Linq-to-SQL: Combining (OR'ing) multiple "Contains" filters?

Question

I'm having some trouble figuring out the best way to do this, and I would appreciate any help.

Basically, I'm setting up a filter that allows the user to look at a history of audit items associated with an arbitrary "filter" of usernames.

The datasource is a SQL Server data base, so I'm taking the IQueryable "source" (either a direct table reference from the db context object, or perhaps an IQueryable that's resulted from additional queries), applying the WHERE filter, and then returning the resultant IQueryable object....but I'm a little stumped as to how to perform OR using this approach.

I've considered going the route of Expressions because I know how to OR those, but I haven't been able to figure out quite how to do that with a "Contains" type evaluation, so I'm currently using a UNION, but I'm afraid this might have negative impact on performance, and I'm wondering if it may not give me exactly what I need if other filters (in addition to user name filtering shown here) are added in an arbirary order.

Here is my sample code:

public override IQueryable<X> ApplyFilter<X>(IQueryable<X> source)
{
    // Take allowed values...
    List<string> searchStrings = new List<string>();

    // <SNIP> (This just populates my list of search strings)

    IQueryable<X> oReturn = null;

    // Step through each iteration, and perform a 'LIKE %value%' query
    string[] searchArray = searchStrings.ToArray();
    for (int i = 0; i < searchArray.Length; i++)
    {
        string value = searchArray[i];
        if (i == 0)
            // For first step, perform direct WHERE
            oReturn = source.Where(x => x.Username.Contains(value));
        else
            // For additional steps, perform UNION on WHERE
            oReturn = oReturn.Union(source.Where(x => x.Username.Contains(value)));
    }
    return oReturn ?? source;
}

This feels like the wrong way to do things, but it does seem to work, so my question is first, is there a better way to do this? Also, is there a way to do a 'Contains' or 'Like' with Expressions?

(Editted to correct my code: In rolling back to working state in order to post it, I apparently didn't roll back quite far enough :) )

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ETA: Per the solution given, here is my new code (in case anyone reading this is interested):

public override IQueryable<X> ApplyFilter<X>(IQueryable<X> source)
{
    List<string> searchStrings = new List<string>(AllowedValues);

    // <SNIP> build collection of search values 

    string[] searchArray = searchStrings.ToArray();

    Expression<Func<X, bool>> expression = PredicateBuilder.False<X>();

    for (int i = 0; i < searchArray.Length; i++)
    {
        string value = searchArray[i];
        expression = expression.Or(x => x.Username.Contains(value));
    }
    return source.Where(expression);
}

(One caveat I noticed: Following the PredicateBuilder's example, an empty collection of search strings will return false (false || value1 || ... ), whereas in my original version, I was assuming an empty list should just coallesce to the unfiltered source. As I thought about it more, the new version seems to make more sense for my needs, so I adopted that)

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Answer 1

You can use the PredicateBuilder from the LINQkit to dynamically construct your query.