Is there a data annotation for unique constraint in EF Core (code first)?

To update, there is now a code-first annotation.

[Index(nameof(MyProperty), IsUnique = true)] // using Microsoft.EntityFrameworkCore
public class MyClass
{
    [Key, DatabaseGenerated(DatabaseGeneratedOption.None)]
    public Guid Id { get; set; }

    [StringLength(255), Required]
    public string MyProperty { get; set; }
}

In EF Core you could use the extension method HasAlternateKey in fluent API only. There are no data annotations to realize a unique constraint.

This MS doc article - Alternate Keys (Unique Constraints) - will explain how to use and which further possibilities are exist.

A short example from link above:

class MyContext : DbContext
{
    public DbSet<Car> Cars { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Car>()
            .HasAlternateKey(c => c.LicensePlate)
            .HasName("AlternateKey_LicensePlate");
    }
}

class Car
{
    public int CarId { get; set; }
    public string LicensePlate { get; set; }
    public string Make { get; set; }
    public string Model { get; set; }
}

Also it's possible to define an unique index. Therefore, in EF Core you can use the fluent API's extension method HasIndex or the data annotation way with the attribute [Index].

In this MS doc article - Indexes - you will find further information how to use.

Here an example for an unique index with fluent API:

class MyContext : DbContext
{
    public DbSet<Blog> Blogs { get; set; }

    protected override void OnModelCreating(ModelBuilder modelBuilder)
    {
        modelBuilder.Entity<Blog>()
            .HasIndex(b => b.Url)
            .IsUnique();
    }
}

public class Blog
{
    public int BlogId { get; set; }
    public string Url { get; set; }
}

Here the same example, but with data annotation:

[Index(nameof(Url), IsUnique = true)]
public class Blog
{
    public int BlogId { get; set; }
    public string Url { get; set; }
}

---

Update 2021-09-10

• added additional info how to use with data annotation, because it's now available in EF Core;

Update 2021-09-24

• fixed missing IsUnique property in attribute example