I am working with a LINQ to SQL query and have run into an issue where I have 4 optional fields to filter the data result on. By optional, I mean has the choice to enter a value or not. Specifically, a few text boxes that could have a value or have an empty string and a few drop down lists that could have had a value selected or maybe not...

For example:

    using (TagsModelDataContext db = new TagsModelDataContext())
     {
        var query = from tags in db.TagsHeaders
                    where tags.CST.Equals(this.SelectedCust.CustCode.ToUpper()) 
                    && Utility.GetDate(DateTime.Parse(this.txtOrderDateFrom.Text)) <= tags.ORDDTE
                    && Utility.GetDate(DateTime.Parse(this.txtOrderDateTo.Text)) >= tags.ORDDTE
                    select tags;
        this.Results = query.ToADOTable(rec => new object[] { query });
    }

Now I need to add the following fields/filters, but only if they are supplied by the user.

1. Product Number - Comes from another table that can be joined to TagsHeaders.
2. PO Number - a field within the TagsHeaders table.
3. Order Number - Similar to PO #, just different column.
4. Product Status - If the user selected this from a drop down, need to apply selected value here.

The query I already have is working great, but to complete the function, need to be able to add these 4 other items in the where clause, just don't know how!