EF migration for changing data type of columns

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You have a default constraint on your column. You need to first drop the constraint, then alter your column.

public override void Up()
{
    Sql("ALTER TABLE dbo.Received DROP CONSTRAINT DF_Receiv_FromN__25869641");
    AlterColumn("dbo.Received", "FromNo", c => c.String());
    AlterColumn("dbo.Received", "ToNo", c => c.String());
    AlterColumn("dbo.Received", "TicketNo", c => c.String());
}

You will probably have to drop the default constraints on your other columns as well.

I’ve just seen Andrey’s comment (I know – very late) and he is correct. So a more robust approach would be to use something like:

 DECLARE @con nvarchar(128)
 SELECT @con = name
 FROM sys.default_constraints
 WHERE parent_object_id = object_id('dbo.Received')
 AND col_name(parent_object_id, parent_column_id) = 'FromNo';
 IF @con IS NOT NULL
     EXECUTE('ALTER TABLE [dbo].[Received] DROP CONSTRAINT ' + @con)

I know this probably doesn’t help the OP but hopefully it helps anyone else that comes across this issue.

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