Missing Assert Methods in NUnit 4: Resolution Guide

Problem

After upgrading to NUnit 4.x, you may encounter errors indicating missing definitions in the Assert class like:

'Assert' does not contain a definition for 'AreEqual'
'Assert' does not contain a definition for 'IsNotEmpty'
'Assert' does not contain a definition for 'IsTrue'

This occurs because NUnit 4 introduced a breaking change—classic Assert methods (AreEqual, IsTrue, etc.) were removed from the primary Assert class and moved to preserve backward compatibility while promoting a new testing paradigm.

Solutions

:material-code-brackets: Option 1: Use ClassicAssert (Quick Migration)

Add compatibility with older tests by referencing the NUnit.Framework.Legacy namespace:

1. Add this namespace declaration:

   using NUnit.Framework;
   using NUnit.Framework.Legacy; // Add this line

2. Replace Assert with ClassicAssert:

   // Before
   Assert.AreEqual(expected, actual);
   Assert.IsTrue(condition);
   
   // After
   ClassicAssert.AreEqual(expected, actual);
   ClassicAssert.IsTrue(condition);

:material-lightbulb-on: Option 2: Migrate to Assert.That (Recommended)

Adopt NUnit 4's modern constraint model using Assert.That:

// Classic syntax
Assert.AreEqual(expected, actual);
Assert.IsTrue(condition);
Assert.IsNotEmpty(collection);

// Modern equivalent
Assert.That(actual, Is.EqualTo(expected));
Assert.That(condition, Is.True);
Assert.That(collection, Is.Not.Empty);

Common Translations:

Classic Method	Modern Equivalent
Assert.AreEqual(a, b)	Assert.That(b, Is.EqualTo(a))
Assert.IsTrue(x)	Assert.That(x, Is.True)
Assert.IsFalse(x)	Assert.That(x, Is.False)
Assert.IsNull(x)	Assert.That(x, Is.Null)
Assert.IsNotNull(x)	Assert.That(x, Is.Not.Null)
Assert.IsNotEmpty(col)	Assert.That(col, Is.Not.Empty)
Assert.Greater(a, b)	Assert.That(a, Is.GreaterThan(b))

:material-alert: Option 3: Downgrade (Not Recommended)

Temporarily downgrade your NUnit version while transitioning:

1. Uninstall NUnit 4.x via NuGet
2. Install NUnit 3.13.3:

   Install-Package NUnit -Version 3.13.3

Key Takeaways

1. ClassicAssert is a bridge solution: Provides quick migration but delays full modernization
2. Constraint model (Assert.That) is the future: Improves test readability and flexibility
3. Namespace changes are critical:
   • Keep NUnit.Framework
   • Add NUnit.Framework.Legacy if using ClassicAssert

For advanced migration guidance, refer to the official NUnit 4 Migration Documentation.