Build an OR query expression progressively [duplicate]

Answer 1

You would need to construct the filters first, and then combine the filters into a single lambda that you can use as the composed query:

var filters = new List<Expression<Func<YourType, bool>>>();
filters.Add(d => d.TerritoryID == 3);
filters.Add(d => d.CustomerType == "Individual");
...

var lambda = AnyOf(filters.ToArray());
// this is: d => d.TerrotoryID == 3 || d.CustomerType == "Individual";

var data = src.Where(lambda);

Using:

static Expression<Func<T,bool>> AnyOf<T>(
          params Expression<Func<T,bool>>[] expressions)
{
    if (expressions == null || expressions.Length == 0) return x => false;
    if (expressions.Length == 1) return expressions[0];

    var body = expressions[0].Body;
    var param = expressions[0].Parameters.Single();
    for (int i = 1; i < expressions.Length; i++)
    {
        var expr = expressions[i];
        var swappedParam = new SwapVisitor(expr.Parameters.Single(), param)
                            .Visit(expr.Body);
        body = Expression.OrElse(body, swappedParam);
    }
    return Expression.Lambda<Func<T, bool>>(body, param);
}
class SwapVisitor : ExpressionVisitor
{
    private readonly Expression from, to;
    public SwapVisitor(Expression from, Expression to){
        this.from = from;
        this.to = to;
    }
    public override Expression Visit(Expression node)
    {
        return node == from ? to : base.Visit(node);
    }
}

Answer 2

While you can do this sort of thing directly, I use the LinqKit library which makes such progressive query building really easy:

You would end up with:

var context = new AdventureWorksDataContext();
// Step 1
var query = context.Customers.Where(d => d.CustomerType == "Individual");
// Step 2
query = query.Or(d => d.TerritoryID == 3);

The system I'm working on needs a lot of this, and LinqKit is a major life saver. You can find it here.

(Note: I'm not affiliated with LinqKit's developer in anyway - just a fan.)

Answer 3

To accomplish what you ask without using the dynamic linq lib, you can define expressions for each test in the where clause, then use them to build a lambda expression:

Expression<Func<Customer, bool>> isIndividualCustomer = c => c.CustomerType == "Individual";

Expression<Func<Customer, bool>> territoryIdIs3 = c => c.TerritoryID == 3;

Expression<Func<Car, bool>> customerIsIndividualOrOfTerritoryId3 = Expression.Lambda<Func<Customer, bool>>(
    Expression.Or(isIndividualCustomer.Body, territoryIdIs3.Body), isIndividualCustomer.Parameters.Single());

Usage:

var query = context.Customers.Where(customerIsIndividualOrOfTerritoryId3);

Answer 4

if you want it in two steps, you can use union:

var context = new AdventureWorksDataContext();
// Step 1
var query = context.Customers.Where(d => d.CustomerType == "Individual");
// step2 
query = query.Union(context.Customers.Where(d => d.TerritoryID == 3));