Question

Assume you have a table with a nullable varchar column. When you try to filter the table, you would use (pFilter is parameter):

var filter = pFilter;
var dataContext = new DBDataContext();
var result = dataContext.MyTable.Where(x=>x.MyColumn == filter).ToList();

Now, what if there is a keyword that means “All Nulls”. The code would look like:

var filter = pFilter != "[Nulls]" ? pFilter : null;
var dataContext = new DBDataContext();
var result = dataContext.MyTable.Where(x=>x.MyColumn == filter).ToList();

But this doesn’t work. Apparently, a String with value of null is… not null?

However, what do work is this code:

var filter = pFilter != "[Nulls]" ? pFilter : null;
var dataContext = new DBDataContext();
var result = dataContext.MyTable.Where(x=>x.MyColumn == filter || (filter == null && x.MyColumn == null)).ToList();

The workaround did not convinced me, that’s why my question is: What is the best way to deal with nullable string columns in LinqToSql?

Answer 1

Use String.Equals that will make LINQ handle null appropriately on the generated SQL query

var result = dataContext.MyTable
                        .Where(x => String.Equals(x.MyColumn, filter))
                        .ToList();

Edit:

If you use == LINQ will generate the query for the general case WHERE [column] = @parameter but on SQL NULL does not match NULL, the proper way to test for NULL is [column] IS NULL.

With String.Equals LINQ has enough information to translate the method to the appropiate sentence in each case, what means:

if you pass a non-null string it will be

WHERE ([column] IS NOT NULL) AND ([column] = @parameter)

and if it is null

WHERE [column] IS NULL