DatabaseGeneratedOption.Identity not generating an Id

问题

Using EntityFramework code-first, I've created a simple Foo table. Here's my entity:

public class Foo
{
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public virtual string Id { get; set; }

    public virtual string Name { get; set; }
}

However whenever I try to insert a new row, I get a Cannot insert the value NULL into column 'Id'. Why is this happening when I've added a DatabaseGenerated attribute? Deleting and recreating my table makes no difference.

回答1:

Identities for string column types are not supported with SQL Server. (How do you expect such a string to look like?) To get this working you could - for example - add a computed column/user defined function in SQL Server that formats a string from an ordinary int identity column - as shown here.

回答2:

you forgot the Key attribute. and there is no need to use virtual for primary key.
as mentioned by Slauma you can't use Identity for string datatype.

Try this code:

public class Foo
{
    [Key]
    [DatabaseGenerated(DatabaseGeneratedOption.Identity)]
    public int Id { get; set; }

    public virtual string Name { get; set; }
}

回答3:

For anybody reading this in 2020, 'Identity' attributes for string column types are supported using Entity Framework. For example, decorating a string type property in your C# class with the [DatabaseGenerated(DatabaseGeneratedOption.Identity)] attribute tag, then letting Entity Framework create the table through a database-first migration, and inserting that record through your program with Entity Framework methods (AddAsync(), SaveAsync()), auto-generates a string that looks like e.g. 'b652daab-9bb9-5143-a1ae-97a89232ea38'.

MS SQL Server will not auto generate this value however (I trialled it with an Insert SQL statement). Instead, the program / Entity Framework seems to generate it.

回答4:

I know that is an old thread, but for anyone in the future:

DatabaseGenerated(DatabaseGeneratedOption.Identity)

does not generate any values, it simply let EF know that DB suppose to generate the value as described in the MS documentation. Therefore you must either set a default value in the database, in your model or assign an actual value in the controller.

来源：https://stackoverflow.com/questions/22027701/databasegeneratedoption-identity-not-generating-an-id

标签

asp.net-mvc

entity-framework