SharePoint : How to check for null with a CAML Query?

Question

I have this CAML:

query.Query = @"<Where><Eq><FieldRef Name='MessageID' /><Value Type='Text'></Value></Eq></Where>"; 

This checks if the value of MessageID = string.empty()

What I would like to check for is null.... not empty string...

Is this possible with CAML?

Answer 1

CAML has the IsNull operator,so the query would be:

query.Query = @"<Where><IsNull><FieldRef Name='MessageID' /></IsNull></Where>" 

Answer 2

Needed an equivalent to String.IsNullOrEmpty(Description). Ended up with this:

<And>   <IsNotNull>       <FieldRef Name='Description' />      </IsNotNull>     <Neq>       <FieldRef Name='Description' />       <Value Type='Text'></Value>     </Neq> </And>