The structure of the expression "flat", not "tree" LINQ

Question

I use some code (available here on MSDN) to dynamically assemble LINQ expressions containing multiple OR clauses.

Corresponding code

var equals = values.Select(value => (Expression)Expression.Equal(valueSelector.Body, Expression.Constant(value, typeof(TValue))));

var body = equals.Aggregate<Expression>((accumulate, equal) => Expression.Or(accumulate, equal));

This generates a LINQ expression that looks something like this:

(((((ID = 5) OR (ID = 4)) OR (ID = 3)) OR (ID = 2)) OR (ID = 1))

I use this expression to limit recursion (100), so I want to create an expression that looks like this:

(ID = 5) OR (ID = 4) OR (ID = 3) OR (ID = 2) OR (ID = 1)

How do I change the expression build code to do this?

Answer 1

, OR, , . :

 Your code               Better
 ---------              --------
    OR                     OR
 #1    OR              OR      OR
     #2  OR          #1  #2  #3  #4
       #3  #4

, ( ). #:

Expression GenerateTree(List<Expression> exprs, int start, int end) {
  // End of the recursive processing - return single element
  if (start == end) return exprs[start];

  // Split the list between two parts of (roughly the same size)
  var mid = start + (end - start)/2;
  // Process the two parts recursively and join them using OR
  var left = GenerateTree(exprs, start, mid);
  var right = GenerateTree(exprs, mid+1, end);
  return Expression.Or(left, right);
}

// Then call it like this:
var equalsList = equals.ToList();
var body = GenerateTree(equalsList, 0, equalsList.Length);

, , .

Answer 2

LINQ , ? , .

, : , , - :

var query = from item in source
            where idCollection.Contains(item.Id)
            ...