CRM Plug-in C#, How to set lookup field according to another lookup field in post-event?

Question

I have a custom entity student, each student has a department, and each department belongs to a campus (department and campus are lookup fields).

What I am trying to do is to create a new account and choose a department for him.

Then the plugin changes the campus according to the selected department.

This is my code, could someone explain to me which steps I need to do.

        var context = serviceProvider.GetService(typeof(IPluginExecutionContext)) as IPluginExecutionContext;

        Entity student = context.InputParameters["Target"] as Entity;
        string Department = string.Empty;

        if (student.Contains("karam_department"))
        {
            Department = student.GetAttributeValue<>("karam_department");
        }

Answer 1

I would recommend you to do this in pre-event of create/update, hence you can set the campus attribute in target itself, this will avoid another service.Update . You have to just query the corresponding campus from the selected department, then set in target Entity.

var context = serviceProvider.GetService(typeof(IPluginExecutionContext)) as IPluginExecutionContext;

Entity student = context.InputParameters["Target"] as Entity;

//get the department from target
EntityReference department = student.GetAttributeValue<EntityReference>("karam_department");

if (department != null)
{
//retrieve the campus from department
Entity deptEntity = service.Retrieve("karam_department", Department.Id, new ColumnSet("karam_campus"));

//set the campus attribute in target itself
student["karam_campus"] = deptEntity.GetAttributeValue<EntityReference>("karam_campus");

}