Cannot insert explicit value for identity column in table 'TABLE' when IDENTITY_INSERT is set to OFF

I have an existing database table that doesn’t allow identity inserts. When I try to add a new item to the database, I get the following error message:

{"Cannot insert explicit value for identity column in table 'ITEMS' when IDENTITY_INSERT is set to OFF."}

My simplified service code:

public Item AddItem(Item item)
{
        db.Items.Add(item);

        if(db.SaveChanges() > 0)
        {
            return item;
        }

        return null;
    }
}

My OnModelCreating override:

protected override void OnModelCreating(ModelBuilder builder)
{
    builder.Entity<Items>()
        .Key(e => e.Id);

    builder.Entity<Items>(e =>
       {
           e.Metadata.SqlServer().Table = "ITEMS";
           e.Property(c => c.Id).Metadata.SqlServer().Column = "ITEM_ID";
           e.Property(c => c.Id).Metadata.SqlServer().ColumnType = "numeric(18,0)";
           e.Property(c => c.Id).GenerateValueOnAdd(true);
           e.Property(c => c.Name).Metadata.SqlServer().Column = "ITEM_NAME";
           e.Property(c => c.Name).MaxLength(50).Required();
           e.Property(c => c.LastModified).Metadata.SqlServer().Column = "LAST_MODIFIED_DTM";
       });
}

And lastly, the SQL Command that is being executed against SqlServer:

--The data may be truncated and may not represent the query that was run on the server
USE [database];

GO

--Type and value data was not available for the following variables. Their values have been set to defaults.
DECLARE @p0 AS SQL_VARIANT;
DECLARE @p1 AS SQL_VARIANT;
DECLARE @p2 AS SQL_VARIANT;
SET @p0 = NULL;
SET @p1 = NULL;
SET @p2 = NULL;

SET NOCOUNT OFF;
INSERT INTO [ITEMS] ([ITEM_ID], [LAST_MODIFIED_DTM], [ITEM_NAME])
VALUES (@p0, @p1, @p2);
SELECT @@ROWCOUNT;

Because Identity Insert is turned off, I can’t have the Item ID included when an insert occurs. Is there a proper way to deal with this scenario?

[jpwits]

@scottengle You can generate your own Id, this will be an issue on a multi user system, too keep it in sync, actually kind of impossible, in that case u can auto generate a guid as your Id, but this is dirty on a central db.

[jpwits]

@scottengle , have not investigated this, but that e.Property(c => c.Id).GenerateValueOnAdd(true); is probably only gonna work with Identity Insert ‘ON’, else what will generate the ID?