How do I reuse an Expression on a single object in another Expression

Question

I feel like I am missing something simple, but I have not found the documentation that answers my question.

I have recently been decomposing some of the linq projection

Answer 1

You can use LINQKit to expand the expressions that you have within other expressions:

private static Expression<Func<Department, DepartmentDto>> CreateDepartmentDtoUnexpanded = d => new DepartmentDto
{
    Manager = CreatePersonDto.Invoke(d.Manager),
    Employees = d.Employees.Select(employee => CreatePersonDto.Invoke(employee))
        .ToList(),
};
public static Expression<Func<Department, DepartmentDto>> CreateDepartmentDto = CreateDepartmentDtoUnexpanded.Expand();

Answer 2

You can:

public static Expression<Func<Department, DepartmentDto>> CreateDepartmentDto = d =>
     new DepartmentDto
        {
            Manager = CreatePersonDto.Compile()(d.Manager),
            Employees = d.Employees.Select(CreatePersonDto.Compile()).ToList() // Or declare Employees as IEnumerable and avoid ToList conversion
        };

Clearly, instead of calling two times Compile method you can store the compiled Func