c# - Lambda expression weirdness in a LINQ to SQL 'where' condition -

I am working on an application that uses repository patterns with my data source. In my repository I expose the following method:

  public IEnumerable & lt; T & gt; Find (convert & lt; Func & lt; T, bool & gt; & gt; ou)) Returns _ Convert GateTable & lt; T & gt; (). Where (where); }  

I am able to call by saying:

  repository & lt; Users & gt; (). Find (U => true);  

But if I'm trying (when the search is zero)

  the repository & lt; Users & gt; (). (U = & gt; String.EasanOrcti Search)? U.UserName.Contains (search): true);  

I get an error:

The value can not be empty The parameter name: text

I thought That Lambda's expression will execute the same thing because the value of search is zero, but this is not clearly the case.

How do I fix this problem?

To add to other answers, if you is adhering to this ideal To do this, you can do so instead:

  repository & lt; Users & gt; (U => String.EsanLolecti (search) || u.UserName.Contains (search));  

Before applying it blindly with eyes, if you are to do so, then read Adam's answer to learn about the result.