How can I do it ? I don't want to add same record in database with Entity Framework

Question

What methods can I use Find, FirstOrDefault, Any....

public virtual void Add(T entity)
{
        using (MovieAppContext _context = new MovieAppContext())
        {
            var record = _context.Set<T>().Find(entity); // does not work I got e

            if (record == null)
            {
                _context.Set<T>().Add(entity);
                _context.SaveChanges();
            }
        }
}

Answer 1

Given your scope here, DbSet<T>.Find<T>(params object[]) is probably not the method you should be using.

---

The following solution uses IQueryable<T>.Any<T>(Expression<Func<T, bool>>) , a Linq Extension Method that accepts a Predicate\Function.

Code Sample using it

using (MovieAppContext _context = new MovieAppContext())
{
    bool queryResult = _context.Set<T>().Any((e) => e.Name.Equals(entity.Name, StringComparison.OrdinalIgnoreCase));

    if (queryResult)
    {
        _context.Set<T>().Add(entity);
        _context.SaveChanges();
    }
}

Here, we are defining an anonymous function delegate, Func<T, bool> , that is in turn being used to define a Linq Expression, Expression<Func<T, bool>> .

---

Note

I defined this function predicate expression to evaluate on the entity names (a made-up property of this type; that in this example portrays a unique constraint.)

Also, an alternative to using anonymous functions declaration for this expression is to declare it, then pass it.

That looks like the following:

...
Expression<Func<T, bool>> WhereEntityNameMatches = (e) => e.Name.Equals(entity.Name, StringComparison.OrdinalIgnoreCase));
    
bool queryResult = _context.Set<T>().Any(WhereEntityNameMatches);
...

Which may help out with reusability and scope considerations.

Answer 2

For consideration, An alternative here would be to not attempt the query lookup and check at all.

Instead, you would do a try-catch for DbUpdateException in the attempt to save changes after\following this Add operation.

try 
{
    _context.Set<T>().Add(entity);
    _context.SaveChanges();
} 
catch (DbUpdateException) 
{
    var isRaisedByUniqueIndexOrConstraint = new[] { "2601", "2627" }.Contains(ex.GetBaseException().Data["HelpLink.EvtID"]) ?? false;

    if (!isRaisedByUniqueIndexOrConstraint)
        throw;

    // Could suppress and\or handle attempted duplicate record adding
}

This relies on those index\constraints existing, so you would first define a Unique Index on the column(s) (as well as your entity type using Data Annotations: IndexAttribute ).

---

Note

Part of this answer\solution purposes getting the Error Number from ADO.NET SqlClient Base Exception to further scope and qualify the cause of exception. The Error Numbers, "2601", "2627" , are hard-coded in this example.