建设“平”，而不是“树”LINQ EX pressions

问题描述：

我用一些code（可此处 MSDN上）动态地构建一个包含多个OR'条款'LINQ EX pressions。

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

相关code是

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));

这会产生一个LINQ EX pression，看起来是这样的：

This generates a LINQ expression that looks something like this:

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

我用这个前pression击球时的递归限制（100），所以我想产生一个前pression看起来是这样的：

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)

我将如何修改EX pression建设code做到这一点？

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

答

您需要修改的产生，因此它建立了或 s的序列的营养均衡的树，而不是其中左子树是一个单一的前pression和右子树包含所有剩余的元素。图形：

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

正如你可以看到，即使在这个简单的情况下，更好的方法是不一样深深地（递归嵌套）。在code，以产生更好的EX pression树可以写成一个递归方法在C＃：

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);

我没有尝试在code，所以有可能会出现一些小错误，但它应该表现出的主意。

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

建设“平”，而不是“树”LINQ EX pressions

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