Linq IQueryable short circuiting an empty search parameter

I have a generic repository with the following method

IQueryable<T> GetAllByFilter(Expression<Func<T, bool>> expression);

I'm now trying to provide a search feature through the front end, where one or more parameters might have been entered or 开发者_如何转开发left blank. I'm having problems short-circuiting the expression for empty parameters.

The problem can be demonstrated by calling the following example on the repository:

public IEnumerable<Foo> Search(string param)
{
    var filteredFoos = _fooRepository.GetAllByFilter(
          f => string.IsNullOrEmpty(param) || f.Something == param );

    return filteredFoos.ToList(); // throws exception
}

Enumerating the query with ToList() throws a System.NullReferenceException if param is null.

I neither understand this nor know how to fix it, so any pointers appreciated. Thanks.

UPDATE: in response to the comments below, I added a null check. My actual code looks like this now

var test1 = _repository.GetAllByFilter(
     r => r != null && 
         (string.IsNullOrEmpty(param) 
              || (r.Field != null && r.Field.IndexOf(param.Trim()) != -1)));

var test2 = test1.ToList(); // exception here

I'm still not seeing where the problem could be.

EDIT: in response to comment, the generic repository GetAllByFilter code:

public IQueryable<T> GetAllByFilter(Expression<Func<T, bool>> expression)
{
    return _dataContext.GetTable<T>().Where(expression);
}

note that if I run a simple GetAll query

public IQueryable<T> GetAll()
 {
     return _dataContext.GetTable<T>();
 }

on the same table, no null records are returned (as expected).

---

keep it simple:

public IEnumerable<Foo> Search(string param)
{
    if (string.IsNullOrEmpty(param))
    {
        return this.fooRepository.GetAll().ToArray();
    }

    return this.fooRepository.GetAllByFilter(o => o.Field.Contains(param.Trim())).ToArray();
}

---

cake.

    public IEnumerable<Foo> Search(string param)
    {
        Expression<Func<Foo, bool>> shortCircuit = a => true;
        Expression<Func<Foo, bool>> normal = a => a.Something == param;

        var filteredFoos = _fooRepository.GetAllByFilter(
            string.IsNullOrEmpty(param) ? shortCircuit : normal);

        return filteredFoos.ToList(); // no more exception.
    }

You just gotta remember, you can't throw anything into those IQueryable methods and expect them to understand. You can probably make shortCircuit expression to static.