在EntityFramework 6.1后可以直接使用

[Index("TitleIndex", IsUnique = true)] public string Title { get; set; }

在旧版本中，

Unfortunately you can't define it as unique key in code first because EF doesn't support unique keys at all (it is hopefully planned for next major release). What you can do is to create custom database intializer and add unique index manually by calling SQL command:

public class MyInitializer : CreateDatabaseIfNotExists
     
      {  protected override void Seed(MyContext context)  {    context.Database.ExecuteSqlCommand("CREATE UNIQUE INDEX IX_Category_Title ON Categories (Title)");  }}
     

And you must set this initializer in the bootstrap of your application.

Database.SetInitializer
     
      (new MyInitializer());
     

http://stackoverflow.com/questions/5701608/unique-key-with-ef-code-first