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Let's say I'm planning to sell a .NET application in my website. If I need to use a library A (a reference to the compiled assembly, not the source code)...

Under which license could be A so I can do this?

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What are your objectives? – ohho Mar 25 '11 at 9:35

3 Answers

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If the library is licensed under LGPL (Lesser General Public License)

You're free to use the library however you want as long as you don't modify it or create any derived works from it.

Whether a work that uses an LGPL program is a derivative work or not is a legal issue. A standalone executable that dynamically links to a library is generally accepted as not being a derivative work (in LGPL). It would be considered a "work that uses the library" and paragraph 5 of the LGPL applies.

Here's Paragraph 5:

A program that contains no derivative of any portion of the Library, but is designed to work with the Library by being compiled or linked with it, is called a "work that uses the Library". Such a work, in isolation, is not a derivative work of the Library, and therefore falls outside the scope of this License.

That means, don't modify and compile the source. Just use the dll that the site provides and you can use it on whatever you want (even proprietary software).

Most open source libraries should be covered by this license (unless they're authored by a GPL nazi).

The MIT License is the other licence you may encounter a lot

MIT is significantly more liberal than the GPL (General Public License) or even the LGPL (Lesser General Public License).

It's so simple I can quote the whole thing directly here:

Copyright (C) by

Permission is hereby granted, free of charge, to any person obtaining a copy of this software and associated documentation files (the "Software"), to deal in the Software without restriction, including without limitation the rights to use, copy, modify, merge, publish, distribute, sublicense, and/or sell copies of the Software, and to permit persons to whom the Software is furnished to do so, subject to the following conditions:

The above copyright notice and this permission notice shall be included in all copies or substantial portions of the Software.

THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY, FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM, OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN THE SOFTWARE.

Basically, you can do what ever the hell you want with it for free (including modify and sell it) as long as you maintain the rights that anybody can do whatever the hell they want with it for free.

When I write open source software I prefer this license because it doesn't contain the 'viral' traits and (IMHO) legal sleaziness that GPL does. If I'm going to go to the effort to make it free, I might as well make it truly free.

Note: With GPL if you use it, not only do you also have to license your project under GPL but you have to share the source of any changes you made to the source. I don't see much freedom in that.

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up vote 1 down vote

There are many licenses that would fit this, see the licenses under "Release changes under a different license" here: Comparison of free software licences

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You didn't say that you intend to keep your own software closed source (people do sell open source software), but I assume that's what you meant or you probably wouldn't have asked the question.

Practically all open source software that is distributed as a library (i.e. you don't have to recompile their code to use it, just link to it) has a license that permits your intended use, but you should give it a once over or post the details here when you select a specific one, because there are exceptions, mysql being perhaps the most notable. If you are also distributing the library, not just instructing your users to download it from the original site, you usually also need to provide a link to download the library's source, even if you don't have to open your own source. Also, there may be attribution requirements to keep in mind.

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This advice does not agree with the standard interpretation of the GPL as I understand it. – Alison Mar 25 '11 at 9:15
@Alison, you're right, but we're talking about libraries here, not regular software. Almost no open source libraries use the GPL for precisely these sorts of reasons. – Karl Bielefeldt Mar 25 '11 at 12:10
@Alison @Karl The library license you're referring to is the LGPL (Lesser General Public License) which is the one where you can link to a library freely (Ie you can use it however you want even in proprietary software without including the source code or license) as long as the library isn't modified in any way from its original (if it is it's considered a derivative work and needs to include source/licence/ect with distribution). LGPL is what the majority of the people writing libraries use. – Evan Plaice Mar 25 '11 at 13:17
@Karl @Evan Sorry, read that in a hurry - like you say most have licences that make linking ok, but again this isn't a given. MySQL is possibly a poor example, as MySQL connectors have a linking exception in their licence. – Alison Mar 25 '11 at 13:26
@Alison MySQL is also controlled by a company who sells a derivative version of it for profit. Open source projects that are developed under the thumb of a large corporation (and have very expensive pay versions that are nothing but licenses that unlock its full capabilities) usually have tricky licensing schemes with hidden holes. It's important to recognize this and understand the details before making a selection. – Evan Plaice Mar 25 '11 at 13:32
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