What are the arguments for creating you own ORM layer? [closed]

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The advantages of ORM are pretty clear. But I noticed that some companies prefer to build their own home made ORM. Why?

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There are only two arguments that I can possibly see for ever hand-rolling your ORM (and these have happened to me in the past, which forced me to write my own):

1. The company refuses to use Open Source software because of liabilities they assume might creep into their application.

2. The company refuses to spend money on a commercial ORM.

Any other argument (like the quality of Entity Framework is too poor for us to use it) is completely moot. No matter how bad Entity Framework (or whatever other ORM you may be referring to) is, you're not going to come close to the robustness and reliability by hand rolling your own.

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As O/R mappers are very complex pieces of software, writing your own which goes beyond the typical datareader wrapper and pre-fab SQL query executor will take a lot of time (think 6+ months full time at least). That's not the biggest problem. The biggest problem is that once you go with your own O/R mapper, you have to maintain it for the rest of the time the application using it is in production. Which can be a long time. Make no mistake, maintaining an O/R mapper yourself is not a simple task: you have to re-invent every trick O/R mapper developers already know about and have solved themselves.

Last but not least: doing this yourself should not be done on a billable contract. After all, you're writing infrastructure code which is already available elsewhere.

I know I'm biased (I wrote LLBLGen Pro), but I also am one of the few people in this industry who has written a full O/R mapper framework and knows what it takes to get a decent one up and running with good performance and a great feature set.

Simply do the math: if it takes 1000$ to get an o/r mapper framework license (or less) and you can get started right away with the application of your customer, how many hours do you get for that 1000$ so you can built the O/R mapper without costing the company any money? And maintain it? No way you can do it for that money.

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If you have an in-house database that has evolved to have a bad schema, it can be simpler to write your own ORM layer than try and get an out of the box solution to play nice with it.

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In my opinion, ORMs are specialized and purposed to solve typical problems. If you want some more generic solution (e.g. for much more complex queries) or just different functionality you can either modify existing solution (what for various reasons often isn't the best choice) or create your own.

ORMs also limit you by forcing you to use their conventions and accept their limitations.