I find myself repeating the same little code pattern time after time when building a LINQ expression based on a number of optional filtering criteria. Using standard if statements seems a bit verbose to me.

if (SearchControlMain.CategoryID.HasValue)
    query = query.Where(q => q.CategoryID == SearchControlMain.CategoryID);

if (SearchControlMain.TypeID.HasValue)
    query = query.Where(q => q.TypeID == SearchControlMain.TypeID);

if (SearchControlMain.PostingID.HasValue)
    query = query.Where(q => q.PostingID == SearchControlMain.PostingID);

Completely inlining the if statements doesn't do much for readability.

if (SearchControlMain.CategoryID.HasValue) query = query.Where(q => q.CategoryID == SearchControlMain.CategoryID);
if (SearchControlMain.TypeID.HasValue) query = query.Where(q => q.TypeID == SearchControlMain.TypeID);
if (SearchControlMain.PostingID.HasValue) query = query.Where(q => q.PostingID == SearchControlMain.PostingID);

So I created a couple of quick extension methods on both the IEnumerable and IQueryable forms of Where. I named my new extension WhereIf but I struggled a bit as to whether to name it just Where and wind up with an overload.

Now you can code:

query = query.WhereIf(SearchControlMain.CategoryID.HasValue, q => q.CategoryID == SearchControlMain.CategoryID);
query = query.WhereIf(SearchControlMain.TypeID.HasValue, q => q.TypeID == SearchControlMain.TypeID);
query = query.WhereIf(SearchControlMain.PostingID.HasValue, q => q.PostingID == SearchControlMain.PostingID);

Here are the extension methods:

public static IEnumerable<TSource> WhereIf<TSource>(this IEnumerable<TSource> source, bool condition, Func<TSource, bool> predicate)
{
    if (condition)
        return source.Where(predicate);
    else
        return source;
}

public static IEnumerable<TSource> WhereIf<TSource>(this IEnumerable<TSource> source, bool condition, Func<TSource, int, bool> predicate)
{
    if (condition)
        return source.Where(predicate);
    else
        return source;
}

public static IQueryable<TSource> WhereIf<TSource>(this IQueryable<TSource> source, bool condition, Expression<Func<TSource, bool>> predicate)
{
    if (condition)
        return source.Where(predicate);
    else
        return source;
}

public static IQueryable<TSource> WhereIf<TSource>(this IQueryable<TSource> source, bool condition, Expression<Func<TSource, int, bool>> predicate)
{
    if (condition)
        return source.Where(predicate);
    else
        return source;
}