EF set nvarchar(450) in case of having index for string property

[sa-es-ir]

I have a model like this:

public class Student
{
    public int Id { get; set; }

    public string UniqueCode { get; set; }
}

Configuration:

 modelBuilder.Entity<Student>()
            .HasKey(x => x.Id);

  modelBuilder.Entity<Student>()
            .HasIndex(x => x.UniqueCode)
            .IsUnique();

And generated migrations:

migrationBuilder.CreateTable(
                name: "Students",
                columns: table => new
                {
                    Id = table.Column<int>(type: "int", nullable: false)
                        .Annotation("SqlServer:Identity", "1, 1"),
                    UniqueCode = table.Column<string>(type: "nvarchar(450)", nullable: true)
                },
                constraints: table =>
                {
                    table.PrimaryKey("PK_Students", x => x.Id);
                });

            migrationBuilder.CreateIndex(
                name: "IX_Students_UniqueCode",
                table: "Students",
                column: "UniqueCode",
                unique: true,
                filter: "[UniqueCode] IS NOT NULL");

EF set the string property to nvarchar(450), I know maybe EF is wise enough to know that SQL will not allow creating an index on nvarchar(max) but why 450 chars? why not at least give a warning during generating migrations?

Include provider and version information

EF Core version: EF 7.0
Database provider: Microsoft.EntityFrameworkCore.SqlServer
Target framework: .NET 7.0
Operating system: Windows

[ErikEJ]

Max index size is 900 characters .

[sa-es-ir]

The behavior is a little confusing, I couldn't find any document about it, and no hint during migrations, As @ErikEJ mentioned the max size for an index is 900 chars.

[ajcvickers]

@sa-es-ir The max size was chosen such that the column would work as part of a composite key or index, with other columns potentially taking up some of the index space. However, this is only a default. Feel free to configure a different size as needed.

[sa-es-ir]

Thanks, @ajcvickers
Yeah, I always set max length for my strings but I just wondered how this number (450) was chosen.
Can I say that it only happens if I set an index on the string property?

[ajcvickers]

@sa-es-ir 450 is also used for properties of primary, alternate, and foreign keys.

[sa-es-ir]

Great and thanks for the clarification @ajcvickers