Linq Query ignore empty parameters

Question

How do I get Linq to ignore any parameters that are empty? So Lastname, Firstname, etc? If I have data in all parameters it works fine...

refinedresult = From x In theresult _
                    Where x.<thelastname>.Value.TestPhoneElement(LastName) And _
                    x.<thefirstname>.Value.TestPhoneElement(FirstName) And _
                    x.<id>.Value.TestPhoneElement(Id) And _
                    x.<number>.Value.TestPhoneElement(Telephone) And _
                    x.<location>.Value.TestPhoneElement(Location) And _
                    x.<building>.Value.TestPhoneElement(building) And _
                    x.<department>.Value.TestPhoneElement(Department) _
                    Select x


Public Function TestPhoneElement(ByVal parent As String, ByVal value2compare As String) As Boolean
'find out if a value is null, if not then compare the passed value to see if it starts with
Dim ret As Boolean = False

If String.IsNullOrEmpty(parent) Then
    Return False
End If
If String.IsNullOrEmpty(value2compare) Then
    Return ret
Else
    ret = parent.ToLower.StartsWith(value2compare.ToLower.Trim)
End If

Return ret
End Function

Answer 1

Just to make sure I understand what you want: You want an IEnumerable of XElements x returned where at least one of the child elements' values matching the corresponding string variable. So by ignore you mean that your extension method would return false. So I deduce that if it's not working fine, then an empty parameter causes true to (mistakenly) be returned by TestPhoneElement, and hence you get false positives. Meaning, if a parameter is an emptystring or nothing, it always returns true, and hence you are getting items in your result that you shouldn't be getting.

My thinking is this:

1. Only ret = parent.ToLower.StartsWith(value2compare.ToLower.Trim) could possibly return true.
2. value2compare.ToLower.Trim() would certainly cause the problem you indicate.
3. String.IsNullOrEmpty(value2compare) has to be returning false.

I believe that the second parameter that you're passing into TestPhoneElement must in fact be a string containing at least one space. This way, String.IsNullOrEmpty(value2compare) returns false. Then in the final line value2compare.ToLower.Trim evaluates to an empty string because you trimmed it, and ret = parent.ToLower.StartsWith(value2compare.ToLower.Trim) evaluates to true because every string begins with an empty string.

So, trim value2compare when it first comes in, or change you 2nd conditional to:

If String.IsNullOrEmpty(value2compare.trim()) Then

and you should be good.

Edit:

solution based on clarified situation

You want an empty string passed into the extension method to result in True then, right? Also, I updated the extension method to allow for slightly cleaner code. The key though is that you want any blank string passed in to result in True being returned:

refinedresult = From x In theresult _
            Where x.<thelastname>.MatchesOrIsBlank(LastName) And _
                x.<thefirstname>.MatchesOrIsBlank(FirstName) And _
                x.<id>.MatchesOrIsBlank(Id) And _
                x.<number>.MatchesOrIsBlank(Telephone) And _
                x.<location>.MatchesOrIsBlank(Location) And _
                x.<building>.MatchesOrIsBlank(Building) And _
                x.<department>.MatchesOrIsBlank(Department) _
            Select x

And:

Public Function TestPhoneElement(ByVal xE As XElement, ByVal value2compare As String) As Boolean
    Return xE.Value.ToLower.StartsWith(value2compare.ToLower.Trim)
End Function