Dynamic lambda expression for where clause

I have a dynamic list of objects on which I am using lambda expression where clause to filter items. For example, just consider that it have 3 properties, foo, bar and baz

class item // let this be current class in dynamic item list
{
    bool foo;
    string bar;
    string baz;
}

Now if I want to filter item list where foo is false I can use following expression

var filtered = itemList.Where("!foo");

I can even filter the list by strings value as

var filtered = itemList.Where("bar==\"value\""); \\all items with bar = value

What I want to actually check is if item in list have a specific string value not null of white space. I tried following code

var filtered = itemList.Where("!String.IsNullOrWhiteSpace(baz)");

It threw an error

Expression of type 'System.Func`2[DynamicType,System.Object]' cannot be used for parameter of type 'System.String' of method 'Boolean IsNullOrWhiteSpace(System.String)'

Though I succeeded to get result by following query

var filtered = itemList.Where("baz!=null && baz!=\"\"");

I wanted to confirm if there is a way I can use String.IsNullOrWhiteSpace() in this query.