Implement the Select for a custom LINQ provider - linq

Question

I understand pretty the way implementing Linq to something however there is a little problem that I encounter here is the situation, I'm implementing a Linq to Tree object the tree it self is simple Tree implementation so I started by creating the Queryable object
public class TreeContext<T> : IOrderedQueryable where T :IComparable<T>
{
private Tree<T> _root;
private Expression _expression;
private IQueryProvider _provider;
public TreeContext(Tree<T> root)
{
_root = root;
Provider = new TreeProvider<T>(_root);
}
public TreeContext(IQueryProvider provider, Expression expression)
{
Provider = provider;
Expression = expression;
}
#region Model
public IEnumerable<TResult> Select<TResult>(Func<T,TResult> predicate)
{
var param = predicate.Method.GetParameters()[0];
return null;
}
public IEnumerable<T> Where(Func<T, bool> predicate)
{
Provider.Execute<IEnumerable>(Expression);
return null;
}
#endregion
public Expression Expression
{
private set { _expression = value; }
get { return _expression; }
}
public Type ElementType => typeof(T);
public IQueryProvider Provider
{
private set { _provider = value; }
get { return _provider; }
}
public IEnumerator<T> GetEnumerator()
=> GetEnumerator();
IEnumerator IEnumerable.GetEnumerator()
=>Provider.Execute<IEnumerable>(Expression).GetEnumerator();
}
Then I implemented the QueryProvider and here resides the problem, exactly at the select method implementation
public class TreeProvider<T> : IQueryProvider where T:IComparable<T>
{
private Tree<T> _root;
public TreeProvider(Tree<T> root)
=> _root = root;
public IQueryable CreateQuery(Expression expression)
=> new TreeContext<T>(this, expression);
public IQueryable<TElement> CreateQuery<TElement>(Expression expression)
=>throw new NotImplementedException();
public object Execute(Expression expression)
=> new TreeQueryContext<T>(_root).Execute(expression);
public TResult Execute<TResult>(Expression expression)
{
throw new NotImplementedException();
}
}
The select implementation is not provided for this custom provider so that I can write a request like
from el in context select el
I added a method select
IEnumerable Select<T,TResult>(Func<T,TResult> selector)
The problem is that I didn't find a way the transform this selector to an expression so that I pass it to the execute method of the QueryProvider
Maybe I didn't understand well how to develop a custom LINQ to custom provider and I didn't find any good and complete tutorial on that in C#

Answer 1

If you want to create LINQ Provider - you have to deal always with Expression Tree to create and parse query.
Select and Where in your TreeContext are not needed at all. Standard LINQ Extensions has these methods and will automatically prepare Expression Tree for you. Focus on TreeProvider implementation.
Since you are new in this area, I would suggest to base your LINQ Provider on Relinq. It will simplify your life from start and will show possible ways for implementation. It has base classes for Provider and Query implementation, but it is not required to derive from them.
Just try
var queryParser = new QueryParser(new ExpressionTreeParser(
ExpressionTreeParser.CreateDefaultNodeTypeProvider(),
ExpressionTreeParser.CreateDefaultProcessor(
ExpressionTransformerRegistry.CreateDefault(),
null)));
var queryModel = parser.GetParsedQuery(expression);
And Relinq will give you QueryModel, which is much easier to parse.