Contains Constraint

ContainsConstraint is a smart constraint that adapts its behavior based on the type being tested. When the actual value is a string, it tests for a substring. When the actual value is a collection, it tests for item membership.

Usage

Does.Contain(object expected)
Does.Not.Contain(object expected)

Modifiers

.IgnoreCase                  // Case-insensitive matching (strings and collections)
.IgnoreWhiteSpace            // Ignore whitespace in item equality (collections only)
.IgnoreLineEndingFormat      // Ignore line endings in item equality (collections only)
Note

.IgnoreWhiteSpace and .IgnoreLineEndingFormat only work when testing collection membership, not substring containment. For strings, these modifiers throw an InvalidOperationException.

Examples

String Containment

When the actual value is a string, Does.Contain tests for a substring:

[Test]
public void ContainsConstraint_String_Examples()
{
    Assert.That("Hello World", Does.Contain("World"));
    Assert.That("Hello World", Does.Contain("world").IgnoreCase);
    Assert.That("Hello World", Does.Not.Contain("Goodbye"));
}

Collection Membership

When the actual value is a collection, Does.Contain tests for item membership:

[Test]
public void ContainsConstraint_Collection_Examples()
{
    int[] numbers = { 1, 2, 3, 4, 5 };
    Assert.That(numbers, Does.Contain(3));
    Assert.That(numbers, Does.Not.Contain(99));

    string[] names = { "Alice", "Bob", "Charlie" };
    Assert.That(names, Does.Contain("Bob"));
    Assert.That(names, Does.Contain("bob").IgnoreCase);
}

How It Works

ContainsConstraint defers its behavior until the actual value's type is known:

Actual Type Behavior
string Uses SubstringConstraint to test for substring
Collection/IEnumerable Uses SomeItemsConstraint to test for item presence

Notes

  1. When testing strings, the expected value must also be a string. For type safety, consider using Does.Contain(string) explicitly.

  2. Important: .IgnoreWhiteSpace and .IgnoreLineEndingFormat only work with collections, not strings. When used with string containment, they throw InvalidOperationException. This is because SubstringConstraint doesn't support these modifiers, while EqualConstraint (used for collection item comparison) does.

  3. For collections of strings, .IgnoreWhiteSpace affects item equality comparison:

    // Works: checks if any item equals "hello world" ignoring whitespace
Assert.That(new[] { "hello  world" }, Does.Contain("hello world").IgnoreWhiteSpace);

  4. For more explicit control, use the specific constraints directly:

See Also