Test with NO expected exception
Solution 1:
Assert.DoesNotThrow(() => { /* custom code block here*/});
OR just method
Assert.DoesNotThrow(() => CallMymethod());
For more details see NUnit Exception Asserts
Solution 2:
Using NUnit 3.0 Constraint Model type assertions the code would look as follows:
Assert.That(() => SomeMethod(actual), Throws.Nothing);
This example is taken from NUnit wiki.
Solution 3:
Not throwing an exception is the normal course of action. Your test will successfully verify that an exception is not thrown.