Append to a Func in LINQ - linq

Question

I have a method with a projection
public IQueryable<EmpDTO> GetEmployee(Func<Employee, EmpDTO> projection = null)
{
if(projection == null)
projection = emp => new EmpDTO {
Id = emp.Id,
Name = emp.Name,
Salary = emp.Salary,
};
return entities.Employees.Where(e => e.Salary > 10000).Select(projection);
}
It can be extended as follows:
query = classInstance.GetEmployee(emp => new EmpDTO {
Id = emp.Id,
Name = emp.Name,
Salary = emp.Salary,
Address = emp.Address
});
How can I APPEND only the "Address" field to the Func without
rewriting the entire fields (repeated fields Id, Name, Salary)

Answer 1

Using Expression, you can build a new lambda to initialize the fields:
public IQueryable<EmpDTO> GetEmployee(Expression<Func<Employee, EmpDTO>> addProj = null) {
Expression<Func<Employee, EmpDTO>> projection = emp => new EmpDTO {
Id = emp.Id,
Name = emp.Name,
Salary = emp.Salary,
};
if (addProj != null) {
var pBody = ((MemberInitExpression)projection.Body);
var newBindings = new ReadOnlyCollection<MemberBinding>(pBody.Bindings.Concat(((MemberInitExpression)addProj.Body).Bindings).ToList());
var newBody = Expression.MemberInit(pBody.NewExpression, newBindings);
projection = (Expression<Func<Employee, EmpDTO>>) Expression.Lambda(newBody, projection.Parameters);
}
return entities.Employees.Where(e => e.Salary > 10000).Select(projection);
}
You could also build the entire lambda from scratch, but that seemed like more work to me. Plus you can encapsulate the combine init logic into an extension method:
public static Expression<Func<TIn, TOut>> Add<TIn, TOut>(this Expression<Func<TIn, TOut>> proj, Expression<Func<TIn, TOut>> addProj = null) {
if (addProj != null) {
var pBody = ((MemberInitExpression)proj.Body);
var newBindings = new ReadOnlyCollection<MemberBinding>(pBody.Bindings.Concat(((MemberInitExpression)addProj.Body).Bindings).ToList());
var newBody = Expression.MemberInit(pBody.NewExpression, newBindings);
proj = (Expression<Func<TIn, TOut>>)Expression.Lambda(newBody, proj.Parameters);
}
return proj;
}
which reduces the GetEmployee body to:
public IQueryable<EmpDTO> GetEmployee(Expression<Func<Employee, EmpDTO>> addProj = null) {
Expression<Func<Employee, EmpDTO>> projection = emp => new EmpDTO {
Id = emp.Id,
Name = emp.Name,
Salary = emp.Salary,
};
if (addProj != null)
projection = projection.Add(addProj);
return entities.Employees.Where(e => e.Salary > 10000).Select(projection);
}