C# “internal” access modifier when doing unit testing

I'm new in unit testing and I'm trying to figure out if I should start using more of 'internal' access modifier. I know that if we use 'internal' and set the assembly variable 'InternalsVisibleTo', we can test functions that we don't want to declare public from the testing project. This makes me think that I should just always use 'internal' because at least each project (should?) has it's own testing project. Can you guys tell me a reason why I shouldn't do this? When should I use 'private'?

EricSchaefer

Internal classes need to be tested and there is an assemby attribute:

using System.Runtime.CompilerServices;

[assembly:InternalsVisibleTo("MyTests")]

Add this to the project info file, e.g. Properties\AssemblyInfo.cs.

Brian Rasmussen

If you want to test private methods, have a look at PrivateObject and PrivateType in the Microsoft.VisualStudio.TestTools.UnitTesting namespace. They offer easy to use wrappers around the necessary reflection code.

Docs: PrivateType, PrivateObject

For VS2017 & 2019, you can find these by downloading the MSTest.TestFramework nuget

You can use private as well and you can call private methods with reflection. If you're using Visual Studio Team Suite it has some nice functionality that will generate a proxy to call your private methods for you. Here's a code project article that demonstrates how you can do the work yourself to unit test private and protected methods:

http://www.codeproject.com/KB/cs/testnonpublicmembers.aspx

In terms of which access modifier you should use, my general rule of thumb is start with private and escalate as needed. That way you will expose as little of the internal details of your class as are truly needed and it helps keep the implementation details hidden, as they should be.

Keep using private by default. If a member shouldn't be exposed beyond that type, it shouldn't be exposed beyond that type, even to within the same project. This keeps things safer and tidier - when you're using the object, it's clearer which methods you're meant to be able to use.

Having said that, I think it's reasonable to make naturally-private methods internal for test purposes sometimes. I prefer that to using reflection, which is refactoring-unfriendly.

One thing to consider might be a "ForTest" suffix:

internal void DoThisForTest(string name)
{
    DoThis(name);
}

private void DoThis(string name)
{
    // Real implementation
}

Then when you're using the class within the same project, it's obvious (now and in the future) that you shouldn't really be using this method - it's only there for test purposes. This is a bit hacky, and not something I do myself, but it's at least worth consideration.

Adding to Eric's answer, you can also configure this in the csproj file:

<ItemGroup>
    <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
      <_Parameter1>MyTests</_Parameter1>
    </AssemblyAttribute>
</ItemGroup>

Or if you have one test project per project to be tested, you could do something like this in your Directory.Build.props file:

<ItemGroup>
    <AssemblyAttribute Include="System.Runtime.CompilerServices.InternalsVisibleTo">
      <_Parameter1>$(MSBuildProjectName).Test</_Parameter1>
    </AssemblyAttribute>
</ItemGroup>

See: https://stackoverflow.com/a/49978185/1678053

来源：https://stackoverflow.com/questions/358196/c-sharp-internal-access-modifier-when-doing-unit-testing

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