GetEmployees()
{
return new List<Employee>()
{
new Employee()
{
EmployeeId=1,
Name="Jaliya Udagedara",
Department = Department.GetDepartments().First(d=>d.DepartmentId==1)
},
new Employee()
{
EmployeeId=1,
Name="John Smith",
Department = Department.GetDepartments().First(d=>d.DepartmentId==2)
},
new Employee()
{
EmployeeId=1,
Name="Jane Smith",
Department = Department.GetDepartments().First(d=>d.DepartmentId==1)
}
};
}
}
public class Department
{
public int DepartmentId { get; set; }
public string Name { get; set; }
public static List<Department> GetDepartments()
{
return new List<Department>()
{
new Department(){DepartmentId=1,Name="Microsoft Visual Studio"},
new Department(){DepartmentId=2,Name="Microsoft SQL Server"}
};
}
}
Inside the classes, there are two helper methods which will return some data. Now let’s say we want to get all the employees who is working in department 1. We can write the following LINQ query.
IEnumerable<Employee> employees = Employee.GetEmployees().Where(e => e.Department.DepartmentId == 1);
Now how we can we write the same in using expression trees. Let’s find out.
First step is to create the expression tree that represents the parameter to the predicate which is “e”.
ParameterExpression pe = Expression.Parameter(typeof(Employee), "e");
Now comes the place where we need to access the nested property.
Expression left = Expression.Property(pe, typeof(Employee).GetProperty("Department"));
left = Expression.Property(left, typeof(Department).GetProperty("DepartmentId"));
In above two lines, first line is for accessing the Department property of the Employee. Then in the second line we are accessing the property DepartmentId and note that what has been passed as the first parameter of Expression.Property() method. It’s the Department property of the Employee. So what means here is we are accessing the DepartmentId property of the Department. Now you must be getting how it works, by following the above approach, we can access any property in any level.
Then it’s pretty much a simple task ahead. We have to compose the right side of the expression, compile the expression tree into executable code and finally pass it to Where extension method. For that, let's write the following.
Expression right = Expression.Constant(1, typeof(int));
Expression expression = Expression.Equal(left, right);
var lambda = Expression.Lambda<Func<Employee, bool>>(expression, pe).Compile();
IEnumerable<Employee> employees = Employee.GetEmployees().Where(lambda);
So the following is how it looks likes when all above parts comes together.
ParameterExpression pe = Expression.Parameter(typeof(Employee), "e");
Expression left = Expression.Property(pe, typeof(Employee).GetProperty("Department"));
left = Expression.Property(left, typeof(Department).GetProperty("DepartmentId"));
Expression right = Expression.Constant(1, typeof(int));
Expression expression = Expression.Equal(left, right);
var lambda = Expression.Lambda<Func<Employee, bool>>(expression, pe).Compile();
IEnumerable<Employee> employees = Employee.GetEmployees().Where(lambda);
And this is the output when we foreach on “employees”.
| http://lh4.ggpht.com/-Hl91iLkI2Do/U7ZHHUvwN-I/AAAAAAAACfs/8vNixWLt8hU/image_thumb%25255B4%25255D.png?imgmax=800 |
| Result |
You can download the full sample from MSDN Code Gallery, Enjoy!
Happy Coding.