这不是：

public class ApplicationUser : IdentityUser
{
    [required,MaxLength(15)]
    public string UserName { get; set; }

}

或这个：

modelBuilder.Entity< ApplicationUser>().Property(x => x.UserName).HasMaxLength(15);

作品.

我需要这个,因为在nvarchar(max)上设置索引会给我这个错误消息：

Column ‘UserName’ in table ‘dbo.AspNetUsers’ is of a type that is
invalid for use as a key column in an index.

为了详细,我试图设置这样的索引：

public override void Up()
{
    CreateIndex("dbo.AspNetUsers","UserName",true,"IX_UserName");
}

public override void Down()
{
    DropIndex("dbo.AspNetUsers","IX_UserName");
}