Migration Guide for NUnit 4.0 (#827)

* Migration Guide for NUnit 4.0 * Linting fixes * Remove wrong example of a 3rd party library depending on nunit * Add TOC entry for article * Small phrasing update * Change to note formatting * minor word change * Fix typo * minor formatting * NUnit4 -> NUnit 4.x * Update link name. * Minor word choice * Add space * Update other image paths * Turn into non-version-specific migration guide * Add link in NUnit 4 article * formatting updates * Fix note formatting --------- Co-authored-by: Sean Killeen <[email protected]>
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diff --git a/docs/articles/nunit/Towards-NUnit4.md b/docs/articles/nunit/Towards-NUnit4.md
@@ -9,12 +9,17 @@ NUnit 4 has been long-awaited, and we are now starting to see its outline taking
 scheme. This entails aiming to release version 4 as soon as possible and subsequently accelerating the pace of new major
 releases compared to previous versions.
 
-We'd like to bring your attention to several interesting aspects of NUnit 4. First and foremost, there is a crucial
-[NUnit 4 planning issue](https://github.com/nunit/nunit/issues/3325) that we want to highlight.  Additionally, we have
-an upcoming [release notes
-page](https://github.com/nunit/docs/blob/62c43cbbd32b8424c974d5ec50d5463a5c4cd621/docs/articles/nunit/release-notes/framework.md),
-currently in the form of a PR (Pull Request). If you're interested in changes related to supported frameworks and assert
-messages, we've compiled a [list of issues](https://github.com/nunit/nunit/issues/4431) for your reference.  
+We'd like to bring your attention to several interesting aspects of NUnit 4.
+
+* First and foremost, there is a crucial [NUnit 4 planning issue](https://github.com/nunit/nunit/issues/3325) that we
+  want to highlight.
+* Additionally, we have an upcoming [release notes
+  page](https://github.com/nunit/docs/blob/62c43cbbd32b8424c974d5ec50d5463a5c4cd621/docs/articles/nunit/release-notes/framework.md),
+  currently in the form of a PR (Pull Request).
+* If you're interested in changes related to supported frameworks and assert messages, we've compiled a [list of
+  issues](https://github.com/nunit/nunit/issues/4431) for your reference.  
+* We have also created a [Migration Guide](xref:migrationguidance) starting with 4.x that we hope will assist in
+  navigating any breaking changes.
 
 Moreover, we have created a milestone for version 4, [which you can find a list of open issues for
 here](https://github.com/nunit/nunit/issues?q=is%3Aopen+is%3Aissue+milestone%3A4.0). This milestone could be useful in

diff --git a/docs/articles/nunit/release-notes/Nunit4.0-MigrationGuide.md b/docs/articles/nunit/release-notes/Nunit4.0-MigrationGuide.md
@@ -0,0 +1,182 @@
+---
+uid: migrationguidance
+---
+
+# Migration Guidance
+
+## 3.x -> 4.x
+
+NUnit 4.0 has a few [breaking changes](../release-notes/breaking-changes.md#nunit-40) making it neither binary nor
+source code compatible with NUnit 3.14.0
+
+* Change to [Classic Asserts](../writing-tests/assertions/assertion-models/classic.md)
+* Removal of `Assert.That` overloads with _format_ specification and `params`.
+
+### Classic Assert migration
+
+There are different ways to migrate these to NUnit 4.0
+
+* Convert Classic Assert to the [Constraint model](../writing-tests/assertions/assertion-models/constraint.md)
+* Update source code to new namespace and class name
+* Using `global using` aliases
+
+In the sections below we use the following simple test as an example:
+
+```csharp
+public class Tests
+{
+    [Test]
+    public void TestSomeCalculation()
+    {
+        int actual = SomeCalculation();
+        Assert.AreEqual(42, actual, "Expected {0} to be 42", actual);
+        StringAssert.StartsWith("42", actualText, "Expected '{0}' to start with '42'", actualText);
+    }
+
+    private static int SomeCalculation() => 42;
+}
+```
+
+#### Convert Classic Assert to Constraint Model
+
+Although the code can be converted manually, that is a lot of work.
+
+Luckily, the [NUnit.Analyzer](https://www.nuget.org/packages/NUnit.Analyzers) has had rules and associated code fixes
+for a while now. Version _3.10.0_ knows about the 2nd non-backward compatible change and will convert the _format_
+specification and `params` into a `FormattableString`.
+
+> [!NOTE]
+> **Caveat**: The analyzers only run when the code compiles, so execute and act on the analyzer _before_
+> upgrading `nunit` to version `4.0.0`!
+
+In our example code, the analyzer will flag the `Assert.AreEqual` as shown below:
+
+![NUnit.Analyzer Classic Assert Warning](../../../images/NUnit.Analyzer-ClassicAssert-Warning.png)
+
+Running the associated `Transform to constraint model` code fix in Visual Studio will convert the code into:
+
+```csharp
+public class Tests
+{
+    [Test]
+    public void TestSomeCalculation()
+    {
+        int actual = SomeCalculation();
+        Assert.That(actual, Is.EqualTo(42), $"Expected {actual} to be 42");
+    }
+
+    private static int SomeCalculation() => 42;
+}
+```
+
+The analyzer code fix supports _Batch Fixing_:
+
+![NUnit.Analyzer Classic Assert CodeFix](../../../images/NUnit.Analyzer-ClassicAssert-CodeFix.png)
+
+This allows changing all corresponding `Assert` usages for a document, project or a complete solution.
+
+There are many classic asserts and most of these come with a separate code analyzer rule and code fix. Although it
+allows full configuration to what classic asserts to keep or convert, it means that a developer has to repeat this
+process multiple times, once for each assert method.
+
+NUnit.Analyzer also has code fixers for `CollectionAssert` and `StringAssert`.
+
+```csharp
+string actualText = actual.ToString();
+StringAssert.StartsWith("42", actualText, "Expected '{0}' to start with '42'", actualText);
+```
+
+Will be converted into:
+
+```csharp
+string actualText = actual.ToString();
+Assert.That(actualText, Does.StartWith("42"), $"Expected '{actualText}' to start with '42'");
+```
+
+There are no code fixers for `FileAssert` and `DirectoryAssert`. They could be added, but we don't expect these to be
+used too much.
+
+#### Convert Classic Assert into NUnit 4.x equivalent
+
+If you want to keep the Classic Asserts and not convert them to the constraint model -- but do want to use the new NUnit
+4.x naming -- you'll need to update the code manually.
+
+The NUnit.Analyzer can't help here as the code either doesn't compile before the change or after, depending on what
+version of `nunit` you are compiling with.
+
+If you _only_ use classic asserts, you can get away with a couple of global substitutes:
+
+1. Convert `using NUnit.Framework;` into _both_ `using NUnit.Framework;using NUnit.Framework.Legacy;`
+
+   Depending on your editor you can automatically insert a newline between the two `using` statements.
+1. Convert `Assert` into `ClassicAssert`.
+
+   This global substitute will also convert those asserts that have not changed. You can narrow the scope of this
+substitute to do only the asserts that need converting, but there are quite a lot.
+
+   1. Convert `Assert.AreEqual` into `ClassicAssert.AreEqual`.
+   1. Convert `Assert.True` into `ClassicAssert.True`.
+   1. Similar for `IsTrue`, `False`, `IsFalse`, `Greater`, `Less`, ...
+
+   Depending on what is less work, alternatively you can reverse the substitution of those that shouldn't be have been
+   changed:
+   1. Convert `ClassicAssert.That` into `Assert.That`.
+   1. Convert `ClassicAssert.Fail` into `Assert.Fail`.
+   1. Convert `ClassicAssert.Throws` into `Assert.Throws`.
+   1. Etc.
+
+   Or if you use Visual Studio, it will raise an
+   [IDE0002](https://learn.microsoft.com/en-us/dotnet/fundamentals/code-analysis/style-rules/ide0002) with a code fix
+   that can convert all of those that are not considered _classic_ back to assert in one swoop:
+
+   ![ClassicAssert to Assert](../../../images/IDE0002-ClassicAssert-into-Assert.png)
+
+#### Use `global using` aliases
+
+If you use SDK 6.0 or newer that supports C#10, you can upgrade without modifying any actual tests by adding the
+following aliases to `GlobalUsings.cs`
+
+```csharp
+global using Assert = NUnit.Framework.Legacy.ClassicAssert;
+global using CollectionAssert = NUnit.Framework.Legacy.CollectionAssert;
+global using StringAssert = NUnit.Framework.Legacy.StringAssert;
+global using DirectoryAssert = NUnit.Framework.Legacy.DirectoryAssert;
+global using FileAssert = NUnit.Framework.Legacy.FileAssert;
+```
+
+Note that this doesn't mean you have to target .NET 6.0. This also works if targeting .NET Framework as it is purely
+done on the source code level.
+
+### Assert.That with _format_ specification and `params` overload conversion
+
+These overloads were removed to allow for better messages in case of failure. See [The "Towards NUnit 4"
+article](../Towards-NUnit4.md#improved-assert-result-messages) for more information.
+
+NUnit 4.x has been optimized such that these formattable strings only get formatted in case the test is failing.
+
+```csharp
+int actual = SomeCalculation();
+Assert.That(actual, Is.EqualTo(42), "Expected {0} to be 42", actual);
+```
+
+Needs to be converted into:
+
+```csharp
+int actual = SomeCalculation();
+Assert.That(actual, Is.EqualTo(42), $"Expected {actual} to be 42");
+```
+
+To make this transition easier, the Nunit.Analyzer has been updated with a new rule and corresponding code-fix:
+
+![Replace Format Specification with Formattable String](../../../images/NUnit.Analyzer-ReplaceFormatSpecification.png)
+
+### Using NUnit Extension libraries
+
+If your code doesn't call `nunit` asserts directly but uses a local `NUnitExtension` library or a 3rd party one then
+that dependency needs to be upgraded _before_ you can upgrade your own code.
+
+If the library is not NUnit 4.0 compliant, you will get error messages like:
+
+```txt
+System.MissingMethodException : Method not found: 'Void NUnit.Framework.Assert.That(!!0, NUnit.Framework.Constraints.IResolveConstraint, System.String, System.Object[])'.
+```
diff --git a/docs/articles/nunit/release-notes/breaking-changes.md b/docs/articles/nunit/release-notes/breaking-changes.md
@@ -4,6 +4,18 @@ uid: breakingchanges
 
 # Breaking Changes
 
+## NUnit 4.0
+
+* The [Classic Asserts](../writing-tests/assertions/assertion-models/classic.md) have been moved to a separate library
+  and their namespace and their class name were renamed to: `NUnit.Framework.Legacy.ClassicAssert`.
+* The standalone assert classes have also been moved to the `NUnit.Framework.Legacy` namespace. These classes are:
+  * [CollectionAssert](../writing-tests/assertions/classic-assertions/Collection-Assert.md)
+  * [StringAssert](../writing-tests/assertions/classic-assertions/String-Assert.md)
+  * [DirectoryAssert](../writing-tests/assertions/classic-assertions/Directory-Assert.md)
+  * [FileAssert](../writing-tests/assertions/classic-assertions/File-Assert.md)
+* Assert.That overloads with _format_ specification and `params` have been removed in favor of an overload using
+  `FormattableString`.
+
 ## NUnit 3.10
 
 * `NUnit.Framework.Constraints.NUnitEqualityComparer.Default` was deprecated in favor of `new NUnitEqualityComparer()`.

diff --git a/docs/articles/nunit/release-notes/toc.yml b/docs/articles/nunit/release-notes/toc.yml
@@ -2,6 +2,8 @@
   href: framework.md
 - name: Console and Engine
   topicUid: consoleenginereleasenotes
+- name: Migration Guidance
+  topicUid: migrationguidance
 - name: Breaking Changes
   topicUid: breakingchanges
 - name: Pre-3.5 Release notes

diff --git a/docs/images/IDE0002-ClassicAssert-into-Assert.png b/docs/images/IDE0002-ClassicAssert-into-Assert.png
diff --git a/docs/images/NUnit.Analyzer-ClassicAssert-CodeFix.png b/docs/images/NUnit.Analyzer-ClassicAssert-CodeFix.png
diff --git a/docs/images/NUnit.Analyzer-ClassicAssert-Warning.png b/docs/images/NUnit.Analyzer-ClassicAssert-Warning.png
diff --git a/docs/images/NUnit.Analyzer-ReplaceFormatSpecification.png b/docs/images/NUnit.Analyzer-ReplaceFormatSpecification.png