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+# Custom constraints 
+
+`NUnit` allows you to write test assertions in a more descriptive and human readable way using the [Assert.That](https://github.com/nunit/docs/wiki/Assertions) mechanism, where the first parameter is an object under test and the second parameter describes conditions that the object has to meet. 
+
+## Is
+
+We’ve extended `NUnit` API with a custom constraint type and declared an overlay `Is` class. To resolve ambiguity between the original implementation and the custom one you must explicitly declare it with a using statement or via addressing through the full type name `UnityEngine.TestTools.Constraints.Is`.
+
+### Static Methods
+
+| Syntax               | Description                                                  |
+| -------------------- | ------------------------------------------------------------ |
+| `AllocatingGCMemory` | A constraint type that invokes the delegate you provide as the parameter of `Assert.That` and checks whether it causes any GC memory allocations. It passes if any GC memory is allocated and fails if not. |
+
+## Example
+
+```c#
+using Is = UnityEngine.TestTools.Constraints.Is;
+
+class MyTestClass
+{
+    [Test]
+    public void MyTest()
+    {
+        Assert.That(() => {
+            var i = new int[500];
+        }, Is.AllocatingGCMemory());
+    }
+}
+```
+