How to use linq to sql to select one column from row

Question

I am using this C# with linq to sql:

string currentLabel = from s2f in stream2FieldTypesTable
                      where s2f.s2fID == item.s2fID
                      && (s2f.s2fLabel != item.s2fLabel || s2f.s2fIsRequired != item.s2fIsRequired)
                      select s2f.s2fLabel;

I am getting a compiler error saying i can't assign type System.Linq.IQueryable<string> to string.

I tried this code:

string currentLabel = from s2f in stream2FieldTypesTable
                      where s2f.s2fID == item.s2fID
                      && (s2f.s2fLabel != item.s2fLabel || s2f.s2fIsRequired != item.s2fIsRequired)
                      select s2f.s2fLabel.ToString();

And that returns the same error. I'm sure this is a simple thing. what am I missing? I just want the first s2fLabel that matches the where clause.

Answer 1

Actual type of your query will be IEnumerable. LINQ has a concept of deffered execution, in order to get you query actually exectuted you need to call a method that will iterate over IEnumerable:

    string currentLabel = (from s2f in stream2FieldTypesTable
                          where s2f.s2fID == item.s2fID
                          && (s2f.s2fLabel != item.s2fLabel || s2f.s2fIsRequired != item.s2fIsRequired)
                          select s2f.s2fLabel)
                         .FirstOrDefault();

For getting exact one instanse it can be FirstOrDefault() method or SignleOrDefault() or just First() or Single(). The only difference is that methods without "OrDefault()" will throw an exception if enumeration will not satisfy their expectations, and methods with "OrDefault()" will just return null.

Edit

The dirrerence between Single and First is that Single expects exact one element in the collection and First expects at least one element.