在没有竞争条件的情况下使用Entity Framework实现if-not-exists-insert

// 实体类
public class User
{
    public int Id { get; set; }
    public string Name { get; set; }
}

// 数据库上下文
public class MyDbContext : DbContext
{
    public DbSet<User> Users { get; set; }

    protected override void OnConfiguring(DbContextOptionsBuilder optionsBuilder)
    {
        optionsBuilder.UseSqlServer("连接字符串");
    }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<User>()
            .HasIndex(u => u.Name)
            .IsUnique();
    }
}

// 使用示例
public class Program
{
    public static void Main()
    {
        using (var context = new MyDbContext())
        {
            string userName = "John";

            // 查询是否存在符合条件的记录
            var existingUser = context.Users.FirstOrDefault(u => u.Name == userName);

            if (existingUser == null)
            {
                // 不存在，则执行插入操作
                var newUser = new User { Name = userName };
                context.Users.Add(newUser);
                context.SaveChanges();
            }
        }
    }
}