How do you write assertions in NUnit?

NUnit uses constraint-based assertions with Assert.That() for readable and expressive test validation.

Basic Assertions:

[Test]
public void TestAssertions()
{
    int result = Calculator.Add(2, 3);
    
    // Equality
    Assert.That(result, Is.EqualTo(5));
    
    // Comparison
    Assert.That(result, Is.GreaterThan(0));
    Assert.That(result, Is.LessThanOrEqualTo(10));
    
    // Null/Not Null
    Assert.That(result, Is.Not.Null);
    
    // Boolean
    Assert.That(result > 0, Is.True);
    
    // String assertions
    string message = "Hello World";
    Assert.That(message, Does.Contain("World"));
    Assert.That(message, Does.StartWith("Hello"));
}