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A Theory is a special type of test, used to verify a general
statement about the system under development. Normal tests are
*example-based*. That is, the developer supplies one or
more examples of inputs and expected outputs either within the
code of the test or - in the case of
Parameterized Tests
- as arguments to the test method. A theory, on the other hand,
makes a general statement that all of its assertions will pass
for all arguments satisfying certain assumptions.

Theories are implemented in NUnit
as methods within a **TestFixture**, which are annotated
with the **TheoryAttribute**. Theory methods must always have
arguments and therefore appears quite similar to
Parameterized Tests at first glance. However, a Theory incorporates additional data sources
for its arguments and allows special processing for assumptions
about that data. The key difference, though, is that theories
make general statements and are more than just a set of examples.

The primary source of data for a **Theory** is the
**Datapoint** or **Datapoints** attribute.
NUnit will use any fields of the required types, which are annotated
with one of these attributes, to provide data for each parameter
of the Theory. NUnit assembles the values for individual arguments
combinatorially to provide test cases for the theory.

In addition to the Datapoint and Datapoints attributes, it is possible to use any of the approaches for supplying data that are recognized on normal parameterized tests. We suggest that this capability not be overused, since it runs counter to the distinction between a test based on examples and a theory. However, it may be useful in order to guarantee that a specific test case is included.

The theory itself is responsible for ensuring that all data supplied
meets its assumptions. It does this by use of the
**Assume.That(...)** construct, which works just like
**Assert.That(...)** but does not cause a failure. If
the assumption is not satisfied for a particular test case, that case
returns an Inconclusive result, rather than a Success or Failure.

The overall result of executing a Theory over a set of test cases is determined as follows:

- If the assumptions are violated for
**all**test cases, then the Theory itself is marked as a failure. - If any Assertion fails, the Theory itself fails.
- If at least
**some**cases pass the stated assumptions, and there are**no**assertion failures or exceptions, then the Theory passes.

In the following example, the theory SquareRootDefinition verifies that the implementation of square root satisies the following definition:

*
"Given a non-negative number, the square root of that number
is always non-negative and, when multiplied by itself, gives
the original number."*

public class SqrtTests { [Datapoint] public double zero = 0; [Datapoint] public double positive = 1; [Datapoint] public double negative = -1; [Datapoint] public double max = double.MaxValue; [Datapoint] public double infinity = double.PositiveInfinity; [Theory] public void SquareRootDefinition(double num) { Assume.That(num >= 0.0 && num < double.MaxValue); double sqrt = Math.Sqrt(num); Assert.That(sqrt >= 0.0); Assert.That(sqrt * sqrt, Is.EqualTo(num).Within(0.000001)); } }