Are you already using SOLID principles in your code? Check out Telerik JustMock Free Edition. It's even available on NuGet!
You may be thinking to yourself, "Why bother when there are so many other alternatives?" I will give you just a few reasons...
JustMock Free is the best free mocking framework (try it out for yourself).
It's supported by a well-known and trusted vendor, Telerik.
It's the brainchild of Mehfuz Hossain, author of awesome open source frameworks such as AutoBox.
Here's the most important reason: if you find yourself in need of an extremely powerful mocking framework that can mock things like static classes, LINQ queries, and inject them regardless of how the code is constructed, your developers will already be familiar with the framework when you use the commercial edition.
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Read more: Looking for a Free .NET Mocking Framework?