CharliePoole edited this page Oct 11, 2016 · 3 revisions
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CollectionOrderedConstraint tests that an IEnumerable is ordered. If the actual value passed does not implement IEnumerable, an exception is thrown.

The constraint supports both simple and property-based ordering (Ordered.By).

Simple Ordering

Simple ordering is based on the values of the items themselves. It is implied when the By modifier is not used.

int[] iarray = new int[] { 1, 2, 3 };
Assert.That(iarray, Is.Ordered);

string[] sarray = new string[] { "c", "b", "a" };
Assert.That(sarray, Is.Ordered.Descending);

The following modifiers are supported:

...Using(IComparer comparer)
...Using<T>(IComparer<T> comparer)
...Using<T>(Comparison<T> comparer)

By default, the order is expected to be ascending.

Property-Based Ordering

Property-based ordering uses one or more properties that are common to every item in the enumeration. It is used when one or more instances of the By modifier appears in the ordering expression.

string[] sarray = new string[] ( "a", "aa", "aaa" );
Assert.That(sarray, Is.Ordered.By("Length"));

string[] sarray2 = new string[] ( "aaa", "aa", "a" );
Assert.That(sarray2, Is.Ordered.Descending.By("Length"));

The following Modifiers are supported:

...By(string propertyName)
...Using(IComparer comparer)
...Using<T>(IComparer<T> comparer)
...Using<T>(Comparison<T> comparer)

Ordering on Multiple Properties

An ordering expression may use multiple By modifiers, each referring to a different proproperty. The following examples assume a collection of items with proerties named A and B.

Assert.That(collection, Is.Ordered.By("A").Then.By("B"));
Assert.That(collection, Is.Ordered.By("A").Then.By("B").Descending);
Assert.That(collection, Is.Ordered.Ascending.By("A").Then.Descending.By("B"));
Assert.That(collection, Is.Ordered.Ascending.By("A").By("B").Descending);
Assert.That(collection, Is.Ordered.Ascending.By("A").Descending.By("B")); // Illegal!


  1. The Then modifier divides the expression into ordering steps. Each step may optionally contain one Ascending or Descending modifier and one Using modifier.
  2. If Then is not used, each new By modifier marks the beginning of a step. The last example statement is illegal because the first group contains both Ascending and Descending. Use of Then is recommended for clarity.