Is it possible to set a unique constraint using Entity Framework Code First?

Use this to define it in config class, if you're avoiding using annotations:

public class YourTableConfig : EntityTypeConfiguration<YourTableEntity>
{
  public YourTableConfig()
  {
    ToTable("YourTableDbName");

    HasKey(u => u.Id);

    Property(c => c.CompanyId).HasColumnType("nvarchar").HasMaxLength(9).IsRequired();

    HasIndex(x => x.CompanyId).IsUnique(); // This sets the unique index
  }
}

It appears that the unique constraint feature that was scheduled to release with Version 6 got pushed to 6.1.

With EF 6.1, you can define a constraint using the Index attribute as shown below:

[Index("IX_FirstAndSecond", 1, IsUnique = true)]
public int FirstColumn { get; set; }

[Index("IX_FirstAndSecond", 2, IsUnique = true)]
public int SecondColumn { get; set; }

OR

You can use Fluent API as shown here in MSDN

Let's say that you want to add the Unique constraint on only one attribute, you can do as following, starting from EF6.1

[Index(IsUnique = true)]
public string Username { get; set; }

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If you have multiple fields that are related to the same index then you are going to use:

Multiple-Column Indexes

Indexes that span multiple columns are specified by using the same name in multiple Index annotations for a given table. When you create multi-column indexes, you need to specify an order for the columns in the index. For example, the following code creates a multi-column index on Rating and BlogId called IX_BlogAndRating. BlogId is the first column in the index and Rating is the second.

public class Post 
{ 
    public int Id { get; set; } 
    public string Title { get; set; } 
    public string Content { get; set; } 
    [Index("IX_BlogIdAndRating", 2)] 
    public int Rating { get; set; } 
    [Index("IX_BlogIdAndRating", 1)] 
    public int BlogId { get; set; } 
}

Please refer to this link for further information.