Question

I’m using some code (available here on MSDN) to dynamically build LINQ expressions containing multiple OR ‘clauses’.

The relevant code is

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’m hitting the recursion limit (100) when using this expression, so I’d like to generate an expression that looks like this:

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

How would I modify the expression building code to do this?

Answer 1

You need to modify the generation so that it builds a ballanced tree instead of a sequence of ORs where the left sub-tree is a single expression and the right sub-tree contains all remaining elements. Graphically:

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

As you can see, even in this simple case, the better approach is not as deeply (recursively nested). The code to generate the better expression tree can be written as a recursive method in C#:

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);

I didn’t try the code, so there may be some minor mistakes, but it should show the idea.