EF 4.1 RTW Change to Default MaxLength in Code First


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We recently announced the release of ADO.NET Entity Framework 4.1 Release Candidate which included the first go-live release of Code First. The final Release to Web (RTW) is planned to be released approximately a month after the Release Candidate (RC) was published. Between RC and RTW we are not adding any new features and are focusing on fixing any bugs or issues reported by folks using the RC.


The Issue

One issue that has been raised by a number of folks is the side effects of changing the default maximum length of strings and arrays from 4000 in CTP5 to 128 in RC.

In particular this causes issues when mapping to an existing database with Code First because the new validation feature will ensure that all string and array data is shorter than 128 before trying to save. This results in a DbEntityValidationException stating:

 “Validation failed for one or more entities. See 'EntityValidationErrors' property for more details.”

Inspecting the validation result states that:

"The field <property name>must be a string or array type with a maximum length of '128'."

Besides the inconvenience of having to explicitly specify the max length of most columns when mapping to an existing database we are hearing that 128 is just too short for a default and we need to chose something more appropriate.


The RC Workaround

There are two ways to resolve this issue in the RC release:

  • Disable validation by setting ‘DbContext.Configuration.ValidateOnSaveEnabled’ to ‘false’
    By doing this Code First will not attempt to verify the length of string/array data and even though it thinks the max length is 128 it will leave it up to the database to decide if it can store that data or not.
  • Configure the correct string length for each string property using the MaxLength data annotation or the HasMaxLength method in the fluent API.
    This is the most technically correct solution, but we realize that configuring a length for every string/array property in your model is often going to result in a lot of repetitive configuration.


The RTW Change

Based on this feedback we are proposing to change the default length of non-key properties to be ‘Max’ (this equates to varchar(max) and nvarchar(max) when running against MS SQL Server). Key properties and TPH discriminator columns will remain with a default max length of 128.

SQL Compact does not support ‘Max’ columns, when running against SQL Compact an additional Code First convention will set a default max length of 4000.

Introducing this change will cause a model generated by EF 4.1 RTW to be different from one generated by EF 4.1 RC. This means that in scenarios where Code First is generating the database you will need to re-create the database with the new column types. This only affects scenarios where the default length is used, if you have explicitly configured a maximum length then Code First will continue to honor it. If your model changes as a result of the new max length default you would receive the following exception when running the application:

“The model backing the <derived context type name> context has changed since the database was created. Either manually delete/update the database, or call Database.SetInitializer with an IDatabaseInitializer instance. For example, the RecreateDatabaseIfModelChanges strategy will automatically delete and recreate the database, and optionally seed it with new data."


As always, we thank you for your continued feedback that is helping us shape this exciting release.

Rowan Miller
Program Manager
ADO.NET Entity Framework