c# - Extension method can't be used in LINQ Expression Tree -

i have tried use extension method in linq query following way (ef code first):

 user in db.users  join data in db.data on user.id equals data.user_id  data.available.tobool() 

the db.users , db.data variables dbset types.

then got following error:

an exception of type 'system.notsupportedexception' occurred in userdata.dll not handled in user code

additional information: linq entities not recognize method 'boolean tobool(system.string)' method, , method cannot translated store expression.

the extension method simple:

 public static bool tobool(this string source)  {       return source=="aa";  } 

so looked expression trees , created expression:

pivate readonly system.linq.expressions.expression<func<data, bool>> proper = d => d.name.tobool(); 

now use this:

 user in db.users   join data in db.data.where(proper) on user.id equals data.user_id 

i still have same error.

what else should use?

you can use extension methods this:

public static iqueryable<user> hasdata(this iqueryable<user> u) {   return u.datas.any(); } 

used like:

var u=db.users.hasdata(); 

but return users have data, assuming have navigation property on users data called datas (and yes, know plural of datum data)