EF Code First migration maxLength being set to 4000 instead of max - entity-framework

Question

tl;dr: I have a column that should be NVARCHAR(MAX) but scaffolding a migration with Add-Migration is giving me a column with max length 4000 in Up(). What do I have to do to get this to be MAX?
Consider the following model:
public class Foo
{
public int Id { get; set; }
[Required]
[StringLength(100)]
public string Name { get; set; }
[Required]
[DataType(DataType.MultilineText)]
public string Memo { get; set; }
[Required]
[DataType(DataType.EmailAddress)]
public string Email { get; set; }
}
I have two conventions set up in Context.OnModelCreating:
A convention which sets the default maxLength of any string property not otherwise configured to 1024, with the line:
Properties<string>().Configure(c => c.HasMaxLength(1024));
An attribute-based convention which reads the DataTypeAttribute and sets some properties accordingly, namely the column type and length:
switch (attribute.DataType)
{
case DataType.MultilineText:
configuration.HasColumnType("nvarchar").IsMaxLength();
break;
case DataType.EmailAddress:
configuration.HasColumnType("nvarchar").HasMaxLength(255);
break;
}
I tested this all before setting up migrations and it worked beautifully, giving me the following table - note that Memo is being created as NVARCHAR(MAX):
CREATE TABLE dbo.Foo (
Id INT IDENTITY NOT NULL,
Name NVARCHAR(100) NOT NULL,
Memo NVARCHAR(MAX) NOT NULL,
Email NVARCHAR(255) NOT NULL,
CONSTRAINT [PK_Foo] PRIMARY KEY (Id)
)
Then I enabled migrations and added one, and got the following table definition in Up():
CreateTable(
"dbo.Foo",
c => new
{
Id = c.Int(nullable: false, identity: true),
Name = c.String(nullable: false, maxLength: 100),
Memo = c.String(nullable: false, maxLength: 4000),
Email = c.String(nullable: false, maxLength: 255),
})
.PrimaryKey(t => t.Id)
Wha? Where'd that maxLength: 4000 come from? It's not the MAX I would expect, nor is it even the 1024 I configured as the default with my first convention. It's certainly respecting the 100 and 255 of Name and Email (proving my attribute-based convention is working)...
So, is this a bug in the framework, or is there something about migrations that I'm not understanding? And either way, what can I do to get Code First Migrations to respect my IsMaxLength()?

Answer 1

Apparently IsMaxLength() has a different meaning in the context vs in a migration. In the context, it creates a column of NVARCHAR(MAX) but once you enable migrations this is interpreted as "the biggest possible value the column allows, except for MAX". As #marc_s points out in a comment, this is 4000 for a nvarchar column.
Indeed, while I have read this works outside of migrations, setting HasMaxLength(int.MaxValue) and calling Add-Migration gives me the following error:
(0,0) : error 0026: MaxLength '2147483647' is not valid. Length must be between '1' and '4000' for 'nvarchar' type.
Playing around a little more, I decided to try HasColumnType("nvarchar(MAX)"), which to my surprise worked! So replacing the call in the convention above works.
I can't find any way to read out the result of IsMaxLength() anywhere where I can set the type (for example IStoreModelConvention is too late), so it's not the prettiest solution, but at least I can move forward.