linq query with dynamic predicates in where clause joined by OR - linq

Question

you can easily create dynamic queries in c# if you add more restrictions to the current query.
var list = new List<Item>();
var q = list.AsQueryable();
q = q.Where(x => x.Size == 3);
q = q.Where(x => x.Color == "blue");
In this case, every new predicate is added performing an AND operation with the previous. The previous result is equivalent to:
q = list.Where(x => x.Size == 3 && x.Color == "blue");
Is it possible to achieve the same result but with OR instead of AND?
q = list.Where(x => x.Size == 3 || x.Color == "blue");
The idea is to have a variable number of expressions that are joined with OR operator.
Expected result would need to be written in some how similar to the following pseudo code:
var conditions = new List<Func<Item, bool>>();
And later iterate conditions to build something like:
foreach(var condition in conditions)
finalExpression += finalExpression || condition;

Answer 1

Another possible solution to this, especially when someone doesn't want to use an external library is using expression trees.
Add a following expression extension:
public static Expression<Func<T, bool>> Or<T>(
this Expression<Func<T, bool>> expr1,
Expression<Func<T, bool>> expr2)
{
var invokedExpr = Expression.Invoke(expr2, expr1.Parameters);
return Expression.Lambda<Func<T, bool>>(
Expression.OrElse(expr1.Body, invokedExpr), expr1.Parameters);
}
As an example imagine you have a Container entity, which has InnerContainer nested entity with two properties Name and Id. Then you can use it in the following way:
Expression<Func<Container, bool>> whereQuery = c => c.InnerContainer.Name == "Name1";
whereQuery = whereQuery.Or(c => c.InnerContainer.Name == "Name2");
whereQuery = whereQuery.Or(c => c.InnerContainer.Id == 0);
var result = query
.Where(whereQuery)
.ToList();
Materializing such query will result in the following SQL:
SELECT [x].[Id], [x].[InnerContainerId]
FROM [Containers] AS [x]
LEFT JOIN [InnerContainer] AS [x.InnerContainer] ON [x].[InnerContainerId] = [x.InnerContainer].[Id]
WHERE [x.InnerContainer].[Name] IN (N'Name1', N'Name2') OR ([x].[InnerContainerId] = 0)
This way you can hold lambdas in a collection and loop through them.

Answer 2

Thanks to Raphaël Althaus that gave the following link:
http://www.albahari.com/nutshell/predicatebuilder.aspx
Predicate builder is the solution. You can use it installing LinqKit from Nuget. In that url you can find also the implementation of this class.
Note: in order to make this work with LinqToSql or LinqToEntities the IQueriable Object must be transformed using "AsExpandable()" method, for memory objects it's not required