Adding or to linq query

Looks like you could be looking for a Predicate builder Or you can just stack some .Where clause as linq is lazy.

As your question lacks of context, imagine querying a Person table based on a form (2 fields/1 checkbox).

// Where Stacking
if (!string.IsNullOrEmpty(FieldA.Text))
{// OR
    result = query.Where(item => item.Something.Contains(FieldA.Text));
}
if (!string.IsNullOrEmpty(FieldB.Text))
{// AND
    result = result.Where(item => item.SomethingElse.Contains(FieldB.Text));
}

// Predicate
if (CheckBox1.Checked || CheckBox2.Checked || CheckBox3.Checked))
{//Here your initialise your predicate to false because you are going to do OR
    var predicate = PredicateBuilder.False<SearchItem>();
    if(CheckBox1.Checked) predicate = predicate.Or(item => item.Foo == 1);
    if(CheckBox2.Checked) predicate = predicate.Or(item => item.Foo == 2);
    if(CheckBox3.Checked) predicate = predicate.Or(item => item.Foo == 3);
    result = result.Where(predicate);
}

Stacking Where is a good solution but can produce a little overheat but nothing that is really noticable.

PS: Article of Jon about nesting where https://codeblog.jonskeet.uk/2011/06/16/linq-to-objects-and-the-performance-of-nested-quot-where-quot-calls/

You should use LinqKit.PredicateBuilder class

Example:

var predicate = PredicateBuilder.True<Product>();
if(inputPrice > 100)
   predicate = predicate.And (p => p.Price > 100);
if(inputName == "S%")
   predicate = predicate.And (p => p.Name.StartsWith("S"));

Finally,

var result = products.Where(predicate);

More details Wiki

IEnumerable and IQueryable to some extent will allow you to chain conditions

Query.Where(f=> some-condition || some_other_condition);
// or
var q = Query.Where(f=> some-condition)
if(stuff_needs_to_happen)
  q = q.Where(f => stuff_that_needs_to_happen)

The reason for the "to some extent" is that with IQueryable the underlying query language may not support some of the conditions in the predicate (custom functions and so on).