Are you developing a .NET library and want to make sure that the public API of your library is always consistent and versioned correctly? How do you check that changes have not caused a change in the public API of your library?

I’ve tried a combination of the PublicApiGenerator library and approval tests (in my case Verify) to do this. This combination ensures that if my library’s public API changes, I know about it and am forced to think about it:

  • whether the change is in line with the library’s vision
  • whether the new `public’ thing means I’ll have to take care of it (I will anyway 😊)
  • whether the change is properly documented
  • whether the right version of the library is chosen (hasn’t there been a breaking change?)
  • …

Let’s have a simple library that contains one class Calculator with one method Add:

public class Calculator
{
    public int Add(int a, int b) => a + b;
}

In the test project we add PublicApiGenerator and Verify (for me it is Verify.XUnit):

dotnet add package PublicApiGenerator
dotnet add package Verify.Xunit

And let’s write a test to check if the library’s public API has changed:

public class ApiVersionChangeTest
{
    [Fact]
    public Task ApproveApiVersion()
    {
        var publicApi = typeof(Calculator)
            .Assembly
            .GeneratePublicApi(); // 👈 Generate Public API string

        return Verify(publicApi); // 👈 Verify public API
    }
}

When we add a new method Subtract to the Calculator class:

public class Calculator
{
    public int Add(int a, int b) => a + b;
    public int Subtract(int a, int b) => a - b;
}

The test fails and we see that the public API has changed:

 ········public·Calculator()·{·}
 ········public·int·Add(int·a,·int·b)
+········public·int·Subtract(int·a,·int·b)·{·}