How to access a property which is a class using Expressions

Question

I have a class called "User" which has a property defined as a class "Department" (i also want to know what is it called, LOL). I want to access the property in the "Department" class called "Name" (User.Department.Name). It throws a NulLReferenceException when I input the arguement into the compiled expression.

I have this code below. Thanks in advance.

        ParameterExpression paramExpr = Expression.Parameter(typeof(User), "user");
        MemberExpression depPropExpr = MemberExpression.Property(paramExpr, "Department");
        MemberExpression depNamePropExpr = MemberExpression.Property(depPropExpr, "Name");
        ConstantExpression constantExpression = Expression.Constant("SBCA");

        var expression = Expression.Assign(depNamePropExpr, constantExpression); var compiledExpression = Expression.Lambda<Action<User>>(expression, new[] { paramExpr }).Compile();
        compiledExpression(user);

Answer 1

As you can see in this .NET Fiddle your code works fine.

public class Program
{

    public static void Main()
    {
        var user = new User();
        user.Department = new Department();
        user.Department.Name = "hello";
        Console.WriteLine("Before: " + user.Department.Name);

        ParameterExpression paramExpr = Expression.Parameter(typeof(User), "user");
        MemberExpression depPropExpr = MemberExpression.Property(paramExpr, "Department");
        MemberExpression depNamePropExpr = MemberExpression.Property(depPropExpr, "Name");
        ConstantExpression constantExpression = Expression.Constant("SBCA");

        var expression = Expression.Assign(depNamePropExpr, constantExpression);

        var compiledExpression = Expression.Lambda<Action<User>>(expression, new[] { paramExpr }).Compile();
        compiledExpression(user);
        Console.WriteLine("After: " + user.Department.Name);
    }
}

public class User
{
    public Department Department { get; set; }
}

public class Department
{
    public string Name { get; set; }    
}

Output:

Before: hello
After: SBCA

The problem (the NullReferenceException) is that user.Department is null, where your expression is doing user.Department.Name = "SBCA";.
You can test this by removing the assignment of user.Department = new...