Creating OR expressions in LINQ

As everybody who has read my blog before knows, I love LINQ and miss it when coding in other languages, so it’s nice when I get a chance to use it again. When I come back to it with fresh eyes, I notice some things aren’t as easy as they should be — and this time is no exception.

Background

People often need to build up LINQ expressions at runtime based on filters or criteria a user has selected. Adding criteria is incredibly easy, as you can chain operations together on the IQueryable interface, e.g.

if (customerActive)
  query = query.Where(c => c.IsActive);
if (customerCountry != null)
  query = query.Where(c => c.Country == customerCountry);

This is great if you want to AND things together, which is often the case in building up filters. Other operations, like Contains, are useful in allowing many options against a specific field.

Problem

But what about offering a choice involving either of two properties? One way to write it would be:

if (customerActive) {
  if (customerEnterprise) {
    query = query.Where(c => c.IsActive || c.IsEnterprise);
  } else {
    query = query.Where(c => c.IsActive);
  } else {
    if (customerEnterprise) {
      query = query.Where(c => c.IsEnterprise);
    }
  }
}

This code is already hard to read and is going to increase in complexity for each new option.

Solution

With some helper code, we can combine two expression predicates into a single OrElse for use in a Where condition.

class Or {
  public static Expression<Func<T, bool>> Combine<T>(Expression<Func<T, bool>> left, Expression<Func<T, bool>> right) {
    if (left == null && right == null) throw new ArgumentException("At least one argument must not be null");
    if (left == null) return right;
    if (right == null) return left;

    var parameter = Expression.Parameter(typeof(T), "p");
    var combined = new ParameterReplacer(parameter).Visit(Expression.OrElse(left.Body, right.Body));
    return Expression.Lambda<Func<T, bool>>(combined, parameter);
  }

  class ParameterReplacer : ExpressionVisitor {
    readonly ParameterExpression parameter;

    internal ParameterReplacer(ParameterExpression parameter) {
      this.parameter = parameter;
    }

    protected override Expression VisitParameter(ParameterExpression node) {
      return parameter;
    }
  }
}

Now we can:

Expression<Func<Customer, bool>> criteria = null;
if (customerActive)
    criteria = Or.Combine(criteria, c => c.IsActive == customerActive);
if (customerEnterprise)
    criteria = Or.Combine(criteria, c => c.IsEnterprise == customerEnterprise);

How it works

There are two parts to this working. The first combines the two separate predicates into a single OR lambda, taken care of by the Expression.OrElse method.

That alone can’t quite do what we need because the two expressions each have a separate parameter variable. The visitor replaces both of these with a new parameter we’ll pass into the new Lambda expression we’re creating that combines both conditions.

Caveats

The code, as it stands, creates an unbalanced tree. Ideally, the tree should be balanced to limit the depth LINQ providers must traverse to translate the query. I doubt you’ll run into the dreaded Stack Overflow (the message, not the site), but if you do, that’s why.

An overload that takes an Enumerable of Expression<Func<T, bool>> and produces a balanced tree is left as an exercise to the reader

[)amien