我认为描述是唯一的。因此，您必须自己搜索匹配的描述。它是这样的：

I assume that description is unique. So you'll have to search for a matching description yourself. It is something like this:

var descr = "something";
var tabB = context.TableB.FirstOrDefault(f => f.Description == descr );

if (tabB == null)
{
    // Assume TableB_ID is autonumber / identity in database.
    var newTabB = new TableB() { Description = descr };
    TableA.TableB = newTabB ;
    // Not sure if this line is required.
    context.TableB.Add(newTabB);
}
else
    TableA.TableB = tabB;

context.SaveChanges();